Voltage drop while driving - Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum



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  #1  
Old 04-08-2011, 01:52 AM
MOZTANG MOZTANG is offline
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2005 Chevy TrailBlazer LT
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Voltage drop while driving

Gentleman (and ladies?),

I've been having a rough time figuring out what is wrong with my vehicle. My biggest concern at the moment is the voltage drop that occurs while driving. Occasionally while driving, the voltage in my truck will go from 14.x volts to about 12.5V as soon as I get off the gas. The voltage itself varies from about 14.0-14.4 volts usually, but still isn't very consistent. My biggest concern is stalling while driving, and not having adequate control of my vehicle. It is rarely an issue, but when it does it causes me a great deal of stress. I've been battling this issue for over a year now, but it has gotten bad recently.

About two weeks ago, I replaced the:
Fan clutch
Water pump
Idler pully
Alternator
Serpentine belt
Spark plugs
Battery

I feel like I have adequately done the checkdown for electronic components. The next thing on my list would be to upgrade "The Big 3" but last time I checked all of my connections looked great. The unusual part is how dramatically the voltage drops after I take my foot off the gas, even if the throttle is open just a bit. If there was a parasitic draw somehow correlated to my engine, I imagine it having to pull some serious amperage (or prevent the charging system to do its job) to affect my voltage that badly while driving.

Additionally, I've had my truck stall on me at a few red lights. It seems to be worse during warm weather as well, but that may be confounded with something else.

Any ideas or help is much appreciated. You guys seem to know your stuff.
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  #2  
Old 04-08-2011, 01:58 AM
Ralph_T's Avatar
Ralph_T Ralph_T is offline
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Originally Posted by MOZTANG View Post
Gentleman (and ladies?),

I've been having a rough time figuring....
I would start with the reason you stalled. That is not normal...unless you were out of gas. Did you happen to pull any codes from your vehicle?
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Old 04-08-2011, 02:06 AM
MOZTANG MOZTANG is offline
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Originally Posted by Ralph_T View Post
I would start with the reason you stalled. That is not normal...unless you were out of gas. Did you happen to pull any codes from your vehicle?
The only code I have ever thrown was a P0128 for a faulty thermostat. I replaced that as well (forgot to mention it on the list, along with a coolant flush and fill.)

The problem seems to always be lurking there. But the few times I've actually stalled, it has always been at a red light. When it happens while stopped, I notice my voltage dropping below 14v and the car idling heavy. In the worst instances, it affects my RPMs at idle to the point where it can stall. Also, any additional load on the car (power windows, seat heater, sun roof, blower motor) puts an enormous load on the system to the point where the voltmeter registers significant change.

Hey, thanks for the quick reply!
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  #4  
Old 04-08-2011, 02:15 AM
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Ralph_T Ralph_T is offline
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Quote:
Originally Posted by MOZTANG View Post
The only code I have ever thrown was a P0128 for a faulty thermostat. I replaced that as well (forgot to mention it on the list, along with a coolant flush and fill.)

The problem seems to always be lurking there. But the few times I've actually stalled, it has always been at a red light. When it happens while stopped, I notice my voltage dropping below 14v and the car idling heavy. In the worst instances, it affects my RPMs at idle to the point where it can stall. Also, any additional load on the car (power windows, seat heater, sun roof, blower motor) puts an enormous load on the system to the point where the voltmeter registers significant change.

Hey, thanks for the quick reply!
Do this. While at idle in your driveway start the car. Let it idle for as long as needed until you notice an RMP drop. The RPM's should hold steady at 650. If you notice a drop in RPM and the%
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Old 04-08-2011, 02:25 AM
MOZTANG MOZTANG is offline
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Originally Posted by Ralph_T View Post
Do this. While at idle in your driveway start the car. Let it idle for as long as needed until you notice an RMP drop or voltage drop. Do they coincide with each other?
More often than not, I see a voltage drop with no additional effects. Without provoking it at idle, the RPMs seem unaffected. Even when I put extra load on the system (stereo up, windows up/down, heater on) I see a more dramatic effect on the voltage which in turn affects the RPMs noticeably. However, regardless of how much accessory power I use, I cannot force my car to stall.

To be more precise: I do notice them coinciding with each other, but everytime I witness it, the voltage drop comes first. This is why I am more inclined to think it is more electrical than mechanical, but I have exhausted all reasonable explanations.
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  #6  
Old 04-08-2011, 02:40 AM
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Ralph_T Ralph_T is offline
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Quote:
Originally Posted by MOZTANG View Post
More often than not, I see a voltage drop with no additional effects. Without provoking it at idle, the RPMs seem unaffected. Even when I put extra load on the system (stereo up, windows up/down, heater on) I see a more dramatic effect on the voltage which in turn affects the RPMs noticeably. However, regardless of how much accessory power I use, I cannot force my car to stall.

