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Old 11-15-2020, 05:42 PM   #1
badlands122
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'70 GMC Electrical problems

Like most, I HAVE NO CLUE where to start looking for electrical problems. IName:  IMG_2368.jpg
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I have replaced the front harness, now I don't get anything - well, I did get one park light(DS) then the [ark light on the PS, now nothing. I'm sure I missed the 100 or so grounding straps, I purchased more, but don't know where they go. I did find the one under the cab to the frame and engine to the frame. I see one on the frame PS front by the radiator core, but it's just the screw, I don't know where that one goes. Before we get into all of that, I have the vehicle stripped out ready to go to the paint shop- is it a single circuit system where EVERYTHING needs to hooked up or can I get the lights(some of them anyway) to work???
In the pictures are the junction block and the 2 wires I have, one from the harness and the other is from the starter. The schematic shows another wire going to the alternator. Should I add this wire to mine???
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Old 11-15-2020, 05:54 PM   #2
dmjlambert
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Re: '70 GMC Electrical problems

What is the history behind this truck? You said you have the vehicle stripped out ready to go to the paint shop. Was everything working fine before you did that? What did you strip out? What exactly are you interested in fixing in the electric system? Just the parking lights? Do you have a test light probe?

or multimeter?
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Old 11-15-2020, 08:55 PM   #3
badlands122
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Re: '70 GMC Electrical problems

It was my Father's before it was handed over to me. Everything was in working order prior to it being dismantled for repair. I changed the engine, front and rear wiring harness, I didn't change the dash wiring or the fuse box. I used a multi-meter to chase down some bad fuses. Before I removed the fenders to redo the inner fenders, I had 2 park lights and the horn working. I put the bezel with the gauges in it to see what was working. All the gauges are new, the battery and the oil press gauge work. This has been work in progress.... LONG progress.....






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Old 11-15-2020, 11:50 PM   #4
franken
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Re: '70 GMC Electrical problems

I'd get any ground issues resolved, then start working problems one at a time.
I there's a general lack of things working, start at the battery, measure voltage, then start working your way to the ignition switch and so on.
The junction on the fender is what powers everything other than the starter motor. There should be aa fusible link between the battery and junction.You'd be wise to make sure it's there.
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Old 11-16-2020, 12:17 PM   #5
pjmoreland
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Re: '70 GMC Electrical problems

Did your truck have a bezel with gauges in it originally?
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Old 11-16-2020, 01:06 PM   #6
RichardJ
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Re: '70 GMC Electrical problems

>>pictures are the junction block and the 2 wires I have, one from the harness and the other is from the starter. The schematic shows another wire going to the alternator. Should I add this wire to mine???<<

The other , should be the pigtail on the battery cable. The battery cable goes to the starter.
That harness wire is the wire that goes to the alternator.

Is your new harness for EX Reg Alternator or In Reg Alternator.
Your wire diagram above is for original wiring with Ex Reg. If the new harness has changes to that or anything else, you're pretty much on your own. By that, I mean if the harness manufacturer didn't include a new diagram, you'll have to start tracing wires.
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