SOLVED! Fuse #28 Strikes again....Ugggghhh....

[c good]

Previously had trouble with this but thought it was remedied in October of 2021 with replacement of the PCM from PCM of N. Carolina. Mileage at the time was 190K. Now it has 200K miles. In the past 10k miles and almost three years it's been running perfectly. Better than ever before. Until today. Uggghhhh....

The dreaded fuse 28 blew and left me stranded on the side of the road. Luckily only 35 miles from home and I have AAA Premier so was able to get it towed home on a flatbed truck.

Similar to other times, I put a new fuse in while I was waiting for the tow truck to arrive and it instantly blew it again.

Loaded it up on the tow truck. Driver left it off in my driveway. Take a few minutes to look everything over, confirm the blown fuse. Replace it with a new one. Turn the key. Starts right up. A little bit of a rough idle until the PCM relearns and resets and it's all good again.

The fact that it's a blown fuse again, even with the new PCM, I'm now leaning towards a direct intermittant short on that circuit.

Fuse 28 powers the PCM, the fuel injectors, the coil packs, and the fuel pump. Am I missing anything?

Looks like I'll be pulling apart some wiring harnesses looking for a partially broken wire. Pink will be the one I'm following. There are no signs of rodents chewing on anything and it is always garaged with no evidence of rodents in my garage.

Any other suggestions would be appreciated.

 

[Mooseman]

And if anybody wants to reread the 4 pages of posts in the previous thread...

SOLVED! - Fuse 28 PCM-1 blows randomly

Engine stops and won't even turn over when it happens. Replace fuse,, starts right up and runs no troubles. I've got 60 miles on since it happened no troubles. 5 years ago I had a similar problem and it was never figured out 100% but was told it may be a bad coil so put all new OEM Delphi...

gmtnation.com

Maybe it's time to just throw in the towel and replace the whole harness unless you can find the short. It may be close to the PCM given after replacing it it was fine for a while.

 

[budwich]

time to check the coils again. :-( The only suggestion would be a "heat related event" such that once cooled down, it goes back to "good" for a "little while". You know about the fuse and the event... next time it happens, get out a meter and check the output of the fuse location (with reference to ground) to get a resistance reading of the subtending circuit. Its one thing to be a short but another to be "lower resistance" than the circuit expects. You can also do the same test even if its not failed just check what the circuits current condition is. Of course, your history on this is greater than my suggestion so take it with a grain of salt...

Not remembering much of history, did you "decompose" the fuse box layers to ensure that the tracking is good?

 

[c good]

I did not look at the next layer down below the fuses and relays. I was pretty sure the new PCM had solved the problem. Especially after the GM Dealership found nothing wrong then it blew a fuse again while in their parking lot while I was picking it up. I told the tech I had a new PCM that was VIN matched and he said let's give it a try. Then almost 3 years of Envoy Bliss....Until today.

I might look under there next. In typical fashion. It's running perfect again. When it blew it was inland where it was hotter than normal......after returning to the coast....15-20 degrees cooler. It started right up and hasn't blown the fuse yet.

I have a complete set of new coils. I may start substituting them in one at a time.

 

[budwich]

A resistance measurement will help to tell you how close you are to the "fuse blow" outcome which might characterize what kind of fault you are looking for... ie. a weaken component or full short event.

ADDED: I reread your original "problem life history"... brings back "fond memories"...

I think the best thing for you to try at this point is doing the resistance measurement at the output of the fuse 28 position in the fuse box. That will give you a base line. Then, if and when you get the blown fuse, do the same test again... so have your DMM handy at all times. Don't do any towing or vehicle moving assuming the vehicle is in a safe location (ie. off the highway) Compare the results to your "baseline". It is likely to have low / very low resistance. Next disconnect the PCM cable that feeds the power to the pcm (ie. c1). Again, measure the resistance at the fuse... did it change? That may tell you if your issue is at the PCM or some other location / cable (ie. injectors or coils).

 

[c good]

How would I do that? I have multi meter. Just don't know where and how to hook it up for that test.

 

[c good]

Could a fuel pump as it gets old, but still functioning just fine cause a fuse to blow ?

