Need some help solving electrical flickering

[coops09gxp]

Hello all, I'm new to the Envoy scene and really happy with it for the most part. I'm having a rather perplexing issue and I'm not sure if it's electrical or motor.

What I have going on encompasses a number of things but it seems to all tie together. With the vehicle running if I turn on my dome light it flickers in time with a slight miss in the engine that is on the passenger side. Actually all the lights flicker in sync with the slight miss, this includes headlights, dash lights, all of the lights. My passenger rear back up light comes on very slowly and also very dim when I press the unlock button. The driver side back up light comes on instantly and bright. I also have a fuel gauge that seems to have a mind of its own and just goes all over the place, from full to empty, and then somewhere in between.

My first thought is a bad or flakey ground somewhere in the system. It just seems that to many things are tied in with the slight miss, dome light flicker, fuel gauge, and back up light. The other thing I have noticed is that the radio seems to forget things sometimes. Some days I will hop in and the time is correct. Other days it will have changed all on its own to Eastern Time, Western Time, all over the place. The radio presets stay ok but it just seems to be a little forgetful.

I'm hoping for a little help or ideas as to what could be causing the issues. I'm all ears here so fire away with your suggestions and ideas!!

Thanks,

Tony

 

[The_Roadie]

Welcome! You've come to the right place.

Grounds are a good thing to start looking at, especially the one near the battery to the inner fender sheet metal. And the battery cables.

How old is the battery?

Can you take the alternator and battery to your local parts store to be load tested?

Fuel level sender is a separate issue localized to the sender in the fuel tank. Could you fill out your vehicle profile with year and so forth? There was a recall that might apply to your vehicle. Are you the first or a subsequent owner? Does GM have your ownership on record in case of recall communications?

 

[coops09gxp]

Hello Roadie!

I was turned to this website from the "other" by [somebody we know and love]. I can take it to a local parts store and have them load test the battery and alternator. I believe I'm the third owner of the Envoy. From what I have seen it was very well taken care of by the previous owners!

I'm not sure of the age of the battery, but it does look damn near new. I can pull the hold down and take a look to see if there is a date stamp on it. Thanks for the pointers about checking the ground near the battery and the inner fender. I will check those first.

I'm certain GM has my ownership on record since I just got a recall notice for a door switch short circuit recall.

If there is any other information that I can provide to help in resolving the issue let me know and I will get that info posted asap!

Thanks,

Tony

 

[coops09gxp]

Can anyone tell me what the wire in the top photo is for? It looks asthough it hooks to the lug on the underhood fuse block. The lug on the fuse block has +12V on it. I can't find what the purpose of this lug is in either the Chilton or Haynes manual.

The other two pics are the grounds under the hood that I checked, them measured out good.

The battery load checked alright and the alternator did as well. One other thing I have noticed is that the voltmeter on the dash with drop down to about 12V when the engine is running. It will do this under any and all driving conditions. It will drop down for a few seconds and them come back up. When it does this though, all the lights dim and anything else electrical running slows as well.

I haven't had a chance to dive under the vehicle yet to continue my hunt for grounds. Any input or advice would be great!!!

Thanks,

Tony

 

[dmanns67]

Can anyone tell me what the wire in the top photo is for?

Check out this link.

http://gmtnation.com/f23/what[red-wire]-2173/

 

[Mark20]

BTW the gray harness clip has a spot on it to hold the wire and keep it from jiggling around.

 

[coops09gxp]

Dmanns67, thanks for the link!! Guess you could say it was my right of passage lol! There was some good ideas in that link as well about uses for that post on the fuse block!

Mark20, thanks for that tip, I didn't even notice that while I was under the hood.

I did some more playing around with things tonight. I noticed that the lights flickering is in rythme with the miss throughout the rpm range. I held the throttle at different points and the lights would keep time with the miss. The higher the rpm, the faster the lights would flicker. If I would rev the motor up and just let off, the lights would get steady for a second and once it settled back into idle they would start flickering again.

Other than that I haven't had time to get to anything else. If there is anymore thoughts on the matter please, I'm all ears!!

