94 GMC 4x4 350 Extended Cab Electrical Problems - Lights then Heater

I've seen some of my problem in other posts but don't know how to proceed. I'd appreciate any help you can provide.
Symptoms:
Headlights off:
When indicating left, 4-way flashers come on.
When indicating right no problem. Stepping on brake: Radio and heater control lights dim.
Headlights on:
When indicating left: Left indcator light comes on but does not flash When indicating right no problem
What I did:
Cleaned out trailer harness connector. Disconnected and reconnected battery
Result: No change...however at some point thereafter everything seemed to work OK (don't know why)
The story continues....
Now intermittently , the lights on my heater control start to all flash. When this is happenning I get no heat. I can move the fan from the window to the floor but no heat no matter where I've set the temperature. Eventually the heater control lights will stop flashing but still no heat. If I try to make an adjustment, the heater control lights will start flashing again.
Sometimes the lights on the heater control don't flash and I get all the heat I want.
My indicator lights have not malfunctioned since that problem corrected itself.
So what's with the heater? I know I should check the ground but don't really know where to look. Anything else that might be causing the problem?
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Dude, you've got a busted or missing ground cable. The cab has to be grounded to frame, the battery has to be grounded to the frame ... its all about the ground
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From other posts for other problems I guessed it might be a ground issue but how (where) do I start looking for the source of the ground problem? Under the dash? Where? Under the hood? Where? Trailer harness? Where?

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Greetings,
Your truck has several places where items are grounded. The first place to look are the major spots - 1. Negative cable to the battery (replace the end if necessary with new one from auto parts store) 2. Ground from neg. battery cable to chassis/frame (follow the black cable to it's hidden end) 3. Look for a braided ground strap on the firewall from the engine that provides contact for the cab (remove and clean both ends - this is the most likely culprit) 4. Braided ground strap from frame to engine (probably can only be seen from below)
Those are the primary places. Secondary grounds run from each individual electrically powered item, but since you have multiple items working strangely try the major ones first.
Cheers - Jonathan

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Thank you. That's just the sort of thing I was looking for. I appreciate your help.

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Jonathan wrote:

Well I disagree with your characterization of primary and secondary grounds. There is no such thing. It does sound like a ground issue though.
I would look for an issue with the lighting grounds. And since lights dim when you step on brake, I would start in the rear of the vehicle, with the harness just as you did.
Then I would look at the ground for each tail light. Typically the rear lights come together and ground at the same point on the body either on the left or the right, but in this car they might have individual grounds one on each side. It shouldnt be too hard to find the ground point for a tail light assembly, also since the trailer harness is probably tying into it. but sometimes trailer harnesses tie into an underbody harness and end up grounding underhood on the chassis.
Is there any different behavior if the trailer is plugged in or not plugged in?
im not sure what a heater control is. If you explain what it is and what it does and the imputs it has then I might be able to offer more places to check.
CL

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Thank you,


CL Gilbert
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Greetings,
The difference between what I call a "primary" ground versus a "secondary" ground are best explained by this example.
If an object (such as the frame of the vehicle or the chassis) is connected by a direct physical line to the negative side of the battery, then that is a primary ground. If an item (such as a light bulb) is first grounded to the chassis which is then in turn connected to the negative terminal of the battery then that is a secondary ground.
You can attach a ground wire to any part of the chassis you like, but if the chassis isn't directly connected to the neg. battery terminal then you won't complete the cirucuit. If the chassis ground is dirty or making poor contact, you could have symptoms the OP described. A chassis ground is especially important because on many trucks the chassis bolts are damped by rubber bushings which not only absorb some impact and movement, but also electrically isolate the chassis from the frame and engine as a secondary issue. This is the reason why the braided ground straps are used - to ensure a continuous electrical connection between the engine, chassis, frame and battery.
Cheers - Jonathan

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I kind of figured that was where you were coming from. Your definitions are valid I suppose, though the battery tends to have but a single ground to the chassis, and a 2nd to the alternator. I figured that based on your examples which involved mostly things underhood our outside of the passenger cabin. And your right about trucks and not having 'unibody' I think its called.
In any event, Im going to put my money on "secondary" ground. If you loose one of those primari grounds things get very bad, and there are rarely secondary paths for the current to find its way home. Since turn signals are sometimes between two positives, you can see odd things like this when one of their associated grounds come aloose.
Jonathan wrote:

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Thank you,


CL Gilbert
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