I also read a thread somewhere a guy with a 2002 Trailblazer had a bad fuel pump relay that was popping the 28 fuse. He had a hard short though. Easily diagnosed by replacing the relay and it worked.

Mine is intermittent. That's what's sooo frustrating. I replaced the relay and will test the old one to see if that might be it.

 

[budwich]

How would I do that? I have multi meter. Just don't know where and how to hook it up for that test.

Pull the fuse on the block. Put the meter in DC volts scale. Put the black lead on the negative battery terminal. With the red lead, touch the socket area where the fuse was. This is a check for voltage. It should read at one of the points 0 while the other should read 12v. The 12v one is the INCOMING voltage. The other side is the outgoing voltage (ie. 0) because there is no fuse in there. Now, put the meter in resistance mode. Leave the black lead at the negative battery terminal and put the red lead at the "outgoing" socket location that you just found. This reading is the resistance to ground that the power is seeing IF and WHEN it is applied. IF the reading is close to zero, then you have a short... bad. IF the reading is higher, 10s of ohms then the subtending circuit is likely good and a new fuse will not blow when inserted. BUT do not insert a new fuse, if the reading is near ZERO. Instead, as suggested, pull the C1 connector on the PCM. Does the reading change? Depending on the results, you might have isolated where the faulty section is. Hope that helps.

 

[TJBaker57]

Could a fuel pump as it gets old, but still functioning just fine cause a fuse to blow

The fuel pump is NOT powered from fuse #28. Recheck your wiring diagram.

 

[c good]

Thanks Budwich..Excellent info..I'll give that a try.

Thanks for the heads up TJBaker 57. For some reason I thought the fuel pump was on the same circuit. I did recheck the diagram and you are correct.

At least that's one thing less to check. So it's just the coil packs, the injectors, and the PCM1 on fuse 28.

Can a coil pack begin to fail, pull too much amperage just short of blowing the fuse and still run correctly without storing a misfire code? Then blow the fuse?

 

[c good]

IIRC....PCM C1 is closest to the firewall. Looks like pinout #19 is the pink wire ign 1 voltage?

 

[c good]

Did the recommended Budwich test. Found the power side of fuse 28 when keyed on. Power is on the driver's side. Then put multimeter to ohms, put red lead into outgoing side (passenger side). Put black lead to ground on battery. Got no reading at all. Screen read OL...so no indication of continuity at all...thus no short.

I started thinking I wanted to keep it simple so started with the basics. Coil packs seem to be the most obvious and common problem. I have a whole bunch of coil packs I've collected over the years. I did some ohm/resistance tests on the ones in the Envoy and found one where the numbers were a little high compared to all the other ones. So...maybe it's a coil?

The problem is the thing starts and runs just fine after its been off for a while. The last time it ran for 3 years and 10K miles.

I'm still curious where the correlation is with the fuel pump as there are a few other people that have replaced the fuel pum after having 28 blow and it "solved" the problem.

I wonder if it might just be worth putting in a new, quality fuel pump. I believe the one that's in it is original. So...200K miles on it.

 

[budwich]

hmmm.... I would have thought that you should have got some reading as opposed to an "open / infinity" reading. I assume at this point in time, your vehicle is starting / running as opposed to a blown fuse situation. It might the scale that your meter was on during the reading. Depending on the meter, you might have to select a scale (ie. ohms, kohms, mohms, auto... etc). You might have to try that but it is easy to check since you now have identified the outgoing circuit point.

I am going to pull mine to check.

 

[budwich]

I pulled my fuse. I think you have a probe contact issue with your reading. The probe is probably too large to make contact with fuse socket terminal and thus your reading is "open". You might need to make a "thin wire adjunct" to allow for good connection. My reading was around 10K ohms... which is kind of expected.

 

[budwich]

Could a fuel pump as it gets old, but still functioning just fine cause a fuse to blow ?

I also read a thread somewhere a guy with a 2002 Trailblazer had a bad fuel pump relay that was popping the 28 fuse. He had a hard short though. Easily diagnosed by replacing the relay and it worked.