Thanks,

Tony

 

[coops09gxp]

Hello all,

So I thought I would add to this and sort of update things a bit in my quest to solve this problem. So far I have yet to find the culprit of the electrical flickering. I have been checking grounds on the vehicle and installing serrated star washers to help promote good contact on every ground that I have checked so far. Yesterday I replaced the fuel pump in the Envoy and upon removing the neg battery cable the headlight washers went off. I thought this was incredibly odd for these to go off when I removed the neg battery cable!!! The other thing I have noticed is with the
4wd and switch on the Envoy. I will be driving along and notice that it is trying to go into either 4hi or 4lo while going down the highway. The switch stays in 2hi all the time. I'm trying to go through the wiring schematic for the Envoy and see if I can find a common point the would effect the lights, miss, and all the other electrical stuff going on with the Envoy.

I'm leaning towards the BCM as it seem many systems are tied into this. I'm trying to find where the BCM can be somehow linked to the engine miss. If anyone can steer me in the correct direction or offer up some advice it would be great appreciated!!!

Oh, forgot to mention, did the plugs as well yesterday. It did take some time but it really isn't that bad when you jack it up and remove the pass. front wheel. Remove the two nuts that hold the AC lines to the pass valve cover and it will give you enough room, BARELY to get to the front plug on the pass. side. I have pretty meaty hands to if you have skinny fingers and hands it may work even better for you. I have noticed the idle has improved a good deal since doing the plugs.

Thanks for any help that you folks can provide and if anyone has any questions about removing the gas tank or doing the plugs feel free to ask away or shoot me a pm. Have a great Sunday folks!!

Tony

 

[IllogicTC]

I don't think the BCM ties into the miss, however the flickering does tie into the miss.

As for other electrical gremlins, that is rather odd for things to turn on with the negative cable coming off. How new is the battery?

The (failed) attempts to go into 4LO may be something internal to the switch. I know of people who have removed and disassembled the switch to adjust the contact wiper, and have found tons of corrosion inside. Try running the switch back and forth about 50 times and see if that helps alleviate it (that's a Roadie solution right there ). You may also have an issue with serial communications.

Makes me miss Iowa man, I could come help you I used to live only a half-hour from the QC.

 

[Texan]

When I see flickering lights, my first thoughts are, loose connection, bad ground, or
bad diode in alternator. May require a oscilloscope to determine.

 

[coops09gxp]

Illogic,

You mentioned that the BCM wouldn't tie into the miss but the lights flickering WOULD tie into the miss. Is there some common circuitry, especially grounds, that would tie the lights and engine miss together?? I'm using both a Haynes and Chilton manual to go through the wiring. The one thing I have noticed about both manuals is neither of them give you the slightest clue about where the connections are located. The service manuals I had for my Pontiac G8 actually told you where the connections were and also showed the locations of the grounds.

With the 4wd and the switch, I'm not trying to change it into 4hi or 4lo. For whatever reason the vehicle is trying to engage either 4hi or 4lo on its own. I will give the switch turning method a shot and see if that helps it. Otherwise I will just replace the switch and go from there.

You also mentioned serial communications, can you possibly explain a little how or what the serial communication is used for on this platform and if or how it could tie everything together??

What area did you live in when you were back in this area? Some help and knowledge on this platform would have been great!


Texan,

You brought up a good point about the diode in the alternator, I hadn't even given that a thought! For as many systems that are being affected with the problem it really has me leaning towards the bad or flaky ground. When doing a load test on an alternator would they be able to detect a bad diode?

Thanks for the help folks, the quest for the solution continues. Hope you all have a fast and easy Monday!

Tony

 

[MAY03LT]

When doing a load test on an alternator would they be able to detect a bad diode?

I took a alt with a known bad diode around to see who or what could catch it. Autozones machine couldn't catch it, and the machine at my work (snap on vat45) couldn't catch it. The only thing that caught it was a lab scope (insight) that could do a diode test. The alt output was ok, the only symptom was flickering lights.

 

[coops09gxp]

MAY03LT,

Correct me if I'm wrong here. With the alternator, it is using the diodes as a bridge rectifier circuit to convert AC to DC correct? If this indeed is the case then a bad diode in the alternator could possibly be letting some AC signal pass through to the rest of the system correct? Could this also account for the miss that I'm having and also all of the other lights that are flickering are in time with?

