Never ending voltage issue I can't figure out!

[shepherd92683]

I have a 2008 trailblazer 4.2L. I have had a voltage issue for about a year now with it not staying over 12.6v. TB will start and charge at 14.8 then slowly drop down to 12.6 and NOT recharge. But...it does not do that all the time. I have a thread I posted in a car audio forum that I have gotten some response from but rather than relisting all of it I will post the link. If anyone has options of things to check for please give me some ideas. Thank you!!
http://www.stevemeadedesigns.com/board/topic/186417-excessive-voltage-drop/

 

[The_Roadie]

Hello, and welcome!

You've been given some excellent advice on that forum. Your crimps may be suboptimal, but is there any other reason you need ten different people on two forums to advise you to just upgrade your alternator? Sooner or later your battery is going to drain itself and leave you stranded and that's a risk you don't need to take.

Alternators can get flaky and work sometimes and not others, especially if they are loaded right to max capacity whenever you're running your audio system.

 

[gmcman]

I had a very similiar issue and the first time I noticed this I turned my lights on and rear defrost to load down the alternator, then while the engine is running while I had low volts, I wiggled the signal lead going to the alternator. It's the plastic connector with two wires going to it, listen for the load when you wiggle them.

I pulled mine and cleaned the contacts and It fixed it until the alternator gave out about 3 years later.

Using a 45 deg pick aids in removing the connector, also your alternator could be going out but I would try the signal lead first.

 

[shepherd92683]

I have a new alternator on the way. I have just always heard an alt works or it doesn't. I doesn't just work 'sometimes'. I didn't know for sure if RVC affected my vehicle or not and how to tell. I do know that turning on my headlights and/or rear defrost it only drops voltage and does not recharge like I've read that the rvc will do when those are used. I do believe the alt needs replaced myself and have had for some time. I want to be sure that the investment of an ho alt solves the problem and that I'm either not overlooking something or fixing the wrong thing as I am not a full fledged mechanic

 

[shepherd92683]

I had a very similiar issue and the first time I noticed this I turned my lights on and rear defrost to load down the alternator, then while the engine is running while I had low volts, I wiggled the signal lead going to the alternator. It's the plastic connector with two wires going to it, listen for the load when you wiggle them.

I pulled mine and cleaned the contacts and It fixed it until the alternator gave out about 3 years later.

Using a 45 deg pick aids in removing the connector, also your alternator could be going out but I would try the signal lead first.

I did disconnect all leads and the 4 pin connector to be sure of that and saw no difference... I think I've just about tried it all. Any way the PCM could be telling my alt not to charge at a higher voltage?

 

[The_Roadie]

Most of us have the old alternators with built-in voltage regulators, and only two control wires going to it. One goes FROM the alternator to the PCM to report back the field coil duty cycle so the PCM knows how hard the alternator is working. If it sees the alternator working as hard as it can, and the voltage measured at the PCM is continues to go down, it starts shedding load starting with the stock head unit audio system then the HVAC system, then lights, all in an attempt to save the last battery energy for the engine. I'm not sure how the 2008 alternator works, not having a shop manual for the later years.

The other single control wire goes from the PCM to the alternator, and is a simple suppression wire to hold off the alternator from starting to recharge a battery under frigid conditions, where the engine is cold, the battery is cold, and by holding off the alternator from starting up for 10-30 seconds, it gives the engine a few seconds to stabilize its RPM before its also asked to send 2-3 HP through the belt to the alternator. That's the only condition I know about where the engine is running, but the alternator won't be putting out 13.8+ volts.

 

[coolasice]

Some customers may comment that the voltmeter is fluctuating between 12 and 14 volts on their full size pickup or utility vehicle. Starting with the 2005 model year, light duty full size pickups and utilities are equipped with a Regulated Voltage Control (RVC) system. This system reduces the targeted output of the generator to 12.6-13.1 volts when in "Fuel Economy Mode" to improve fuel economy. The generator may exit "Fuel Economy Mode" if additional voltage is required. This will cause the voltmeter to fluctuate between 12 and 14 volts as opposed to non-regulated systems that usually maintain a more consistent reading of 14 volts. This fluctuation with the RVC system is normal system operation and NO repairs should be attempted.

 

[bobdec]

+1 with above my '07 Envoy has a 'Generator Voltage Control Module' mounted behind the battery. It has a 10 pin connector and a current probe (loop) around the negative cable to battery. It takes over control that the PCM used to do. It is on the class 2 serial buss and feeds the voltage info to the dash indicator and gauge and reports failures (DTC's) to the PCM '07 or ECM '08 . As you said it varies voltage a couple of volts while driving by pulsing the L terminal on/off (duty cycle). My voltage gauge will drop into mid 12's during H'way driving, but never saw it lower than the 13's running around town. I see the module in the '07 and '08 manuals not in '06 and earlier.

