Volts Drop to 9 w/AC on. Sometimes

Budmiester

Original poster
Member
Jan 14, 2012
8
This has happened a total three times in the past 4 or so months, but twice in the past week. Last week, episode #2, the 'Voy had been sitting for five days. Driving home from the airport I noticed the Amp meter @ 9 Volts. Shut all accessories of. Volts stay set @ 9vdc. Get home, shut it off, restart, and everything is fine. (Ya I know, I should have left it on, then checked it, we were getting back in town from a bitch of a trip and just automatically shut it down, DOH!) Started it back up, 14volts. With the voltmeter I get 11.89 at the battery, engine off. 13.9 with engine on. Battery is about 18 months old. Did it again today, wife driving home from work, stuck in construction traffic, drops to 9vdc again. Idle is smooth as a sewing machine, no lights dimming, everything works fine. She did shut off all electrical items, just in case, (she's so smart), makes it home, same thing. Restart, 14vdc. Before I go out and replace the alternator, my thinking is the voltage regulator is starting to shoot craps, but wanted to know if anyone else had seen anything like this.
 

n0kfb

Member
Dec 8, 2011
104
Check your battery with your multimeter when the dash gauge reads 9 volts. I'd wager your alternator, voltage regulator and battery are all just fine.

I'm guessing ignition switch.

-- Dan Meyer :coffee:
 

RayVoy

Member
Nov 20, 2011
939
The way it's acting, you might have a loose/dirty connection.

You might also have a problem with a funny little feature that these trucks have; when it is cold, the PCM stops the alt from charging for a few seconds when the truck is started, we are guessing, but we think it is to let the engine reach a happy idle state before the load of the alt is added.

Anyway, there might (small might) be a problem in this area, check the connections first and then we can look at this other circuit.
 

Chickenhawk

Member
Dec 6, 2011
782
Your battery voltage is REALLY low when not running. I would guess there is a loose cable or, more likely, the battery is toast.

A voltage check of the battery is not conclusive, simply because a slight drop from the usual 12.6 volts or so indicates a huge drop in battery capacity. The only way to test it for sure is to do a load test.

This takes less time than it takes to read this sentence. Let us know what it reads. (The readout will be in CCA.)
 

Budmiester

Original poster
Member
Jan 14, 2012
8
Pulled both the alternator and battery out and had them tested. Both passed with no problems. The meter is dropping off everyday now, but only on the way home from work. Does not do it in the mornings. Weird. I'll hook up the scan tool this weekend and get a real time read out. Does the dash amp meter use a stepper motor?
 

Matt

Member
Dec 2, 2011
4,033
Yes it does. But if it holds steady and doesn't wander around, I would say that is not your problem...mine does, but I use the Torque app and keep an eye on my voltage.
 

christo829

Member
Dec 7, 2011
509
Fairfax, Virginia
Budmiester said:
Pulled both the alternator and battery out and had them tested. Both passed with no problems. The meter is dropping off everyday now, but only on the way home from work. Does not do it in the mornings. Weird. I'll hook up the scan tool this weekend and get a real time read out. Does the dash amp meter use a stepper motor?

Yes, all the gages have stepper motors. Does it always return to zero when you shut the truck off?

Also, check your ground connections. As others have pointed out, it sounds as if you've got a bad connection.
If the battery and alt checked out good, then something is inhibiting the battery from receiving the proper charge.
Grounds are one likely culprit. Make sure all connections are clean and tight.

Not really sure about the morning/evening correlation. Might be temperature affecting a connection? Yeah, just
a guess, but might be something to at least check.

Good Luck!

Chris
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Budmiester said:
...dash amp meter use a stepper motor?
It does, but to be precise, it's not an ammeter, but a voltmeter. We don't have a gauge that used to be in some older vehicles that would measure the current going to or from the battery.
 

Budmiester

Original poster
Member
Jan 14, 2012
8
Update: After cleaning the grounds up, no signs of corrosion build up, the voltage still remains about the same. 13.9 volts running. No fluctuations. All week.
Now this morning, I fill up and when I restart to leave, the gas gauge stays on "E" with the low fuel warning on, it was not on before. Driving to work the gauge comes off Empty, but never goes above were it was prior to filling up. I shut the engine down and re-started a couple of times. Along with double checking the gas cap. My gremlin seems to have moved......
 

Budmiester

Original poster
Member
Jan 14, 2012
8
And the fun continues. This afternoon on the way home from work, gas gauge fine, volt meter drops off.
 