Mine is intermittent. That's what's sooo frustrating. I replaced the relay and will test the old one to see if that might be it.

I think you might be on to something... maybe.... but your question is somewhat misplaced.

AS TJbacker57 points out, the FUEL PUMP is not driven by the PCM.... BUT as you further point out, the other "poster" indicated a BAD FUEL PUMP RELAY was popping the fuse. The relay and the pump are two different things. The fuel pump relay IS driven by the PCM and relies on some of the "juice" from fuse 28 to do its job... I think. Hence, IF the relay gets hot or otherwise, it maybe drawing more amps that "expected" causing the fuse 28 to tip over. That would seem like a plausible event... especially since there is NO direct fusing for the relay...

Having said that, as you have indicated, a "new pcm" seems to have resolved the issue "temporarily". That maybe because the components involved in driving the relay were new / in better spec than the older pcm. Once, a more "heavier than expected" relay draw / operation happened, the "new replacement" became a "victim" again OR the subtending relay circuit has diminished even further.

 

[c good]

Changed the scale. This is what I'm getting.

Also, that's interesting info on the fuel relay being on the PCM1 circuit. I put a fresh relay in also. I think I'll pull the old one apart and inspect for any burnt contacts that would possibly increase draw.

 

[budwich]

Changed the scale. This is what I'm getting.

OK... that seems a bit more "reasonable" as in 23Kohms. That is likely going to be in "operational condition"... meaning a fuse won't blow in that state as that represents about .5 m amp draw on a 15 amp fuse. Of course, that is in a "no engine run" state which is probably the lowest state for the PCM.

The "true event" will be doing the test just after a fuse blow event.

 

[c good]

UPDATE: I took the car around the block a few times. At about 10 miles it blew the fuse again. Luckily I was in a shopping center parking lot when it went as I was expecting trouble.

I put a new 15 amp fuse in, turned the key and nothing. Just the dash lights and guages as normal. No starter noise, no selenoid noise. Nothing. Then I checked the fuse and it was not blown.

The last time I had a similar situation, (minus the blown fuse part) I assumed it was a bad starter, so would tap the starter, actually the selenoid. Tap with a leather mallet and it would start right up. So, luckily I keep the leather mallet in the vehicle. Tapped the selenoid. Started right up.

When I worked on the previous starter problem, this past December, I noticed a dirty and somewhat loose connection where the main Positive High Amp battery cable bolts to the selenoid.

I removed the cable, cleaned the connection, re tightened the cable and it solved the intermittant starter problems I was having. Had no problems with either the starter or the fuse blowing for 7 months. It just seems like theres a direct correlation with the two problems.

Would a bad connection at the selenoid, or a bad selenoid or starter potentially put enough load on the circuit to pop #28. Possibly an internal short in the selenoid or starter?

I'm not seeing in the schematics I have where the starter circuit is anyway related but maybe it's in the PCM and the PCM pulls too much current when the starter or selenoid act up?

I'm going to pull the starter and check it out. I'm going to reorder that AC Delco starter to have on hand also.

Thanks for any brainstorming all. I'm gonna figure this out if I have to strip it down to the frame!

 

[Mooseman]

Check all the grounds, especially on the engine block. My main one from the battery to the block broke into green pus during engine mount replacement. Although mine didn't blow any fuses, it wouldn't crank even though power seemed to be there.

 

[mrrsm]

This Issue becomes extremely important *IF* you keep having Coil Over Plugs Failing on One Particular Cylinder:

(1) The behavior of ALL Coils is directly affected by the "Compression Atmospherics" present just BTDC (Before Top Dead Center) inside every Cylinder.

(2) This is especially TRUE if that Failing Cylinder's "Compression Atmosphere" is particularly LEAN.

(3) LEAN Conditions can cause the Primary and Secondary sides of the COPs to STRUGGLE by allowing the HV (in KV) to have to CLIMB HIGHER AND HIGHER on the Secondary Coil Side in order to get the Electrons to propagate from one side of the Spark Anode-Cathode area to the other during its Induction Discharge at VERY High Voltage.

