lights dimming/ voltmeter irregular.. help [Archive] - Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum

: lights dimming/ voltmeter irregular.. help


ff1751
01-14-2011, 12:20 PM
hi, new here first post. have viewed threads before and got some good info on repairs.
heres my problem. i have an 06 trailblazer, 4.2l 4x4. once in a while the headlights will dim then get bright then dim again. this will last for a minute or two then go away. while this happens the voltmeter drops to about 9V and jumps around. i put a new battery in about 2 months ago, had the alternator tested(supposedly good). battery terminals are tight. the only thing i can think of is a loose or corroded chasis ground? im going to check this tonight and maybe add a second ground strap from the motor to the frame.
any ideas?
thanks in advance.....:thx

wardak33
01-14-2011, 01:17 PM
It does sound like a loose ground connection. Check for areas more specifically under the hood. It sounds like its coming from that area. there are a few ground connections underneath the car on the frame, but they wouldn't cause the whole car to almost die out.

franco2112
01-14-2011, 06:59 PM
It does sound like a loose ground connection. Check for areas more specifically under the hood. It sounds like its coming from that area. there are a few ground connections underneath the car on the frame, but they wouldn't cause the whole car to almost die out.

Im thinking alternator?:suicide:

the roadie
01-14-2011, 09:03 PM
Does your idle RPM ever go down at the same time below 600 RPM? Erratic idle can be the cause of dimming lights and dipping voltages, caused by a dirty throttle body.

Please detail when it happens - at idle, or driving along? Only a few times per driving cycle, but not at idle? If so, how long from when you started driving to the dimming?

ff1751
01-15-2011, 10:25 AM
happens at idle. idle drops down and lights dim. this morning i did the onstar diagnostic thing and it said there was a code for the throttle body, how do you clean the throttle body? please excuse my dumb question, but im learning as i go and dont want to mess anything thing up while trying to fix something else.:duh: any help would be great. cleaning the throttle body, is this something that you need to remove the throttle body for or can it be done on vehicle? or is it as simple as adding a fuel injector cleaner to the gas? i will wait for some advice and direction. :thx

Convoy04
01-15-2011, 08:18 PM
I've had a similiar issue with mine but most of the time mine does it when I first hit the brake, like the brake lights draw all the power.

I_hate_joining
01-26-2011, 07:28 PM
I've had a similiar issue with mine but most of the time mine does it when I first hit the brake, like the brake lights draw all the power.

I just noticed the same problem tonight. I did not observe the voltage drop but I did notice, hear, a drop in RPM when it happened. I will look for the voltage drop too.

Additionally I noticed a few times this AM, with the heat set on 90, ac off, fan on full, in manual mode, directed at the floor only I heard the fan slow for a sec then go back to the normal sound. I thought I was just hearing things today cuz I just got it and any noise out of the norm gets my attention really quick.

I would guess these are related in some way.

Just hope it is and easy and cheap fix.

mootsnyhc88
01-26-2011, 07:54 PM
I had the same problem on my 06 GMC Envoy and i had my throttle body cleaned. It has worked better, but there are still times it does act up. Does anyone think its a engine to body ground? i had also changed my alternator twice new battery and new belt so i have scratched all of those off my list. And they're all factory no after market.

Thanks.

I_hate_joining
01-27-2011, 08:47 PM
Tonight I paid more attention to everything. When I got the dimming I was going less then 40 on curvy hilly roads, the lights dimmed going up hill and cresting, did not notice a change in RPM, hand was blocking it while driving, the volt meter was about 14.2 but dropped to 13.8 when they dimmed then back to 14.2

At idle 600 rpm steady with a bouncing voltage of 14 to 14.2 no dimming.

In doing research on the site waiting a few minutes to run the diagnostics seems to reduce the occurrence, I tried it today and it seemed work, but the jury is still out, I will be doing the Big Three to improve battery to alternator connection and ground, that all seems to help too. It is cheap and easy to do and can't hurt. I was going to wait till spring to clean the throttle body but may do it sooner cuz it cheap, easy and helps eliminate the dimming too, plus it can't hurt. Also going to verify all grounds are tight and in order. The battery is brand new so that is eliminated now.

greeneyerocker
01-27-2011, 11:14 PM
I just cleaned my throttle body tonight due to a rough idle and RPMs dropping intermediately from 600 to 500. I took off the TB and got out the rags. I cleaned it thoroughly with TB cleaner. Let me tell you what, WOW what a difference that made. No more rough idle and no drops in the RPMs. I had noticed my lights dimming faintly the other night, but did not notice it tonight. Could it have all been due to the TB being dirty? I THINK SO!

p.s. I had just cleaned my TB last April and it was almost pitch black this year, so a yearly cleaning wouldn't be that bad!:m2:

I_hate_joining
01-28-2011, 05:17 AM
Having to clean it yearly seems a bit much, in my other TBLT the throttle body was never cleaned and still averaged about 19-21 mpg in mixed driving, on the highway during long drives the ave mpg would get to the 23-24 range. I had 195k on it 5 years and never had the dimming or some of the other issues so I never bothered to clean it.

