Electronically issues only in cab

Wyatt Tankersley · November 2, 2022

So I have a 2003 chevy 1500 with about 293000 miles on it. I have been having voltages drops while driving. The voltage will drop only in the cab to almost 9 volts at times. The abs light l, parking brake light and sometimes the airbag and security light will come on too. This happens when the hvac fan speed is above 1 sometime it even happens on speed one also. It also happens if the music is turned up or if any other accessories are on in the cab. The voltage is normal everywhere else on the truck the only voltage drop is in the cab. I located all of the in cab grounds and cleaned them. I have tried swaping the dash cluster, the hvac module and it has a new transfer case control module. I have also taken apart and checked all the connections on the abs module. It also have a new fan restor. I've tried unplugging molutiply different accessories like the fan motor etc and nothing fixed it. My mechanic friend hooked up his fancy snap on scanner and determined it was not the ecu. I have also tryed putting different alternators on it and nothing worked. When it started I had no after market accessories in the cab. I can't seem to find where the voltage drop is coming from. If anyone has ran into this let me know

asilverblazer · November 2, 2022

Warning lights are coming on because of the low voltage. Sounds like your alternator and/or battery is going bad.

The alternator should keep voltage above 12 volts while running, the battery stores the voltage for when the alternator is producing power.

Problem 1 is the alternator isn't keeping voltage up while running.

Problem 2 is the battery isn't keeping the 12 volts up, while the alternator is out - unless you are running it long enough that any available voltage in the battery gets depleted.

Edited November 2, 2022 by asilverblazer

Wyatt Tankersley · November 2, 2022

I've already tryed putting in a new alternator and battery and neither worked. But everywhere else on the truck under the hood and everywhere else is getting the correct voltage. The ecm is also getting the correct voltage. The voltage starts dropping only in the cab and does not effect anything else on the truck, the head lights don't dim or anything like that

asilverblazer · November 3, 2022

So I must not have read the original post well enough... Sounds like you've isolated it to the cab and checked grounds. I would start trying to isolate circuits if possible. Also check the wire harness where it goes into the cab at the firewall next to the brake booster. Wouldn't hurt to check the BCM either.

Weather conditions make it worse? Water could be leaking into something...

Wyatt Tankersley · November 3, 2022

2 hours ago, asilverblazer said:

So I must not have read the original post well enough... Sounds like you've isolated it to the cab and checked grounds. I would start trying to isolate circuits if possible. Also check the wire harness where it goes into the cab at the firewall next to the brake booster. Wouldn't hurt to check the BCM either.

Weather conditions make it worse? Water could be leaking into something...

I had also looked at the wiring harness going into the cab and all looked good and it doesn't change weather it's cold or hot or raining or sunny

asilverblazer · November 3, 2022

I'm running out of ideas... when it is acting up pull fuses to see if what circuit is causing it or are immune to it. Make a detailed note of what circuits are dropping voltage and are not. Track all the good circuits and bad circuits on a wiring diagram to see what are grouped together or other common points.

What happens if you shut the truck off and restart it?

You checked all the cab grounds, what about cab to chassis?

Wyatt Tankersley · November 3, 2022

1 hour ago, asilverblazer said:

I'm running out of ideas... when it is acting up pull fuses to see if what circuit is causing it or are immune to it. Make a detailed note of what circuits are dropping voltage and are not. Track all the good circuits and bad circuits on a wiring diagram to see what are grouped together or other common points.

What happens if you shut the truck off and restart it?

You checked all the cab grounds, what about cab to chassis?

I will try pulling the fuses and isolating the circuits. If I turn the truck off and restart it still does it. And yes I checked that one and all the other ones under the hood and in the cab and on the frame

1 hour ago, asilverblazer said:

I'm running out of ideas... when it is acting up pull fuses to see if what circuit is causing it or are immune to it. Make a detailed note of what circuits are dropping voltage and are not. Track all the good circuits and bad circuits on a wiring diagram to see what are grouped together or other common points.

What happens if you shut the truck off and restart it?

You checked all the cab grounds, what about cab to chassis?

Also someone recommended I try running a ground strait from the battery negative to the dash what do you think about that

asilverblazer · November 4, 2022

15 hours ago, Wyatt Tankersley said:

Also someone recommended I try running a ground strait from the battery negative to the dash what do you think about that

For sure, cables can rot out inside the insulation and you can't see it.