Crazy electrical issue

dnl_paterson · January 14, 2016

I have a 96 GMC suburban k1500. Sometimes the a/c will shut off and at the same time the parking brake and antilock light come on. It will do this for a few minutes and then both lights go out and the a/c comes back on. Both lights also come on sometimes when I flip on the bright lights at night. I looked over the wiring schematics for my truck but I can't seem to nail down what would be the cause of this. Anyone have any suggestions?

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dnl_paterson · January 15, 2016

I have a 96 GMC suburban k1500. Sometimes the a/c will shut off and at the same time the parking brake and antilock light come on. It will do this for a few minutes and then both lights go out and the a/c comes back on. Both dash lights also come on sometimes when I flip on the bright lights at night. I looked over the wiring schematics for my truck but I can't seem to nail down what would be the cause of this. Anyone have any suggestions?

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THORX4 · January 21, 2016

wow that sound like a battery issue. have it load tested at an auto parts store. can you duplicate the problem?

dnl_paterson · January 22, 2016

wow that sound like a battery issue. have it load tested at an auto parts store. can you duplicate the problem?

Thor thank you for your reply. I have not load tested the battery. I have however, tested the output of the battery with a multimeter. It is 12.4 volts. I have also tested the alternator and it is putting out above the battery voltage. I forgot to mention something else weird. When I hook up my scan tool when these lights are on, it reads “connection error.” When the lights go out, the scanner reads like it normally would.

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dnl_paterson · January 23, 2016

I just load tested the battery. It is good.

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green 788 · January 23, 2016

if battery is good, check wiring from alternator to + battery post.

It does sound like the system is operating on the verge of losing voltage.

If the truck is running, you should be able to disconnect the + battery cable and still have the engine running; the alternator should be providing enough amperage to charge the battery, and operate the electronics.

If the engine won't continue running when you remove the + battery cable, I would suspect that the alternator is not putting out enough power to keep everything satisfied. When the a/c compressor kicks in it's over-loading the system, and when you hit the high beam headlight switch, same thing... overload, and you get the symptoms.

THORX4 · January 23, 2016

did you have the alternator load tested?

dnl_paterson · January 24, 2016

Green, I'm going to unhook the battery while it's running with all the electricals on today and I'll get back to you. Thor, I'm going to do first what I mentioned to Green and see what happens. I may go ahead and load test the alternator down at the auto parts also. Thanks guys and I will let you know what I find out.

dnl_paterson · January 24, 2016

Well, I unhooked the battery, and everything stayed running. I also took it down to advance and had the battery and alternator load tested. Everything checked out OK. There must be a short somewhere? The peculiar thing is, according to the wiring diagram, the a/c, headlight dimmer, antilock and obd 2 port share the same circuit. Could I be on to something here? Do you guys remembering the old floor mounted dimmer switches? I remember when they went bad, you would press them and your headlights and dash flights would go out. I wonder if this could be the case for me. Maybe the dimmer switch is bad and causing all these problems?

green 788 · January 24, 2016

when you say your air conditioner cuts off, are you basing this on the fact that it stops cooling, or are you referring to the blower motor (fan)?

dnl_paterson · January 25, 2016

The blower motor. Everything shuts off just as if I manually turned it off

green 788 · January 25, 2016

I looked over the schematics in my Haynes manual... I can't see the connection between the blower motor and the high beams--other than perhaps a ground, which is what this sounds like.

You won't be able to see where the grounds connect on most wiring schematics, so you'll need something like this image below to show you where to look for ground points. That is probably what is going on, a corroded or loose ground.

dnl_paterson · January 25, 2016

I'm going to check the battery ground cable to engine block connection tomorrow. The ground strap from the block to frame to firewall looks pretty worn. Maybe replacing these will make a difference. I'll let you know what I find. Thanks green.

green 788 · January 25, 2016

yes, do let us know. I had a Ford with a bad ground strap (a Bronco I put a body lift on) and I really never found the bad ground... so I just slaved a new ground from the firewall to the frame, and all was well. It's still working... I've got at least 1 Ford in the family. :o

dnl_paterson · February 11, 2016

Well I ran new ground wires from the block to frame to firewall. I ground all surfaces down to clean metal. I took off the negative battery cable and cleaned where it bolted to the front of the block and reinstalled it tightly. The problem is still there. I don't know what else to check.