MacMan

Member
Mar 3, 2012
194
Budmiester said:
....I fill up .........the gauge comes off Empty, but never goes above were it was prior to filling up. I shut the engine down and re-started a couple of times.......

Had the same problem w/ my '02 back about 5-6 yrs ago. Had to have the fuel pump/fuel level sensor unit in the tank replaced.

No problems since, but I make sure to add a can of Chevron w/ Techron fuel injector cleaner to the tank every 3-4 months. It helps keep the sending unit/pump unit clean.
 

tom1999d

Member
Apr 24, 2012
32
I have the same strange 'random low voltage problem" The volt meter will read 11-12 volts. It NEVER starts there but drifts there after driving a while. It has done this on hot days and cold days. I changed the battery and the alternator. A volt meter across the terminals agrees with the dash meter.

EVERY time I turn the truck off then back on the volts jump to 14-15. I've driven the truck this way for about 5,000 miles. I think it is a gauging problem. When shopping for a new (used) Envoy I noticed several models in the '06 to '08 range all had this phenomenon.
 

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The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
tom1999d said:
...A volt meter across the terminals agrees with the dash meter.... I think it is a gauging problem. When shopping for a new (used) Envoy I noticed several models in the '06 to '08 range all had this phenomenon.
What do you mean? If the dash gauge agrees with your hand held voltmeter it's an electrical issue. Perhaps with the battery ground so the alternator is charging the red wire properly but the black wire to ground isn't connected well to the fender.
 

RayVoy

Member
Nov 20, 2011
939
The charging system, in these trucks, has a quaky feature. Most of us have notice a short period of no charge when the trucks are started on a cold winter morning. The theory is that the PCM allows the engine to run for a few seconds without the load of a charging alternator (I'm not sure we have consensus for the reason). I wonder if your truck might be dropping into this "no charge" condition?

I no longer have my shop manuals (gave them to a friend), so I can't look it up, but it might be something to consider.
 

NewfieEnvoy

Member
Jan 25, 2012
525
tom1999d said:
I have the same strange 'random low voltage problem" The volt meter will read 11-12 volts. It NEVER starts there but drifts there after driving a while. It has done this on hot days and cold days. I changed the battery and the alternator. A volt meter across the terminals agrees with the dash meter.

EVERY time I turn the truck off then back on the volts jump to 14-15. I've driven the truck this way for about 5,000 miles. I think it is a gauging problem. When shopping for a new (used) Envoy I noticed several models in the '06 to '08 range all had this phenomenon.

I have the exact same condition with my 07. Really bothered me a year ago when I got the vehicle but after 13000kms everything's still strong with the same voltages. Been reading what I can and asking questions but I still don't understand the charging system in these trucks. There's certainly a few quirks.
 

jaguarjoe

Member
Nov 22, 2012
73
Just throwing this out here-

Maybe the pulley on the alternator shaft is slipping every now and then. I had a Jag that did that.
 

gmcman

Member
Dec 12, 2011
4,658
I went through this exact same issue about a month or so ago. I wiggled this wire and I could hear the alternator load up, and later removed and cleaned the connection and all is good.
 

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Vicompc

Member
Dec 5, 2011
109
I had a simlar issue with mine, it ended up being the voltage regulator. It would not show up under load tests but if you let run for about 15 min under load you should see the dip.
 

tom1999d

Member
Apr 24, 2012
32
the roadie said:
What do you mean? If the dash gauge agrees with your hand held voltmeter it's an electrical issue. Perhaps with the battery ground so the alternator is charging the red wire properly but the black wire to ground isn't connected well to the fender.

I am getting a volt meter for the cigarette lighter socket. I'll record the dash and the cig meter and when practical my DVM. The DVM is tricky because I have simple probes and getting the max voltage takes a few tries (i.e not low voltage because ther is some scale on the batery connector bolt.)
 

tom1999d

Member
Apr 24, 2012
32
It is random and I can't nail down why. Hot days seem to be sometimes worse than cold days. Belt and alternator are new. The charging light never comes on just an akward needle deflection to the left....

I have not fixed the fuel sending unit. I wonder if this is related...
 

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tom1999d

Member
Apr 24, 2012
32
The in dash gauge seems to agree with a cheap cigarette lighter plug in digital gauge. I do own a DVM but this is more practical for driving around and knowing the voltage.
 

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Bartonmd

Member
Nov 20, 2011
545
tom1999d said:
The in dash gauge seems to agree with a cheap cigarette lighter plug in digital gauge. I do own a DVM but this is more practical for driving around and knowing the voltage.