Thanks for your insight and findings with your alternator, very good to know especially since I took mine to Autozone for a load test.

Tony

 

[IllogicTC]

My thought process is that the miss is bad enough that there is a notable variation in engine speed (especially bringing it below idle), flickering lights may match the part of the engine cycle where it slows down as there is no cylinder firing to keep the mass spinning at the desired speed. This may not always be the case, just something I'd seen before, then again this was on a 4-banger.

If one were to have a lab scope or some super-mega ammeter that goes far beyond the capabilities of your typical OTS meter (typically 10A max), the alternator could be tested by one self. I have a feeling May03LT is onto something.

Serial com ties most modules in the vehicle together. The security system involves several modules (including BCM and PCM) chatting in order to allow an engine start. You'd know if security was the problem as the security lock light would be on or flashing, and of course it won't start. Liftgate module chats to the BCM and the DDM and PDM to tell the doors to unlock when you use the key fob. The radio chats to the BCM I believe to confirm it's installed in the vehicle it's supposed to be, and also chats to the amp in the Bose system in order to make it turn on and I believe also for adjusting tone and fade/balance. BCM even chats to the DDM and PDM when you turn the panel dimmer to tell them how bright to be, rather than having a wire for the lights in them!

 

[coops09gxp]

IllogicTC,

Thanks for the info about the serial comm lines! One question about though, and this is probably gonna be noob stuff for ya lol. What the hell is DDM and PDM?

What you mentioned about the miss got me to thinking in a couple directions at once. I sort of combined what you are saying with what MAY03LT said. The miss is noticable that is for sure, well, to me anyway. I've been a motor head since I was about 4 yrs old so the feel of a miss drives me nuts lol! I can feel the loss in performance and also can feel it at idle. With all the lights flickering in time with the miss and at all rpm ranges it got me thinking. At idle, the lights flickering is much more pronounced, as the rpm increases the lights still flicker but it becomes more faint. Now, my thinking here, one of two possible things could be happening.

As the rpm increases the lights flickering could become more faint just from the increase in rpm and make it appear that they are more steady. Kinda like a strobe light, as you turn the speed up on the strobe the more steady the light will appear The other train of thought I had goes back to what MAY03LT said. If there is a bad diode in the alternator, as the rpm increases this would make the signal coming from the alternator look more and more like a true or better DC signal. There would be less time for any possible AC signal to pass through to the rest of the system thus making it APPEAR that the lights are flickering less as they would be seeing a more consistent or steady voltage supply.

I'm gonna try the alternator tomorrow and report back. I would be tickled shitless if that was the fix for this!!!

Again, thanks for the input and help thus far!!!

Tony

 

[coops09gxp]

Well,just to update things. As much as I was hoping MAY03LT was onto something with the alternator sadly, it was not the issue. Oddly enough, it seems the flickering is more pronounced since installing a new alternator. The lights do seem to be a bit brighter, this may also explain why the flickering seems more pronounced now. The miss is still there as well, and the flickering is still in time with the miss.

I'm not sure if I have mentioned this or not. At random and in all driving conditions I will notice the battery gauge drop down to roughly the 12V range and everything in the vehicle dims badly. It will stay down for maybe 2-3 seconds and then gradually come back up, it has done this since day one also. I don't know at this point, getting kinda frustrated with the damn thing! I will continue trying to find a common link between the miss and how this would affect all of the lighting, and also keep checking the grounds when I have time.

Any and all thoughts or ideas are much welcomed at this point!!! If a solution is found I will certainly update things and reveal the findings.

Thanks,

Tony

 

[Texan]

You all have me straining my brain. How long ago was "from day one"?
The volt meter in the cluster is stepper motor produced, right?
Many years ago I bought a volt meter with a pigtail and lighter socket
plug. How about a lighter socket adapter to a DMM and monitor the real voltage
real time.

 

[coops09gxp]

Texan,

The phrase I used, "since day one" is in reference to when I purchased the vehicle in Dec. of last year. From what I have read, you are correct, the volt meter in the gauge cluster is stepper motor driven. With the DMM, I can just hold it in the lighter socket and monitor the voltage this way to get the real time readings you are speaking of. The vehicle doesn't have to be moving, just running for all of the BS to occur.