(4) If the Spark Discharge cannot occur with relative ease as the Piston approaches TDC with a sub-optimal Fuel-Air Mixture... THE HIGHER VOLTAGE WILL OSCILLATE BACK AND FORTH IN BETWEEN THE SECONDARY COIL BACK THROUGH THE PRIMARY COIL and seek to Find an Alternative Ground. THAT COIL'S COMPONENTS WILL GET HOT AS HELL!

(5) If you find or observe Carbon Tracing along any of the Spark Plugs Rubber Insulation Boots ...or presents upon the outside of the Spark-Plug Porcelain Insulators... then the Ignition Spark is happening sporadically on the PRIMARY Side of the Coil instead of at the Secondary Side... and is NOT occurring inside of the Combustion Chamber.

(6) Chronic, Lean Combustion Chamber "Compression Atmospherics" will eventually cause the energy from the Secondary Coil oscillate back and forth in order to dissipate and cause the Primary Coil to HEAT UP and Short Out Internally.

(7) A Mixture of Air AND AN INSUFFICIENT AMOUNT OF ATOMIZED FUEL will prevent Normal Spark Propagation to occur inside of the Cylinders because the Hydro-Carbon Rich Combustion Atmosphere needed to encourage the Ignition Spark Electrons to pass through and Ignite the Compressed F/A Vapors and reach Ground INSIDE of the Combustion Chamber will not be present.

(8) DO NOT TOUCH THE TOP OF THE COP ALUMINUM FINS OF ANY SUSPECT COP ...UNLESS YOU WANT TO EXPERIENCE BURNING THE TIPs OF YOUR FINGERS!

(9) Swapping Coils between Cylinders will only confirm that the COP in Question has already been damaged....But ...Merely Replacing that Coil Over Plug will NOT solve the problem.

(10) Investigate and Mitigate the Following Issues (not necessarily in the order listed below):

(a) The Poor Conditions preventing proper Fuel-Air-Spark Delivery PRIOR to replacing the COP is the proper approach to take.

(b) Examine Fuel Pressure at the Rail for a Failing Fuel Pump. Examine the EFIs for Clogs or Damage to the EFI Wire Harness by means of an EFI Balance Test in order to explain the reasons the Fuel Spray being either Weak or Absent.

(c) Examine ALL of the Modules leading from one to the other in the sequence that starts at the Air Filter and ends with delivering a Mist of Atomized Gasoline into the Suspected Failing Cylinder as the means of improving ALL of the elements in between necessary to achieve Stoichiometric Balance.

(d) Examine the relationships between the Timing of the Crankshaft (CKP) Signal to the Camshaft (CMP) Signal, as the means of determining the presence of a Stretched or Worn out Timing Chain, Tensioner and Chain Guides. If these conditions prevail, it can ruin any possibility for the Spark Plug to Fire at the proper time and Ignite the Fuel-Air Mixture. Even being "Out Of Whack" by as little as 2-3 Degrees can have a negative effect on performance and drive-ability.

(e) Examine and compare the Cylinders for Dry & Wet Compression Values... looking for Leaking Valves or Loss of Compression from Worn Rings and Compression Imbalances.

(f) Examine and Investigate the snugness of the 10 Intake Manifold Fasteners (4.2L LL8 Motors) and Replace the Old IM Gaskets if needs be to prevent the entry of Un-Metered AIR.

(g) Use the ACDelco TEC (Top Engine Treatment) to break up the Excess Carbon "Mung" build-up in and around the Valve Seats...and loosen up the "gas-gum lacquered in" Top Compression Rings to improve Compression. Change the Oil and Oil Filter at the same time.

(h) Remember that even though the COPs are Electronically Controlled Devices for Fuel and Air Combustion...They Do Not Function in ISOLATION.

