Flickering Head lights and dome lights with engine running

sylvezter

Original poster
Member
Dec 21, 2011
3
I have a 2002 4.2L Trailblazer LS that has started to experience slight flickering headlights, dashboard lights, and dome lights when the engine is running. The flickering is very fast, where the lights flicker probably 5 to 10 times per second. It appears to get worse when you increase the RPM. The voltage fluctuation varies quickly from about 14.2 to 14.5 volts. If you are coasting and the idle is around 1000 RPM the flickering goes away and the voltage stabilizes. (I have a radar detector that has a volt meter readout.)

Another related symptom is sometimes when letting off the gas to decelerate (before braking) the voltage will drop from 14.x to 12.8 for about a second and then return. The lights will dim momentarily.

Sometimes the voltage fluctuation is 0.1 to 0.2 volts and the flickering is not noticeable.

I checked my battery terminal connections. I also checked several ground points. The resistance from the negative battery terminal to the engine chassis and to the frame of the car is approx. 0.1 ohms or less. The alternator puts out about 14.2-14.5 volts. The battery puts out about 12.5 volts (open circuit). Both the alternator and the battery passed with handheld testers at Autozone and Oreilly. The battery is 1 year old.

My alternator is original from the factory. I noticed there is a connector from the alternator that has 2 thin wires going into a wire wrap. I assume one is the turn on signal and the other for the PCM to regulate the alternator voltage?

Could I have a bad voltage regulator, which I think is part of the alternator assembly? Or could my serpentine belt be slipping and wearing out? I don't notice any obvious squealing sounds under the hood. The belt is 9 years old and has 85000 miles on it.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Sounds like a bad diode or voltage regulator inside the alternator. The hand held testers don't look for that sort of behavior IIRC. Just swap it.

Yes, the two wire connector is to the PCM. One is a suppression control where the PCM can hold off the alternator's start-up on very cold mornings where you have a cold battery and use up a lot of its remaining energy to start the cold engine with cold oil. The PCM allows the engine RPM to stabilize at 1000-1200, waits 10-20 seconds, THEN starts the alternator to recharge the battery by using HP from the engine through the serpentine belt. The other wire is a field sensor that tells the PCM how hard the alternator is working - its field duty cycle. If the alternator is working too hard, the PCM can command some load shedding to keep your engine and headlights operating as long as possible as the interior lights, AC, heater, and audio system get shut off. Same thing happens if your alternator fails and the battery is carrying the whole load. Almost nobody discusses this load shedding feature, but I read about it in the shop manual theory of operation.
 
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Shottputter

Member
Dec 15, 2011
1
I have a 02 Envoy, with the same problem. I replaced the Alternator about a week ago and am still having the problem. Battery was replaced about 2 months ago. Any other suggestions as to what may be causing this problem? (fuses to check, PCM reset, Anything else) Also, on start up the lights flicker alot, but this goes away after about 30-45 seconds. When Driving I don't notice any flickering unless I turn on the blower, which in the winter is almost all the time...

I do have some slight sqealing on start up...if that helps
 

sylvezter

Original poster
Member
Dec 21, 2011
3
Shottputter said:
I do have some slight sqealing on start up...if that helps

Did you replace your serpentine belt?

I am going to do some testing on my Trailblazer. I have a new alternator to try out and a new serpentine belt to put in. I will let you know how it goes.
 

Sparky

Member
Dec 4, 2011
12,927
Any updates? I know, christmas and all, but still curious. When I had voltage drops and occasiona dimmed lights on my old car it was the alternator.
 

sylvezter

Original poster
Member
Dec 21, 2011
3
Well, I replaced my alternator and serpentine belt yesterday. After taking out the old alternator, I spinned the pulley on the old alternator, and noticed a slight whine coming from the bearings. I also noted that the plug connections were all clean plug and 4 pin connector. I put in a new Remy Alternator from Advanced Auto Parts.