A little brain strain never hurt anyone did it lol? It's a good thing I'm bald because I probably would have scratched all the hair off my head already brain straining on this thing.

 

[Texan]

I am glad that you have a sense of humor. How about this possibility? Autos use to have
a ground strap between the motor and body. I have no idea if or where a TB might have one.
But it probably does, and it may have a high resistance. Use a ohm meter to measure between
the battery and/or body ground connection and the engine block. Should be minimal ohms.

 

[coops09gxp]

Texan,

Good call on that, I will go out and check that now and see what I get. From reading in the manuals they list 5 ohms as the break point. If you get anything over 5 ohms the ground is either flaky or bad, and anything under 5 ohms in their opinion is a good ground. I will let you know what I find out.

Having a sense of humor is almost a must when working on vehicles! If it wasn't for a sense of humor I probably would have set fire to a few projects many years ago lol.

Any other information or suggestions are very welcome!

Thanks,

Tony

 

[coops09gxp]

Just got done taking some ohm readings at various points and attached to various things. Every single ground I checked was good, less than 1 ohm resistance on all of them. I checked the ground attached to the front of the pass. head to battery and chassis. I also checked from the battery to other various grounds and mix and matching connection style, i.e. battery neg to body, engine block to body, engine block to battery neg. and so on. Everything came up good.

Now, with the lighter voltage, I did some checking there as suggested. The voltage coming out of the front lighter, right below the rear wiper switch read a constant 14.3-14.4 volts. I have found that I can induce the low voltage scenario by lightly and quickly tapping the gas pedal to get the motor off idle. If I do this more than once I can get the volt gauge and the DMM to read the same, they both would drop to about 12.6-12.7V for roughly 2-3 seconds and then it would gradually come back up. I did also notice that while doing the light and quick gas pedal tap manuever, patent pending by the way lol, that the engine would start to stumble and run rough, this would last roughly 5-10 sec. I good romp on the gas pedal would have it return to normal and running somewhat smoothly again.

At this point all I can do is keep trying things and following the suggestions of you folks to TRY and get to the bottom of this issue. At the same time, keep providing feedback for anything I notice that seems abnormal hoping it sets a light bulb off for someone. This really sucks as I know the Envoy isn't running to it's full potential and I would love to sink my hooks into this thing and start making some performance upgrades!!! Kind of a wasted effort right now though.

Keep those thoughts and suggestions coming folks, what may seem trivial to most may be the golden egg of a solution that I'm looking for!

Thanks,

Tony

 

[Texan]

Just a wild idea. What would happen if you removed the sepertine (sp) belt and run
the engine. Do the lights blink. If not I would wonder about the alternator. If they do
must be a ignition problem.

 

[Hypnotoad]

Sounds to me like you have a burned, corroded, or loose wire somewhere.

 

[coops09gxp]

Just did the alternator today. I think it was my first post today talking about the observations after installing the alternator.

 

[Texan]

I understand, but did you run it without the alternator turning. That would isolate the possibility
that the alternator is the problem. If you got a rebuilt alternator, there have been cases where
they were bad out of the box.

Should have thought of this sooner.

 

[kickass audio]

I have a slight flicker in my dome lights only on my envoy and it has done that since I upgraded my alternator. I also upgraded to add AGM batteries and 4 additional batteries to the truck and upgraded the ground & power wires to have 1/0 on top of the stock wires. I never really gave much of a care for it as it only happens if I keep the lights on for longer than 3 seconds which is weird but idc about it.

You should have 0 ohms resistance on the wires at rest. Try to test the resistance of the ground wires under load too, like turn your vents on full speed, defrost on, all the lights on, etc. Sometimes you may have a good reading on a ground connection with 0 current flowing through it but once you pull current through it the resistance spikes up.