(I) Using an Oscilloscope for Wave Form Diagnostics with a Fairly High Sample Refresh rate of around 12 Bits is the preferred method for diagnosing WHICH side of the COP is suffering with this LEAN FUEL DAMAGE. The Image below shows just a few of the COP Tests that a Good "O" - Scope can perform... and take ALL of the Guess Work Out of the Repair Equation. The attached PICO-Scope PDF describes the Top Ten Automotive Oscilloscope Guided Component Tests beneficial for ALL "Diagnosticians" vs. Parts Changers... as the Latter will ultimately wind up Buying additional COPs... and never solve the underlying problem(s):





[Mod edit: this post moved from inactive thread to this one]

 

[c good]

Excellent info Mrrsm. Thanks for the scoop on the coils. I'm leaning more towards an electrical short possibly connected to the starter motor or selenoid. I replaced suspicious coil and it popped the fuse again after about 10 miles. Then after replacing the fuse, it did not blow upon trying to start. The starter motor simply did nothing as it had done in the past. A simple tap on the side of the selenoid with a leather mallet and it started right up. There has to be some kind of correlation there.

I just ordered a new AC/Delco starter and will be installing it soon. Fingers crossed I'm on the right track.

 

[mrrsm]

Just an additional FYI... In The Past...Whenever the Vehicle Stalls, Stumbles and DIES while driving... AND the Blown #28 Fuse is discovered...the Cause & Origin is a Shorted COP. Someone here at GMT Nation (I'm Thinking it was @MAY03LT) had Posted up an image of the COP Circuit vs. its relationship to the PCM that would lead us to believe the problem might be with the PCM.

However, as you just read... It was a Short Circuit in the Primary Windings of the COP that had reared its Ugly Head. The reason that you were able to continue driving right after changing out the Burned #28 Fuse... was because the Interior of the Coil Over Plug Copper Windings had a chance to COOL DOWN and then allow the COP to continue to function, albeit poorly, soon afterward.

 

[c good]

I have always thought it was C.O.P.'s also. Over the years I have replaced two complete sets with good quality Delphi COPs. Yet continued to suffer from this problem. The longest I went was with the latest replacement of the PCM. That was after spending 10 days at the dealership that could find nothing. I was the one that went ahead and bought the PCM.

I hate to buy another set of C.O.P.S. without determining if the current set is good.

I'll see if I can find a shop with an oscilloscope and somebody that knows how to use it on the existing C.O.P.S.

Is there any potential correlation between the starter motor acting up, requiring a light tap on the solenoid with a leather mallet to get it to engage possibly shorting circuit #28?

 

[Mooseman]

If this sequence of events is followed:

- Fuse 28 pops, engine stalls, no crank.
- Fuse 28 replaced, still no crank
- Tap the starter, cranks and runs.

I would assume it's a coincidental problem with the starter as there is a separate fuse and relay for the starter. My bet is a ground problem that seems to rectify itself when you happen to tap the starter where I imagine you have to touch/move the ground wires near the block, or maybe just the vibration. AFAICT, fuse 28 powers the PCM, coils and injectors. That's why when it blows, it doesn't crank as the PCM is missing power.

But I could be wrong. It happens.

 

[mrrsm]

I think the Starter and Solenoid Connections can become problematic if they are either Under---or---Over Tightened via their Eyelet Fasteners ... or ... if there are ANY signs of Green Copper to Aluminum or Copper to Steel-Iron Galvanic Corrosion invading the full length of the B+ Cable (Cupric-Oxide Deposits are DIELECTRIC and increase RESISTANCE TO CURRENT FLOW)... or ... likewise having the CU02 Invasion happening to the Solenoid Cable. Make certain that the Red Power Cable connection at the B+ Battery Post and the Ground Cable Connections are SOLID... but NOT Over-Tightened.

There should never be any reason to replace ALL of the COPs ...all at one time... because they are not connected in series, electrically ...AND... Ordinarily, the problems of having LEAN Conditions that will precipitate a COP Failure are NOT common to ALL cylinders at the very same time.

If you have Random P0300 Codes... that could indicate the problem of having Loose or Failing Intake Manifold "Figure 8" shaped Rubber "O" Ring Style Gaskets that have FLATTENED OUT and are allowing ambient air to leak inside the otherwise Metered Air Stream.