Yours is normal. The later ones in particular have 14V+ to recharge the batter, but then kick down to 12.8-13V when the battery is recharged, to not over-charge it, and to conserve energy, keep the alternator/regulator cooler (last longer), etc.

Mike
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
The other issue that should be mentioned is that in cold weather, the alternator output is suppressed for a few seconds to allow the cold engine and cold oil to stabilize its RPM. THEN the PCM throws the load of the alternator on the belt to recharge the cold battery. So you might see a display of 10-12 V for 5-15 seconds, THEN it jumps up to 14+ for the recharge initial time. This surprises coms foplks who then post up about it.
 

tom1999d

Member
Apr 24, 2012
32
the roadie said:
The other issue that should be mentioned is that in cold weather, the alternator output is suppressed for a few seconds to allow the cold engine and cold oil to stabilize its RPM. THEN the PCM throws the load of the alternator on the belt to recharge the cold battery. So you might see a display of 10-12 V for 5-15 seconds, THEN it jumps up to 14+ for the recharge initial time. This surprises coms foplks who then post up about it.

Thanks Roadie,
The first photo is 3 min after startup on my way to work. The second photo is 12 miles and about 18 minutes later after arriving at work. Both photos were taken at idle in park.

Sometimes the volts go back just above 14 then back down to the 12-13 range. It’s been this way for 4 months. Obviously there is sufficient charging but with cold weather here I do get concerned. It was 14F outside and 28F in my garage this morning.


I'm starting to believe the voltage regulator (bridge rectifier) may have a flakey diode. The replacement alternator is remanufactured. The old alternator, which I still have, exhibited the exact same symptoms. I can get low volts and turn off the ignition, turn the truck back on again and get 15V or better every single time. The electrical reset leads me to think it's a flaky diode. I'm a Mechanical engineer not an electrical engineer so this isn't my strongest suit. I'll read up some more
 

tom1999d

Member
Apr 24, 2012
32
Bartonmd said:
Yours is normal. The later ones in particular have 14V+ to recharge the batter, but then kick down to 12.8-13V when the battery is recharged, to not over-charge it, and to conserve energy, keep the alternator/regulator cooler (last longer), etc.

Mike

I was wondering about that. My old 2002 SLE always stayed at a high voltage. Where did you learn this? I'd like to read up some more. I test drove several Envoys an several had the same issue or symptom.
 

Bartonmd

Member
Nov 20, 2011
545
When you restart the vehicle, it will go back up to 14V to recharge the battery from the start. Mine will drop down to 12.8-13V after a while of driving, then if conditions change (bright lights on, blower on high, wipers running, etc), it'll go back up to 14V. Has done this since it was new.

Mike
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Most of what I know is observations that get me curious, and a cover-to-cover reading of the factory shop manual in 2005. One observation that has never been confirmed is that sometimes the voltage dips for no sensible reason while you're driving along. There *is* a known diagnostic of the SAIS turbine pump that can throw a 30A load on the electrical system, three times, with about 3 seconds between tests, that's checking operation of the secondary air system. It's programmed to occus as you're driving along with a steady RPM, a certain number of minutes after a cold start. That test dims the headlights, usually on the drive home in the winter when it's dark, and people just don't notice it happening in the summertime drive home or the drive TO work. That's an understood phenomenon.

The unexplained voltage droops, I think with absolutely no proof but it's the way I'd design the system if I was in charge, are happening as a way for the PCM to regularly test the battery to make sure it hasn't failed in the middle of a long drive. The PCM has an algorithm in it to retain some driveability in case the alternator dies. If the voltage drops down, and the PCM wire that checks for the alternator's field control duty cycle is indicating the alternator is working as hard as it can, but not keeping up (as if there's a dead diode), then the PCM can command the BCM to do some load shedding. Like the entertainment system, then the HVAC, then interior lights, and last of all the headlights. All to reserve as much of a dwindling battery power to run the engine to let you limp to a place of safety. The Owner's Manual SHOULD have mentioned this critical safety feature, but it doesn't.

I think the programmers were also interested in the case where the alternator is keeping up with the vehicle's current load, but it wants to see *if* the alternator dies, that the battery is still there and capable of running the vehicle for a while. So it turns off the alternator with its suppression control wire, sees what the battery voltage is and if it droops too quickly during the test. As I said, all a conjecture, but it fits the many observations by me and other owners of mysterious alternator dropouts for no good reason, that aren't caused by the turbine air pump test.
 

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