I would try to take off the accessory belt and remove it and run the engine for a minute or two with just the engine and no accessories on the belt attached and see if your light flickers. If it does not then something with your alternator is affecting your electrical. I would then double check the resistance of the wiring with the alternator under as much of a load as you can place it on and see what the reading is. Another easy thing you can do too would be if you can get to the back of your alternator through the wheel well is prob easier, disconnect the harness that controls the on/off of the stator and the duty cycle wire then fire up the truck and see if your dome lights flicker. My lights only flicker at idle and not at any other speed so it really is just something stupid that I could care less about. Do yours flicker faster when you give it a little bit of gas or rev it up and down? If they do then you may have a bad ignition module for one or more of the pistons that is arcing to ground and back feeding power onto the ground side. Sometimes it doesn't really do anything to be noticeable but it does have that chance to go to ground and play with other electronics.

 

[coops09gxp]

Texan,

Good point and I see where you are coming from! The alternator is new, but like you said, you can get bad ones. For no more than it takes to pull the belt I will give that a shot.

Hypnotoad,

I agree with what you are saying and that is what we've been trying to diagnose. The one thing that is linking stuff together is the engine miss. The lights flicker in time with the engine miss. If you know how or where these two systems can be tied together I'm all ears!!! I have been going after the grounds on the vehicle as that is going to be the most common place for all the systems to interact.

Thanks for the input and help folks, the quest continues!!

Tony

 

[coops09gxp]

Kickass Audio,

Very good points about checking the grounds under load, never thought about that!! I have the Denali with the V8 so my alternator is right on top of the engine and easy to get at. I noticed today that I only have two wires going to my alternator, I thought there was suppose to be four.

A few posts back I mentioned again how the lights flicker in time with the engine miss. The higher the rpm the faster the lights flicker. It gets to a point in the rpm range that the flicker becomes so fast that you can barely notice it. Guess the easy solution there is to keep her above 4500rpm and go fast until it sounds exspensive lol Now, as the engine returns to idle from higher rpms the light flicker becomes much more pronounced for a sec or two until it settles into its normal idle.

Good suggestions guys, will have to try these things tomorrow. Any other thoughts, ideas, or suggestions please feel free to post them up.

Thanks,

Tony

 

[IllogicTC]

I'm still going with the miss causing flicker. When you press the gas lightly and increase the RPM a couple hundred, do the lights get brighter or stop flickering?

The alternator in this application is fairly interesting in that it dynamically adjusts load based on several factors, and the miss dropping the RPMs down (I'm imagining at least a consistent 600 or less) could be affecting the system.

Find and fix the miss, since it's going to cause more harm than flickering lights, and if that problem then fixes itself, then you have a 2-for-1 fix.

 

[Texan]

My problem is this, I was a small block V/8 chevy person all my life until I got this I6.
My TB has been amazing (knock on wood) for 133,000 miles. Only normal maintenance.
It very well could be a ignition problem, but I am not familiar with the newer systems
on the V/8.

 

[kickass audio]

If my memory serves me right this engine has the coil packs for each plug right above the header. What you can do if it is just a single pack that is messing up would be to disconnect the electrical harness for it at the ignition pack on the header. Disconnecting the wire going from the coil to the spark plug may prove useless as the ignition pack could be bad and can be arcing out. You could also have a bad wire going from the ignition pack to the spark plug that is close enough to the body of the engine to jump to ground. It will set a misfire on your truck so don't do that test without a scan tool to reset the CEL when you are done testing. I would only do this kind of test at the absolute end of everything. I never advise to throw money at parts without diagnosing the culprit but you can always go to a junk yard or something and get some ignition packs for your truck cheap but the GMT platform in my area is extremely rare to find in a junk yard which I guess is a good thing.

My first step would be to start the engine in the night with it being as dark as possible, do not use any lights under the hood and start the engine. Check all over the side of the engine by the ignition packs and wires for any sparks, if you see them then you have found your bad wire or ignition pack.

I can't speak 100% on this but this should be your engine and if you are stumped on the places to check see the following picture. Those black boxes on the side of the engine that you can see 4 of are your ignition pack and the wires going down are where it goes to your spark plug. Unless you love being shocked like I do, DO NOT grab those wires at all with the engine running as if you have one with a pin hole break on it where the electricity can leak out you will get one hell of a shock.

http://www.justanswer.com/uploads/ebrock63/2009-07-10_170218_2009-07-10_110126.png

 

[coops09gxp]

Illogic,

When I increase the rpm the lights flicker faster in time or rythme with the miss. The idle of the enigne hovers around 650-700rpm and maintains that. I may just start unhooking coilpacks one at a time and see if I can find something this way. It may or may not kick a CEL but I have a decent scan tool for reading and clearing those.