The important thing is to be able to SEE the behavior of ALL SIX COPS... and to be able to SEE the behavior of ALL SIX EFIs on Screen at the same time via a "Channel A" vs. "Channel B" Aligned "PARADE" View appear on the Laptop Screen with the Engine Running at Idle and during Snap Throttle events while looking for any correlation between Low EFI Performance Fuel Delivery as the Probable Cause and Origin behind any particular COP Failure throwing a Bad Wave Form upon the Screen as Proof of this Activity.

THIS is THE Best Training Video to PROVE What can Go WRONG when the Secondary Side of ANY Coil encounters a Diminished Hydro-Carbon Fuel Air Mixture as the Spark Plug Ignition Firing is supposed to occur. Paul "Scanner" Danner is THE MAN!

Duane from "RealFixesRealFast" Echoes this On Topic Discussion IN MUCH GREATER DETAIL:

Examining and Understanding The Primary Ignition Wave Form:

Primary Current Ramp Wave Form:

 

[c good]

Thank you Mooseman and Mrrsm.

With this particular gremlin it never sets a code. That's also why it's particularly frustrating. I'm going to replace the starter regardless since it does give me trouble sometimes.

While I'm in there, I'll check and clean the grounds, ground straps, and battery cables to make sure they all look good and maybe I'll test them for resistance.

I really want to keep driving this Envoy as it is so well maintained in all other respects. Thank you to all for the amazing patience and input. I will keep everyone posted.

 

[budwich]

In your long history of this problem, can you confirm that you have never had fuse 28 blow from a "cold start". That is, when a start attempt is done say after a few hours of sitting.

 

[c good]

Hi Budwich. That is correct. It has always been after running for awhile. It's never just popped the fuse from a cold start. Even letting it sit for 15-20 minutes after it's blown the fuse. I can put in a new fuse and it starts right up.

 

[c good]

Another clue. Fuse 28 pops every time I turn the key now. I unhooked all the coils and it doesn't pop. If I keep any two of them unplugged it won't pop the fuse. Once I connect 5 of them it pops.

There does not seem to be any correlation with any one particular coil.

Starter Motor arrived so will be replacing that next.

 

[Mooseman]

Didn't you replace all of them? Were they Chinese clones? Maybe need to get better quality ones. Of if you have a DMM that can display amps, you could check the draw from each one and total of them. Maybe the total draw is beyond the fuse capacity. Or maybe it could be an injector and pulling the coils brings down the draw under the fuse rating. May need to check both.

 

[budwich]

Hi Budwich. That is correct. It has always been after running for awhile. It's never just popped the fuse from a cold start. Even letting it sit for 15-20 minutes after it's blown the fuse. I can put in a new fuse and it starts right up.

Well I guess the "observation" is out the window now. :-( Kind of good in that you have a "scenario" that allows you to do things. Are you doing the resistance test before you replace the fuse? That check may save you a fuse replacement. Next, with the fuse out, do the same resistance test when turning the key to ON ... which seems to blow the fuse now... you should see some "difference" from your other measurements.

ADDED: if want to keep blowing fuses, you might as well do some additional scenarios... leave all your coils connected but remove your fuel pump relay... what happens to the fuse with key on?

 

[mrrsm]

Suggestion:

Based upon the Idea that, "When Conducting ANY Scientific Experiment... Only Change ONE Variable at a Time"

Observation:

There is very likely a Dead Short in at least ONE or MORE of the SIX Coil Over Plugs. Obtain a "Fist Full" of Replacement Fuses prior to beginning this Test Procedure. Do NOT Exceed the AMP Value listed in the Service Manual in ANY Fuse Positions in BOTH the Front & Rear Fuse Boxes.

Tactics:

(1) Remove and Lay Out ALL Six COPs in line with their present positions on Bench.
(2) Install ONLY ONE COP at a Time and Try to determine if Fuse #28 FAILS.
(3) Avoid Replacing ALL SIX Coil Over Plugs if possible ...and use ONLY OEM COPs.
(4) Conduct Resistance Tests on ALL COPs and remember that the Dead Short can be TRANSIENT with Thermal Variance.

 

[TJBaker57]

Just over 3 years ago there was this post showing a simple means of displaying fuse #28 amperage. Did you ever get this setup and might you use it now??

