2000 GMC electrical problem

Vehicle info:


2000 GMC SIERRA 4DR 1500 V8 5.3L

Mileage 136,025

The history:


I bought this truck 5 years ago. And over the past I would be driving down the road and every one in a while the gauges would get all twitchy and all the lights on the dash would go out for a split second. The engine never skipped a beat. This would happen once about every two - three months. Then about six months ago things got worse. The symptoms listed below started happening about every 3-5 weeks, they would last about a week and then suddenly go away and every thing would work fine again.

Now the problem is steady now.

The symptoms:


*** ABS light is on ***

Blinkers and Hazard lights only work in park or neutral. Works sometimes in drive if I let off the brake just a little bit but only if the truck is not moving. And they blink fast.

Both blinker arrows in cluster have a faint glow. When the left or right blinker is on the rear blinker flashes but both tail lights have a slight blink but in opposite time.

Front appears to be fine.


*** Service 4wheel drive on ***

The transfer case control switch wont work (no lights on) . When I turn on the ignition to the diagnostic position, the lights blink fast one time and then go out. If i do it again with out turning the ignition all the way off the lights stay on. I can then select auto, 2high, 4high, and 4low and I can hear the motor on the Tcase run. But if I start the engine the controls quit working.


*** Fuses & Voltage (engine running)***

FUSE --------- Left side | Right side


Left turn------ 22.0mv ----- 3.5v

Right turn------ 3.5v ------ 28.0mv

VEH B/U------ Varies +/- ----3.4v

W/W pmp----- 3.5v------- 50.mv

Horn --------- 3.5v ------ Varies +/-

RR prk-------- 3.9v-------- 14.1v

LR prk-------- 1.6v-------- 3.4v

Veh stop---- Varies +/- — 3.5v


TRL prk------ 1.6v------- Varies +/-

I’ve had many people suggest the ignition switch but I have 12 volts @ all the wires. I’ve checked/cleaned the major ground wires that I could find going to the frame, I got the one on the fire wall, the left RAD SUP, Left driver side cab mount and the one above the spare tire. I’ve replaced the brake switch and all so tested with a new “all in one” switch but still no solution.

It looks like it maybe a BCM (Body control module) problem but I’m not sure. I cant think of anything else to to try.

The trouble could be due to a bad ground and I was thinking that while reading your post. You could make a ground jumper and tie one end to the negative battery post and then tie the other end to areas you suspect may have a bad ground to see if that makes a difference.

The trouble may also be due to a bad power connection in the power bus somewhere. Check the power getting to the fuses while the engine is running. Start with the power panel under the hood and then the dash fuses to make sure the fuses have good power to them.

You would be wise to get a service manual to help you with this trouble. It is a very good investment to go with your voltmeter. Having those things in hand you will be able to pin down any electrical trouble.

Good idea… I’ll make a ground jumper. I will be working on it today and I will post my findings.

The ABS light is ON might be from low voltage feed to the ABS from the alternator. Attach a voltmeter to the alternator, and run the engine, slowly, up to 3000 rpm and down to idle, while observing the voltage output.

Try pulling and reconnecting the BCM connectors a few times, they have been known to have corrosion build up on some vehicles.

Check the voltage getting to fuses under the hood along with checking the ground connections. Chances are there is a problem with the power bus under the hood rather than a bad ground.

After 35 hours of probing wires I have some useful results. I discovered that I have 4.5 volts running through the frame.

(1) I checked for continuity between the battery ground and the body and that’s good.
(2) I checked for continuity between the frame and the body and that’s good.
(3)I checked for continuity between the frame and the battery ground and that’s no good.

If I run a wire from the frame to the battery ground then all the problems go away and the lights get brighter.

I’ve looked all over to see where the frame gets its ground from but cant seen to find any sort of wire. Where dose it get grounded? Also why would there be 4.5 volts in the frame???

There is resistance between the frame and the body that is causing the voltage to drop to 4.5. I don’t know how the connection between the body and the frame was done originally–it may have been through the body to frame bolts. Over the years, corrosion could have had an effect. However, just make a permanent connection from the battery ground to the frame since this solves your problem.