Texan,

I'm with you, I grew up working on the old style small block Chevy and have made the leap in the last number of years to the new LS family of engines. The LS family of engines from my experience is a great engine, it has its draw backs like anything but it is a nice leap from the old small block of the day.

Kickass Audio,

I'm not new to the LS engine family, but I do thank you for the picture and explanation about where to find the coil packs. You never know, some folks may not know that much and it is this kind of help that makes the forums such a useful resource!! My last vehicle was a 09 Pontiac G8 GXP that I spent a good deal of time working on and upping the go factor on. That had the LS3 engine in it which thankfully did not have the DOD bullshit that is in my Denali. After doing a few different head and cam combos on that car I decided to say screw it and go big. I ended up putting a hand built 418 stroker in the GXP. Needless to say, she ran very nicely to the tune of low 11 sec ET's and 500rwhp!!! The car was my daily driver and yes even in winter but she did great!! The build of that motor was nothing special really, just some well selected parts, small custom cam, and one hell of a great tuner to work with! Sadly though, I sold the car before I could ever get back to the track and really see what she had in her. The high 10's were certainly there for the taking with that car!! Not to bad for a four door daily driver lol.

I know the Envoy certainly is not that car!!! Do I plan to add some go fast goodies to the Envoy in the future, HELL YES!!! My first and foremost goal is to make sure she is running correctly and that my friends is why I am here. Hoping to solve some of the issues that I'm having with my Envoy and at the same time learn and also help when I can!

Thanks for all of the input folks!!! I will try the coil packs tomorrow and see if I can find one that doesn't have much effect when it is unplugged. Oh, the plugs and wires were just replaced this last weekend. Doesn't mean that there couldn't be something wrong with either the plugs or wires but, the same symptoms are still present that were there before doing the plugs and wires. Have a great evening folks and thanks again for all the input thus far!!

Tony

 

[kickass audio]

Also with them being replaced recently double check the actual feed wires going to the coil packs that switch them on and off. Try to get a look at the whole harness run if you can to make sure nothing melted or anything like that to cause a short. Typically if you have something like this happening and it is not alternator related you definitely have a short somewhere in the system.

If this was my truck I would troubleshoot it like this:

1.) Take off the accessory belt or disconnect the harness from the alternator, whichever is easier and run the engine and look for any flickering on the lights. If it is still present proceed to step 2 otherwise replace the alt.
2.) Wait until night and start up the truck and look for any sparks or anything on the wires coming from the coil packs to the plug, replace if any are found.
3.) Look into the wire loom that feeds the signal to the coil packs for any melted spots, cuts, holes, etc in the wire jacket.
4.) Take my handheld o-scope and test the voltage both with the alt charging and without it charging to see if it has a lot of ripple to it.
5.) Test the resistance of your power and ground wires (the big-3 as we call it in the audio industry. ground from batt to engine, ground from batt to frame (fender), power from alt to battery) with a load on them so turn on your electronics to place a heavy load on the wires. Note the resistance of each wire.

 

[NJTB]

Did anyone suggest replacing the battery cables?

 

[The_Roadie]

When I increase the rpm the lights flicker faster in time or rythme with the miss.

I'm still going with the miss causing flicker. .

A scope could compare the two events and see that one happens a few microseconds before the other, to confirm the cause->effect relationship. My gut tells me there's a problem with one plug or coil at high voltage, not depending on RPM. The arc happens in the coil or plug well, not the plug. The missing combustion cycle is experienced as a miss. The electrical disturbance spike travels back along the ignition wiring or ground to confuse the alternator's built-in voltage regulator, which turns off for a few milliseconds, and that voltage droop from alternator voltage to the couple of volt lower battery voltage is seen in the lights as flickering.