SOLVED! - Fuse 28 PCM-1 blows randomly

Engine stops and won't even turn over when it happens. Replace fuse,, starts right up and runs no troubles. I've got 60 miles on since it happened no troubles. 5 years ago I had a similar problem and it was never figured out 100% but was told it may be a bad coil so put all new OEM Delphi...

gmtnation.com

 

[budwich]

Just over 3 years ago there was this post showing a simple means of displaying fuse #28 amperage. Did you ever get this setup and might you use it now??

SOLVED! - Fuse 28 PCM-1 blows randomly

Engine stops and won't even turn over when it happens. Replace fuse,, starts right up and runs no troubles. I've got 60 miles on since it happened no troubles. 5 years ago I had a similar problem and it was never figured out 100% but was told it may be a bad coil so put all new OEM Delphi...

gmtnation.com

I throw one back at you. Did you ever do your "neat current setup" with just key ON? In the referred to thread, I don't believe that was ever stated.

The other problem that I have is with just KEY ON, why is anything being done to coils????

 

[Mooseman]

The other problem that I have is with just KEY ON, why is anything being done to coils????

I think they do get powered on but just not triggered.

 

[budwich]

I think they do get powered on but just not triggered.

OK... but I still would like to know what a key ON reading is then.

 

[Sqrly]

GM is famous for routing eletrical wires over sharp sheet metal edges. I would suggest you keep an eye out for this during your troubleshooting.

 

[TJBaker57]

OK... but I still would like to know what a key ON reading is then.

So as usual it takes me more time to find where I stashed this tool or that gadget than it takes to do the test!

The results for my own 2002 4.2:

The little amp meter I use only displays to XX.X amps, no hundredths so there are rounding considerations.

Fuse 28 at key on draws 0.2 amps for a couple seconds then 0.0 amps.

That puzzled me for few moments. Then I removed the fuel pump relay.

After that there is no amps drawn at all at key ON (engine OFF). Nothing displayed.

Tried 3 different relays to compare readings and they were always the same 0.2 amps for the time it takes to run the fuel pump prime at key to RUN.

Measured the resistance of a relay coil and it read 87 ohms.

So do a little math and expect some additional milliamp draw for the coils and it makes sense to see about a rounded up 0.2 amp draw as the fuel pump primes.

So there is confirmation that the fuel pump relay coil circuit is indeed powered by the power from fuse 28 to the PCM.

At a cold start I peaked at 4.7 amps then settled back to a out 2.2 amps eventually dropping back to the 1.8 amps I previously reported in the old thread as an amp draw at idle.

 

[budwich]

OK... thanks, sorry to make you work so hard.... I don't have one of those "ammeter punch ins".

Based on this "nice testing", I would suggest to you that the coils are all fine although the wiring may not... but if there was a wiring issue, the CGood should have "stumbled" across it as he plugged in various combination of coils ... assuming he didn't pull them totally and just plugged into the same sockets / connectors to test each "pair" as he stated.

I think it would be wise for him to do the suggested test that I mentioned (assuming he has spare fuses), leave all his coils connected and pull the fuel pump relay... turn the key on and "pray"...

ADDED: I mentioned a while back that the fuel pump relay is "UNFUSED" as it gets its powering from the PCM "internals"... it is thus contributing to the fusing of the PCM. See post #15.

 

[c good]

Thanks for the input all.
Here's the latest. Still haven't installed my new positive battery cable as it hasn't arrived yet from Amazon. Thanks to Mrrsm for the part number on that

I did clean up the connections and cut back the cables to get to some Ok copper. Re crimped everything and am getting good power to the starter motor.

I changed out a couple of suspect coils and am no longer popping fuse 28 when I attempt to start.

I turn the key and get nothing when the starter should kick in. I tested the starter relay. When I jump the relay it engages the starter.

I tested #17 fuse that gets power from the key switch. It's tests 12 volts. I tested neutral and park safety switch. It is good.

I'm leaning towards a bad fuse box on the underside. Am stuck on how the cables unplug from the underside?