 

[shepherd92683]

New alt is installed and it changed NOTHING!!!! What else could be the culprit here? I'm losing my mind over this!

 

[IllogicTC]

Have you considered checking out the module mentioned by bobdec? A cursory inspection would include checking all wiring, and examining the module itself for any obvious issues which made itself physically-present.

 

[shepherd92683]

I will give it a shot. Could it be removed if it's the problem or is there any other workaround?

 

[gmcman]

I would get a piercing test probe and/or perform a continuity check on the control wires from the alternator to the PCM. Someone will need to respond on which wires to check at the PCM harness.

I would initially suspect loose or corroded contacts at the alternator harness. Don't remove the harness, wiggle the wires then if that's doesn't work then go with the continuity check.

 

[shepherd92683]

I did a test on disconnecting my sub amp and rear battery and it completely stabilized my voltage. With my mids/hi's screaming it stayed at 14v... I did find this read from mechman on these type of system that have rvc and a Hall effect clamp.

We have a 270 amp unit that is a direct plug in and maintains all factory functions with the PCM controlled regulator. This would be the best way to go if you only want to replace the stock alternator. Performance is never quite as good with a PCM controlled alternator, but it will be a VAST improvement over your stock alternator, and you won't have to worry about anything being damaged. The MAIN thing to remember when installing the unit, is to not add additional grounds, but rather REPLACE the stock 2 gauge ground cable with a 1/0 gauge ground cable, and terminate it to the housing of the alternator instead of the engine block. (we provide a ground terminal on the alternator housing for this reason) A 1/0 gauge cable will fit through the load sensor, and by grounding directly to the alternator, you negate the need for additional engine grounds. This will give you good performance, without causing any problems. The PCM WILL still control the voltage however, so don't be surprised to see fluctuation depending on vehicle speed/temp/load conditions.


I think after I change my grounds things may improve. I will update as I test and I hope this may help someone else in the future that may run into the same issue.

 

[coolasice]

how is everything wired up? did you add additional grounds to the main battery and bypass the current sensor?

 

[06TrailblazerLSS]

Have a red top under the hood & yellow top in trunk & big 3 upgrade & 250 amp HO alternator, and STILL get the occasional dim and low voltage with just 2 L7's

 

[shepherd92683]

Did some rewiring today. I added a 1/0 from battery - to alt through rvc, removed stock ground to engine, a-frame, and fender and replaced them. Only wire going through rvc is to the fender. This was a pita (time consuming) but worth it. I had several corroded connections and I now have new copper lugs soldered at each terminal. Only problem now is that after I did this and restarted my car, my head unit won't turn on...ugh

 

[shepherd92683]

So after getting the head unit running again, my voltage has started going crazy again. I was dropping .5 now it's 1.5v. I wired it exactly as mechman said to so I guess it's time to put it back how it was. Something I've learned is the big 3 is not worth messing with when you have a rvc installed in your vehicle.

 

[coolasice]

do you have a secondary battery? if so, does the ground for that battery run through the RVC? If not, it will not be registering the current drain from that battery and not supplying the correct amperage because of it.

 

[shepherd92683]

I do but it is not installed currently.

 

[coolasice]

have you tried turning on headlights and rear defrost+ hvac on high to see if the loaded alt will output 14?

 

[shepherd92683]

Yes and nothing. I removed the grounds from the alt and made it back to how it was and also no change. I don't get it.

 

[shepherd92683]

Factory grounds are: 2 from battery - through rvc, one goes to fender, other to wheel well area, last ground goes from wheel well area to engine block. I have mine as 2 through the rvc- one to the fender, the other to the block and from the block to the wheel well. Is that small difference really enough to destroy the voltage? I will be changing it back to find out I guess because I can't deal with 12.6 on a brand new 250a alt with a minimal load.

 

[MAY03LT]

Man, that sucks. Even with the stock alt, some of the tests (headlights/defrost/etc) should have sent the voltage up. If there's an internal problem with the GVCM, it's posed to default to 13.8. It sounds like it's not reacting to load, like it's stuck in economy mode. The GVCM has a data list (and codes) that can be read with a high end scan tool. If you know someone with a scan tool it might be worth peeking at the data and recording codes. If you do read data, there's a line for current mode (not like amps current) that should switch when doing the headlight/defrost test. IIRC there are 9 modes of operation.

The pcm is involved with the alt as well, but if it detects a fault it will turn on the check engine light.

