Factory ground wiring

[GermanMonte]

I'm currently chasing down a low voltage condition in my Monte Carlo and I'm eventually running out of ideas. I get 12.5 volts at the battery posts with the engine off and only 10.5 volts to the lights (front and back). I can see the dome light dimming when stepping on the brakes or switching on the lights. When the car is running I get a nice 14.2 volts at the alternator and the same at the battery, which sits in the trunk.

 

The positive side of the battery is connected to the starter with a heavy 3 awg wire. For grounding I've ran a 3 awg wire from the battery to the engine block, another wire from the heads to the frame and a third one from the negative battery post to the body (in the trunk). I've checked the wire from the positive terminal of the alternator to the junction on the front passenger side where the battery was: 0.2 ohms. From there I followed the heavy red wire that runs to the bulkhead connector of the fuse box: 0.1 ohms. I also disassembled both the ignition switch and the light switch which both looked good inside, cleaned the contacts and put them back together.

Still, only 10.5 volts at the lights.

Is there another factory ground wiring or switch / relay that I overlooked? I hope it's not the copper worm freaking out somewhere behind the dash :-/

[Leghome]

Have you cleaned the ground for the headlights?

[Rock]

Grounding problems cause all kinds of strange things. There are grounds behind the dash for the headlight switch and one that connected to the emergency brake. I had all kinds of issues from those 2 grounds. Do you still run the voltage regulator or has it been bypassed? I have bypassed my regulator and opted for an internal regulated alternator.

[GermanMonte]

Headlight grounds are good. Gotta check the grounds behind the dash - thanks for the heads up!

 

I made an experiment today: Supplied the bulbs up front and in the rear with a proper ground wire directly from the battery: neither a change in voltage nor in brightness. After that I removed the auxiliary ground wire and ran a wire from the positive side of the battery to the bulbs: Voila! Voltage came up and so did the brightness of the bulbs. The source of the problem must be on the positive side then I guess. Sounds like I'll have to examine the wiring behind the dash...ugh!

[Mo's70MCs]

A voltage drop problem.. Ideally If memory serves me right, one of the things that I did to my daily driver (Betsy) was to upgrade on the size of wire, from what I believe was a 10AWG to a 6AWG, the wire that goes from the junction block near the battery to the junction on the fusible link near the voltage regulator, talk about changing the problem all together. I used to have issues with dimming lights when stepping on the brakes, and noticeable on the headlights as well as the dash-lights. It also fixed intermittent starting issues I used to have with it, I believe the engineers at GM had a brain fart along that junction. Not that it needs a 6 gauge wire, but it surely fixed everything in doing so. I never tried an #8 I just went for overkill from the get-go.

[GermanMonte]

Thanks for the answers. I'll dig into the wiring next weekend.

[wallaby]

There is a buss bar on the original regulator where voltage is sensed.  The alternator gets its charging signal from this spot. The battery is also hooked here by a wire that crosses over the radiator. Any accessories should be hooked to this buss bar. If any voltage draw occurs, the buss bar will see it and tell the alternator to put out more. Check the connections there.

There was also a 10ga wire that connected the negative battery post to the fender. Grounds are funny... you need to connect things that are isolated by rubber mounts. The engine to frame, body to frame, even engine to body, and of course engine to battery.

I have seen headlight dimmer switches go bad. Lets face it, they are in a bad spot for road water and stuff to get kicked up on them by the front tire.

 

No real solutions here, just things to check.