Lights dim when electrics used.

[smileynialley]

Basically, I've noticed the headlights dim whenever I press the brake pedal.
This kept happening and tonight I noticed the lights would dim when I used the windows.
Then, when I pressed the "all four windows up" button, the lights would pretty much turn off and all my electrics inside the Hummer cut out momentarily. The engine stayed on and all was fine when I turned it off and on again.
The only thing different I've noticed is a high pitched whirring / wining noise coming from the dash board area, which seems to speed up and slow down with the revs.(not sure if these are linked)

Any ideas guys? i'm stumped!
Thought it could be the alternator but the gremlins don't match that problem.

[fyr145]

Could be any number of issues. Lets look at a few of these.

1. Voltage regulator: Most late model units today have the voltage regulator built inside the alternator. To rule this out, you could simply get one of those load testers and a meter and measure the output of the alternator. While your at it, check the tension of the serpentine belt to make sure that you're getting the proper revolutions to even generate any power. Maybe also look at the pulley on the alternator and ensure that its not worn out and making any noises. You can find out how to make certain connections for specific tests on You Tube.

2 Battery: You could have a bad cell in the battery. How old is your battery? (Here in AZ, batteries only last an average of 3-4 years due to the excessive heat.) But large electrical loads from powerful aftermarket audio equipment or using lots of off road lights can also drain a battery. If the battery is not taking a charge, then either the alternator is not sufficiently charging the battery OR the battery is not taking the charge provided by the alternator.

3. Bad or loose Ground(s): Another remote possibility is a ground wire is not secure to the frame. Check all ground locations starting with the engine, then the negative cable on the battery, and so on.

These issues is where I would start to begin to rule systems out. As with any electrical gremlins.......its always best to start with one system at a time and go from the easiest to check to the more difficult ones. Be systematic in your approach.

[Mister Peebody]

Originally Posted by fyr145

Could be any number of issues. Lets look at a few of these.

3. Bad or loose Ground(s): Another remote possibility is a ground wire is not secure to the frame. Check all ground locations starting with the engine, then the negative cable on the battery, and so on.
.

My vote is with #3! Corroded grounds will cause all sorts of strange issues.

[smileynialley]

thanks for the replies,
I have had a look around the engine bay and there is nothing obviously loose or corroded etc.
It seems like the alternator can't produce enough power to power everything in the car so when you use the windows or brake lights etc, the headlights and cabin lights either dim or cut out altogether and the stero cuts out.
No affect on the engine though.

[fyr145]

Ok then, if you have ruled out #3, then the next thing I would check is the battery then the alternator.

As I stated earlier, get a load tester or you can go to an AutoZone, O'rileys, checker, etc. and have them put their unit on the battery and check it. The load tester simulates having various electrical loads being placed on the battery by them flipping a switch or turning a dial depending on the type of tester then seeing if the battery can still maintain proper output even with a big load draw.

The other test is to check that alternator. just because you see the serpentine belt going around and spinning the pulley on the alternator doesn't necessarily mean that its putting out a charge to the battery. As stated earlier as well, most late model alternators are now built inside the alternator. (I upgraded my stock alternator from the factory 145 amp to a 200 amp because I had added the factory brush AND roof lights.) In the old days, the regulator was a separate unit and if it went faulty or bad, you could simply change out the regulator. Not any more.

The only other issues that I can think of if its still doing it after completing the previous steps mentioned above, is to go on a search for any power wires or connections to your accessories that may be touching or possibly arching on the body. Or worn insulation from being pinched or rubbing on something from vibration. Typically, when these issues occur, it would cause a short and blow the fuse.

Sometimes people tie all their accessories into one power power outlet or line. When all these are being used, the outlet cannot support all of them simultaneously thus causing a voltage drop.

Keep us posted on how the battery and alternator testing goes.

[LoJac963]

There should be other frame (chassis) grounds also not just what is obvious in the engine compartment.