 

[shepherd92683]

Thanks MAY03LT. that may need to happen at some point, also thanks for the alt install video. It helped get me along putting the mechman in! For now voltage is back. I removed grounds to the alt, kept the ground to the fender, moved a ground to the strut tower, and from strut tower to the block.

 

[shepherd92683]

I guess when I install my rear battery again I will make a run from the strut tower ground to the back since that ground went through the rvc. I can only HOPE that doesn't cause issues lol

 

[MAY03LT]

Glad to hear that it helped.

Were you able to fit (2) 1/0 runs through the GVCM? It looks like it in the first pic.

 

[shepherd92683]

Glad to hear that it helped.

Were you able to fit (2) 1/0 runs through the GVCM? It looks like it in the first pic.

Not a chance lol. A 1/0 and 4 gauge.

 

[shepherd92683]

After 2 days I'm back to 12.6-12.8 and have changed nothing. I did find that turning the rear defrost on raised it up to 13.5 but it goes right back down as soon as it turns off.

 

[JerryIrons]

But disconnecting your sub amp and rear battery results in correct voltage?

 

[shepherd92683]

Temporarily. It always goes back down after a few days no matter what I do.

 

[coolasice]

If the voltage goes up when you turn on rear defrost then the system is working as intended.

 

[coolasice]

You could just unplug the module near your battery and run without amp sense... It should default to standard charging

 

[BlazingTrails]

Mine does the same thing, as it is intended too. is your battery dead all of the time? how many times have you had to charge/jump start your battery?

 

[shepherd92683]

Never had to jump my battery. After just driving for 3 hours, I noticed as soon as my headlights kicked on I was steady at 13.6. On my 3 he drive yesterday I was at 14.2 the whole time-sometimes a little better. I just don't understand the inconsistency.

 

[BlazingTrails]

It is normal. Everything that pulls power makes it change constantly. They are all like that, to "save gas"

 

[shepherd92683]

Not trying to be a smartass here but does that mean for the first 2 years I owned the vehicle it was always over 13.5 it was abnormal?

 

[bobdec]

I'll throw this up for discussion to others... here is "MY GUESS" because I can' find good logic flow documentation on how the GBCM works...Looking at the wiring on a '08 . The GBCM (Gen Bat Ctl Mod) controls the alternator charge rate duty cycle pulses (5% to 95%) to the 'L' terminal of the alternator.. There is a line from the ECM to the GBCM that looks like it's used to tell the GBCM to increase the duty cycle to the alt. My guess is this is for headlights on, rear defogger, cold starts, DTC startup alt on/off testing , ETC:. Also probably to lower voltage to 12.5 (fuel saver) when cruising long distances at no loads. What the CBCB also does on it's own is monitor the current leaving the battery via the inductance loop that the negative battery wire goes through. What I am GUESSING is the load your audio equipment is putting on the electrical system is not being recognized by the GBCM or the ECM and thus the duty cycle to the alternator 'L' terminal is not increasing. The vehicle is holding 12.5 volts via the ECM monitoring keep alive voltage and sending a control signal to the GBCM, but again the GBCM is not seeing the uncontrolled load of the audio stuff. If you can't get the audio negative through the GBCM loop then somehow there ought to be a way to bypass it. Almost to the point of adding an external voltage regulator and associated circuitry and then having to tune out all the alternator/voltage regulation DTCS. There must be others with an simpler solution to this this problem out there. Intelligent alternator control has been around for quite a few years and high power audio even longer. Key is to include the audio load into the intelligent load monitoring system.. .

 

[BlazingTrails]

Here's what I don't understand about the system as a whole. Increasing the voltage reduces the amperage, so how does it save fuel to drop the voltage? It seems to me that it would just increase the amperage which would actually put more load on the alternator. Either way mine does the same thing. Also are you reading the "idiot" gauge on the dash for the voltage or are you monitoring the system with an external meter? Because if your using the cluster gauge, that would be a fatal flaw on your part. It could be that the stepper motor is wearing out.

 

[shepherd92683]

I have an external meter ran to my amps. The dash and meter remain very consistent. There is an external regulator you can buy but it make the battery light stay on. I've also heard that they cool battery's faster anyway so I'm not real excited about going that route... The times when it stays at 14.2-14.4 is all I want out of all of this. Again, I can't understand under the EXACT same circumstances why some days it will and some it won't. I can't drive with my headlights/rear defrost on forever lol!!

 

[coolasice]

when did you install that battery that is in there now... just recently? maybe your old battery was going south and not holding charge well so the rvc was always trying to charge it...


did you try running with the sensor unplugged?