To be more precise: I do notice them coinciding with each other, but everytime I witness it, the voltage drop comes first. This is why I am more inclined to think it is more electrical than mechanical, but I have exhausted all reasonable explanations.
Well it could be a bad alternator still. I have seen others replace the alternator twice on the forums and their problems disappeared. Not to say they had the same problems as yours.

The only thing that pops to mind is maybe the alternator regulator is failing and the battery is dying/draining and causing the fuel pump to not provide enough pressure to the injectors.

I would do the following.

1) Get a full ignition and charging system test including load testing the battery. Make sure all connections are tight.
2) Check the belt tensioner and belt
3) Check for more codes if any
4) Check fuel pump pressure.

That pretty much exhausts my list of things that come to mind immedately. Hopefully someone else can jump in here and provide a more targeted solution if they have one. Take care.
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Old 04-08-2011, 03:03 AM
MOZTANG MOZTANG is offline
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Originally Posted by Ralph_T View Post
Well it could be a bad alternator still. I have seen others replace the alternator twice on the forums and their problems disappeared. Not to say they had the same problems as yours.

The only thing that pops to mind is maybe the alternator regulator is failing and the battery is dying/draining and causing the fuel pump to not provide enough pressure to the injectors.

I would do the following.

1) Get a full ignition and charging system test including load testing the battery. Make sure all connections are tight.
2) Check the belt tensioner and belt
3) Check for more codes if any
4) Check fuel pump pressure.

That pretty much exhausts my list of things that come to mind immedately. Hopefully someone else can jump in here and provide a more targeted solution if they have one. Take care.
Ralph again, thank you and I hope you have a good night.

Just to follow up:
1.) Battery/alternator seemed fine after testing not too long ago, and I've only visually checked connections. The rest of the system has yet to be checked.
2.) Belt tension seems adequate (~80 twist, 1/2" play up and down)
3.) I've got my own code scanner, plug it in every once in awhile just in case. Nothing yet.
4.) Haven't checked it. I imagine I'm soon due for a new pump or filters. Again, I don't think this is directly causing my problem, but I agree it would definitely make it worse.

I think the next course of action is upgrading my big three this afternoon. I've always got some 4AWG laying around. At the very least it will narrow down the problem.

Thanks!
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  #8  
Old 04-08-2011, 09:13 AM
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tahawus84 tahawus84 is offline
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Just so you know I changed out my alt. 3 times with a reman from autozone. They tested the alt and said it was good, but the 4th time I paid more and got a new unit and its been fine for over a year. The 3rd reman unit seemed to be working out but the lights would pulsate. Changed for a new one and it was fixed.
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  #9  
Old 04-08-2011, 04:28 PM
MOZTANG MOZTANG is offline
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Quote:
Originally Posted by blckshdw View Post
Have you cleaned your throttle body at all? I'm not sure if that would effect you as much while the truck is in motion, but when sitting idle, with your AC/heat on and having RPM/stalling issues is one of the tell tale signs of a dirty throttle body with our I6 engines, especially post battery replacement since your PCM will reset and be using parameters for a clean throttle body. If you haven't done it recently, it may be worth it just cuz it's pretty cheap to do.
I sprayed it down and wiped it real quick when I had my resonator off when doing the plugs. I only got the front of it good. Maybe I'll actually take it off this time. Definitely cheap, easy, and worthwhile. I'll take you up on that. Thank you.
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  #10  
Old 04-08-2011, 04:35 PM
MOZTANG MOZTANG is offline
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Quote:
Originally Posted by tahawus84 View Post
Just so you know I changed out my alt. 3 times with a reman from autozone. They tested the alt and said it was good, but the 4th time I paid more and got a new unit and its been fine for over a year. The 3rd reman unit seemed to be working out but the lights would pulsate. Changed for a new one and it was fixed.
My old one only had 60k on it and I just paid $200 for a new one. I really didn't even want to swap alternators because GM is known for their long-life alts and I didn't want to think it was even close to going.

The idea of going through 4 alternators makes me cringe. I'm thinking I might just take apart the old one and see if I can clean it up, maybe resolder some connections. The more and more I think about it, I am compelled to think the problem is somewhere from the alternator to the battery, or my grounds.
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