You should have 0 ohms resistance on the wires at rest. Try to test the resistance of the ground wires under load too, like turn your vents on full speed, defrost on, all the lights on, etc. Sometimes you may have a good reading on a ground connection with 0 current flowing through it but once you pull current through it the resistance spikes up.

I have to correct this misconception. I got bit with it myself in high school in a science fair project. Before you start speculating, let me tell you this was in 1966, and yes, we did have voltmeters and oscilloscopes that far back.

Measuring resistance using a meter is actually measuring a small voltage, while the meter simultaneously acts as a current source using an internal battery. You can only get a valid resistance measurement on unpowered circuits, because the meter assumes that its current source is the only one acting on the circuit. So let's say the meter is a 100 milliamp current source on a low resistance scale. The exact amount of current depends on the meter design. If you put that meter on a 1 Ohm resistor, it would develop 100 millivolts of voltage drop, and the meter would tell you it read 1 Ohm. If you put the same meter on a short length of wire and the measurement was 10 mV, it would display that as a 100 milliOhm (0.10 Ohm) resistance. If you put that on a 10 Ohm resistor or long-skinny piece of wire, it would measure 1.0 V, and display that as 10 Ohm after the meter did the math.

The math is simple Ohm's Law: V = IR (V is volts, I is current (for historical reasons), and R is resistance in Ohms)

Hopefully you can see the issue. If you put that meter on a powered up piece of wire, with a lot of current already flowing through it, there will be two sources of the voltage that gets measured by the meter. One caused by the 100 mA coming out of the meter, and another caused by the (let's say) 50 Amps flowing from the electrical loads you just put on the wire. Problem is, the meter only can measure voltage, and it doesn't know that there's any more than ITS 100 mA flowing, so when it does the math, it goofs and tells you the resistance of the wire has gone up when it's really just the voltage drop from one end of the wire to the other that's changed.

This also explains how meters can be very inaccurate in the low range. Most meters aren't putting out anywhere near 100mA current, and the voltages they're measuring aren't as large as 0.1 V. When the meter is measuring very tiny voltages on a good wire or ring lug ground connection, it can't be very accurate. You can get a hint about this by shorting the meter leads together. See what the calculated resistance reading is for the shorted leads. You will NEVER get a lower reading on a measurement, and it will probably change a lot because of unavoidable measurement noise.

Wire resistance doesn't change with the current flowing through it. I guarantee it. You can get to wrong conclusions if you think it can. That said, there are microscopic thermal effects that can happen at very high current flowing through small cross-sections of wire or corroded ring lugs or the like. They can involve making the narrowed-down section glow until it welds things together and sometimes fixes the corrrosion. This sort of thing happens to relay contacts all the time - it used to happen to distributor points but nobody has points anymore. There is another effect called the thermoelectric effect, that can cause errors in the 10-30 microvolt regime, but you'll never see these in automotive systems.

My day job has for many years required extremely accurate measurements of very low and high voltage and current values. Metrology is a way of life, not just a hobby, for me.

Any more questions, just ask.

 

[coops09gxp]

Kickass Audio,

I have done #2 and 5 from you list, #5 was done with no load being placed on the circuits. I was simply checking to make sure I had a good contact with the various grounds that I could find under the hood of the vehicle. With the things that I have changed recently, i.e. spark plugs, wires, and alternator, there hasn't been any change to the problem at hand. The only thing I have noticed since doing the alternator is that the lights seem a hair brighter. That being said, the odds of any of those three things being the culprit here are becoming highly unlikely. That is not to say it isn't possible!!! I'm just simply going by the outrageous odds that either the plugs, wires, or alternator would be what is causing the issue still.

From what I have noticed on this forum with our vehicles it seems that corrosion in connectors and contacts is a very common problem. With that in mind, I went through and pulled all the injector, coil pack, and any other connectors I could find and reach under the hood of the Envoy and just manually plugged and unplugged them numerous times to possibly wear off any possible corrosion that could be there in the hopes of maybe finding a flaky contact point. This made zero change to the situation at hand.