The problem is now gone, for the most part. Thank you Roadie for the advice. With typical settings on accessories, low to medium blower, headlights on, the voltage readout is rock steady. (My old alternator would be flickering at these settings with voltage varying +/- 0.2 volts). When I put the blower on high, a very slight flicker in the dome lights occurs and the voltage fluctuates +/- 0.1 volts. This could be attributed to my battery needing to be charged up since putting the new alternator in. (However, the old battery read about 12.4-12.5 volts which is not terrible). But overall, the problem is barely noticeable and is pretty much gone. Does anyone else see these fluctuations with the blower on max and headlights on? I still don't understand what is causing this. Could the PCM be commanding load sharing from some accessories off and on several times per second? I doubt it.

Now for the second problem with letting off the gas to decelerate where the voltage drops down is much improved. I would say it happened a few times but not nearly as severe and frequent when test driving the vehicle with the new alternator. After driving more, the problem seems to be going away.
 

Mark20

Member
Dec 6, 2011
1,630
12.4 - 12.5 volts is very low on the battery. Let it get a good charge or use an external charger.

Sent from my Xoom using Tapatalk
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
I would still remove, wire brush, and reinstall as many grounds as I could find. Checking them with a hand-held meter is not going to be accurate enough to predict the performance at high current. The connections should be 0.001 to 0.005 Ohms (1 to 5 milliOhms) and it takes a more sensitive and expensive meter to go that low.

Here's one battery chart I located, but I note they do NOT give the temperature of the test, nor the any indication that they removed the surface charge just before the test. You can Google for battery voltages yourself, but I would not declare 12.4 to 12.5 as a very bad unit.

View attachment 17507
 

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v7guy

Member
Dec 4, 2011
298
Shottputter said:
I have a 02 Envoy, with the same problem. I replaced the Alternator about a week ago and am still having the problem. Battery was replaced about 2 months ago. Any other suggestions as to what may be causing this problem? (fuses to check, PCM reset, Anything else) Also, on start up the lights flicker alot, but this goes away after about 30-45 seconds. When Driving I don't notice any flickering unless I turn on the blower, which in the winter is almost all the time...

I do have some slight sqealing on start up...if that helps



This sounds like the air pump to me. It draws some pretty serious amperage for the first 30 seconds of start up. Also if you get in and drive off immediately the truck will cycle the air pump several times while driving to verify it's functioning. This can also cause dimming lights while driving, especially when you're driving at night in the winter. If i recall correctly this has been deemed normal operation by many service departments. You can minimize this by following roadies suggestions.
 

moneypit

Member
Dec 7, 2011
214
My daughter had flickering lights problem a week ago and our gauge was moving alot when cold. I renewed 3 mos old alternator then a day or so later she called with a no-start but jumped it and made it home. After a search on this/these sites I decided to renew ign.switch and battery. I had overlooked the fact that the negative post was loose even though the cable bolt tightened right up..the metal thread insert had loosened which I'm sure played a big part in all this. I did throw the new ign.switch in anyways since they eventually fail and are dirt cheap and super easy...for a change:yes:
 

georgel

Member
May 16, 2018
1
Chicago
I wanted to post a simple solution to the "flickering" issue that I had been battling for years. I have had the TB at the dealer numerous times for the issue and they could never offer anything more than the TSB (relay replacement). I have replaced the alternator two times and replace the battery every 3-5 years.

As noted by a number of other good posts, check the grounds. There are a good many along the frame and you will find that some will fix random issues like traction control errors, brack control errors, etc.

I also found that the only 100% positive method to fixing my electrical problems was to add a new set of battery ground cables (large gauge battery cables from NAPA or similar quality) and run one from the battery ground post to the engine block (I used the alternator bracket), from the engine block to the frame (I used a bolt on the shock tower), and the most important one was from the battery ground post to the body (I used a bolt on the body frame above the radiator). The GM factory ground points for the electrical system are very poorly designed.

What I found was that when cleaning the battery posts and inspecting the factory battery cables, there appeared to be corrosion at the factor crimps (I only noticed it on the negative cable). I have owned the TB since 2006 and I am now very happy with the reliability. Too bad it took until 2017 to figure it all out!
 

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