Do you use discount gas?

IMHO it would seem like some thing else would be causing such a build up so fast but WTF do I know I am not a mechanic nor have I ever looked at a schematic, block diagram or function flow diagram that shows how the entire system operates. So I would only be guessing.

Opethfan86
01-28-2011, 06:15 AM
I've too started to experience the dimming and its only happened on the really cold mornings we've had right after start up and driving. I believe I have an original battery and figured it was just the cold reeking havoc on it being old.

I would sit at a light and watch my lights dim, the idle would drop to 400-500 rpm and the volt meter would dip from just over 14 to down near 11 and when i would accelerate everything would come right back to life. Once the truck warmed up, in the last 3/4 of my 7 mile drive home, idling at the last lights it wouldn't be doing it anymore. Pretty sure I am due for a TB cleaning but its been too damn cold :ugh:

I_hate_joining
01-28-2011, 09:56 AM
I've too started to experience the dimming and its only happened on the really cold mornings we've had right after start up and driving. I believe I have an original battery and figured it was just the cold reeking havoc on it being old.

Did you wait a few minutes after you started before you put it in gear?
Roadie mentioned that a diagnostic has to run first other wise it gets rescheduled and when it runs again it will load the system down and cause the dimming. I notice the dimming mainly on the last 5 miles home on hilly back roads going less then 40 mph. Prior to that it was all highway for 45 - 60 minutes.

the roadie
01-28-2011, 10:28 AM
...the idle would drop to 400-500 rpm and the volt meter would dip from just over 14 to down near 11 and when i would accelerate everything would come right back to life. ...Sure sign of a sticky butterfly valve caused by a dirty throttle body. Once your idle is steady, voltage drooping can be caused by other things, but getting a steady idle FIRST is the right path to troubleshoot things.

I_hate_joining
01-28-2011, 11:08 AM
Once your idle is steady, voltage drooping can be caused by other things, but getting a steady idle FIRST is the right path to troubleshoot things.

So if the idle is steady a 600 rpm, the throttle body needing cleaning can be eliminated as a potential problem?

Evening Update
Coming home tonight I took a longer flatter route, no dimming.

So my problem appears to be related to changing load and speed due to the hilly and shorter route I normally take.

I_hate_joining
01-31-2011, 09:10 PM
No dimming tonight, it could be due to dirty throttle body as suggested, I have put in 2.5 ranks of super unleaded since the 20th so when it warms up I will be cleaning the throttle body. Got the Big Three kit today will install next day or so.

Hope this is solved.

Dohcser
04-07-2011, 10:45 AM
So I noticed this happening on very few occasions. However yesterday I was able to replicate the problem for about a good 10 minutes of driving an average of 50mph.
The voltage would read 14 ish Volts. Then when I step on the brake the lights on the dash would immediately dim and the voltage would drop to around 9 or 10 volts.
When at idle it was fine but only when I got up to a decent speed and then pressed on the brake.
I doubt that the brake lights are pulling that much current.
Any help would be appreciated.

the roadie
04-07-2011, 11:26 AM
Start by yanking brake-related fuses while you test the brakes while sitting still. There's no reason the behavior should be different while driving along with a decent RPM. The alternator should be putting out more than enough amps to deal with any load the brake circuits can throw at it without blowing their own fuses.

For testing your electrical system with a hugely heavy load, while at idle or driving along, press all four window switches UP while the windows are already up. Just for a couple of seconds. Stalled window motors are about the largest test load you can put on, and they're fed by auto-resetting circuit breakers, so even if you leave them on too long, it won't cost you a replacement fuse for your error.

Do you have a meter and know how to use it for further experiments?

Dohcser
04-07-2011, 01:45 PM
Start by yanking brake-related fuses while you test the brakes while sitting still. There's no reason the behavior should be different while driving along with a decent RPM. The alternator should be putting out more than enough amps to deal with any load the brake circuits can throw at it without blowing their own fuses.

For testing your electrical system with a hugely heavy load, while at idle or driving along, press all four window switches UP while the windows are already up. Just for a couple of seconds. Stalled window motors are about the largest test load you can put on, and they're fed by auto-resetting circuit breakers, so even if you leave them on too long, it won't cost you a replacement fuse for your error.

Do you have a meter and know how to use it for further experiments?

Yes I do have a DMM.
As for the test, thanks for your suggestion, however the only time I see the drop and dim is while driving, not at a stand still. I have been reading a few post and was thinking of checking my TB this evening Keep your suggestions coming.

DZP
04-07-2011, 03:10 PM
I was having the same type of issues and then one night on the way home my alternator went out completely without as much of a warning. I replaced the alternator a short time ago and all of the problems went away. I even had an intermittent issue with the rear wiper for 6+ months, decided to pull the fuse after not finding the problem. I reinstalled the fuse after the alternator replacement and have not had the problem since. All my electrical system is operating as designed.