The reason I'm leaning more towards a possible bad ground over a short circuit is that a short would more than likely end up blowing a fuse or burning a wire from the increased current draw in-turn creating more or even bigger problems. Bad or poor grounds account for many of the electrical problems that I've seen in cars over the years. Now, this in no way means that I'm correct here or ignoring what you are saying, such is not the case at all!!! I will continue with the list you provided minus the O-scope part. I don't think my employer would be very happy with me walking out with one of the scopes from one of our work benches lol.

NJTB,

You are the first to mention replacing the battery cables. It is a possibility and will be considered.

Roadie,

You bring up a VERY good point and one that I often don't even consider or think about any more. You are correct about how a DMM works! This is how the DMM derives its values for the measurements we take with them. What's the first thing you do when you turn your DMM to the ohm reading, put your two leads together to verify you have good contact and establish the base reading. At least this is what I do with my meters. When I touched the leads of my DMM together before I started checking the grounds I got a measurement of .03 ohms. As I was checking the grounds under the hood in all the various fashions that I did the highest reading I saw was something like .05 ohm. What you mentioned about the miss and the effects that it could possibly have on the rest of the circuit made a lot of sense! This could very easily explain what is happening! I should be able to scrounge up another coil pack or two, from that point I can start swapping them out and see if I can find the culprit that way. It could also very easily explain why the flicker would appear to follow the rpm. As the feedback becomes more rapid due to the increase in rpm and the possible bad coil feeding things back into the circuit it would more than likely make the flickering appear faster. You brought up a very good point with what you said! See, that's why I like all the input from folks, it spurs the mind and gets us thinking a bit and this is how problems get solved!

I haven't had time to do anymore work or testing today, I'm taking care of a parent who recently had a stroke and open heart surgery. I try and get as much done as possible in the limited amount of time that I have. Thanks for all the info, input, and help so far folks!!! A solution will be found it's just a matter of when and what. I will keep you all posted with the testing I'm able to do and report back with the findings. Keep the thoughts and ideas coming folks, it has helped me a great deal learn more about this platform of vehicle.

Thanks and have a great day!

Tony

 

[coops09gxp]

Well, here is a little update on things. I was able to source a different coil pack for the Envoy. I went through and swapped each one out individually with no solution to the problem. I took it one step further and one at a time, I unplugged each coil from the wiring harness and started the vehicle up. Again, this didn't get me any results. I take the Envoy in tomorrow for the door switch short circuit recall, would a Tech II be able to give me any other information or insight that a hand held code reader would not, like serial communication lines getting funky signals? I know someone mentioned it on the first page and that is why I was thinking along those lines. Really starting to run into a wall here with this thing.

Just an off the wall and long shot question. I notice that everyone says to strickly use the ACDelco 41-103 spark plugs, is this just for the I6 or does this also include the 5.3L?? The manual called out specifically for 41-489 or 41-498 ACDelco plugs. For as touchy as these things seem to be about things I just want to make sure the info is correct as I have only seen the 41-103 plugs referenced in I6 posts.

Thanks again for all the input and help thus far!

Tony

 

[Texan]

Just a guess on my part, but AC41-103 is only for the I6. It has a really long threaded portion.

 

[IllogicTC]

41-103 does not fit.

Double check and ask around, but I believe 41-110 will. It's also an iridium plug and has likely superseded the old platinum plug, just as the 41-103 has superseded the old I6 plugs.

 

[BRomanJr]

Dmanns67, thanks for the link!! Guess you could say it was my right of passage lol! There was some good ideas in that link as well about uses for that post on the fuse block!

Mark20, thanks for that tip, I didn't even notice that while I was under the hood.

I did some more playing around with things tonight. I noticed that the lights flickering is in rythme with the miss throughout the rpm range. I held the throttle at different points and the lights would keep time with the miss. The higher the rpm, the faster the lights would flicker. If I would rev the motor up and just let off, the lights would get steady for a second and once it settled back into idle they would start flickering again.

Other than that I haven't had time to get to anything else. If there is anymore thoughts on the matter please, I'm all ears!!

Thanks,

Tony

I'm wondering if you have a shorted (or partially shorted) injector, the behavior "If I would rev the motor up and just let off, the lights would get steady for a second" coincides with the time that the injectors may be turned off or barely activated. I would try to isolate each injector by unplugging and doing a running check.