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86 Corvette Wiring

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Old 07-11-2018, 12:27 AM
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Bharl
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I have an 86 Corvette, recently had a wire get hot and burn the insulation off. Iíve been diagnosing things, hereís what I found, hopefully someone can help.. the plug that goes into the fuel pump relay has continuity between pin A and B (red and black) relay seems to test fine. Thereís an orange wire that comes up, and feeds right into the positive battery terminal through a fusable link (wire that burned) this wire has continuity with post E of the plug for the fuel pump relay. I also found a short in the cabin light circuit, which I fixed for now by pulling the fuse. The orange wire that comes into the positive terminal doesnít cross a ground anywhere, and only gets hot when the ignition is on. Car starts with everything connected, wire still gets hot super fast. Iíve been tracing through miles of wires and am lost. I traced the red wire that shows continuity with the black wire on the plug back to the ECM on the passenger side. Is it possible thereís a short in the ECM thatís causing the short? Is it normal for the red and black wires to show continuity? What else should I be checking? Any help is appreciated!! Iím pulling my hair out..
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Old 07-11-2018, 10:56 AM
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TheBlaster9001
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Originally Posted by Bharl View Post
I have an 86 Corvette, recently had a wire get hot and burn the insulation off. Iíve been diagnosing things, hereís what I found, hopefully someone can help.. the plug that goes into the fuel pump relay has continuity between pin A and B (red and black) relay seems to test fine. Thereís an orange wire that comes up, and feeds right into the positive battery terminal through a fusable link (wire that burned) this wire has continuity with post E of the plug for the fuel pump relay. I also found a short in the cabin light circuit, which I fixed for now by pulling the fuse. The orange wire that comes into the positive terminal doesnít cross a ground anywhere, and only gets hot when the ignition is on. Car starts with everything connected, wire still gets hot super fast. Iíve been tracing through miles of wires and am lost. I traced the red wire that shows continuity with the black wire on the plug back to the ECM on the passenger side. Is it possible thereís a short in the ECM thatís causing the short? Is it normal for the red and black wires to show continuity? What else should I be checking? Any help is appreciated!! Iím pulling my hair out..
I think the wire at the positive battery post is the power for the ECM - if we are talking about the same wire. It could be a short in the ECM that is causing this - I've seen a similar issue on a Dodge. Fried ECM would melt its ground wire and the wire to the TPS.

You could Ohm the ECM wire from close to the wire to the ECM connector. The pin is B1 on the ECM connector. That orange wire gives constant power to the ECM - so that fact that it is only hot at key-on suggests to me that the ECM may be bad, or somewhere in the key on circuit to the ECM has a short.
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Old 07-11-2018, 02:35 PM
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Hot Rod Roy
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Welcome to the Forum!
I think the wiring for your '86 is probably the same as my '84. Here's the diagram for the FP circuit from my '84 Factory Shop/Service Manual (FSM). You'll need to get one for your '86!



Originally Posted by Bharl View Post
The plug that goes into the fuel pump relay has continuity between pin A and B (red and black). There’s an orange wire that comes up, and feeds right into the positive battery terminal through a fusible link (wire that burned) this wire has continuity with post E of the plug for the fuel pump relay. The orange wire that comes into the positive terminal doesn’t cross a ground anywhere, and only gets hot when the ignition is on.
TILT!!
Continuity between FP connector pin A and B is BAD!
When you turn on the key, the FP relay closes, and current goes thru the org wire to relay pin E, to relay pin A (red), then thru your short circuit to the blk (gnd). Look closely at that FP relay socket! It's amazing this car runs!

If that FP socket looks good, the short circuit could be anywhere on that red wire that goes to the ECM or to the FP fuse in the fuse panel at the RH end of your instrument panel. I hope you find it at the FP relay socket! It could also be at the red wire at your oil pressure switch.


Last edited by Hot Rod Roy; 07-11-2018 at 08:22 PM.
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Old 07-11-2018, 04:37 PM
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Thanks for all the replies! I checked the socket, and canít fjnd anything obvious, itís just all mucked up with the grease I imagine was used from the factory. Iíve started tracing the red wire back, had to pull out the wiper motor to get access to most of it. Thereís still a section I canít get at, as itís behind the engine. If that wire runs directly from the FP relat to the ECM I wonder if I couldnít just cut it off at both ends and splice a new wire in without necessarily tracing it all the way through the loom? I thought about doing it in my frustration of unraveling 30 year old electrical tape/goo, but I didnít know if it got spliced off somewhere along the line.
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Old 07-11-2018, 09:03 PM
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Originally Posted by Bharl View Post
I wonder if I couldn’t just cut it off at both ends and splice a new wire in without necessarily tracing it all the way through the loom? I thought about doing it in my frustration of unraveling 30 year old electrical tape/goo.
With the wiring being 32 years old, the insulation will be getting brittle, so it's best not to unwrap a harness, as you'll probably damage other wires. This red wire also goes to the oil pressure switch, which is located to the left of the base of the distributor. That's a hot location, so that may be an area to inspect carefully.

The red wire (ckt 458) goes thru the firewall to get to the ECM and the FP fuse, but my '84 FSM isn't clear how it gets thru the firewall! You might try unplugging the oil pressure switch and the ECM, and removing the FP fuse, and verifying that you still have the short circuit at the FP relay socket, pin A. The firewall connector is at the base of the steering column, to the left of the column.



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Old 07-13-2018, 01:12 AM
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Bharl
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Well, got it figured out. I pulled the fuse for the fuel pump circuit, all was well under the hood. Traced it down to a tan/white wire that went back to the fuel pump. I ended up pulling the fuel pump, and found a ton of resistance across the positive and negative terminals of the pump. How it still ran us beyond me, but itís sounding like a bad pump that was causing the short. Got one ordered up, and hopefully all will be right in the world again! Thanks again everyone for the help!
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Old 07-13-2018, 01:34 AM
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Originally Posted by Bharl View Post
I ended up pulling the fuel pump, and found a ton of resistance across the positive and negative terminals of the pump.
When you say "a ton of resistance", are you saying the resistance of the pump was very low, or very high? ( I usually measure resistance in "ohms" rather than "pounds". )

The FP SHOULD have a very low resistance across its terminals. If the FP was causing a very high current flow, the FP fuse should have blown. Was the FP fuse the proper rating? It should be a 10 A fuse. Do you have a way to run the FP on 12 v. and measure how much current the pump uses?

I suspect the FP is not your problem.

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Old 07-13-2018, 10:14 AM
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Iíll try running the pump, I suppose j can check the draw by hooking it to the battery and seeing what kind of amperage itís drawing. When I checked the ohms across the old pump it was around 150 ohms, seemed awful high. Parts store ended up having one (of course the plastic tube on the top was broken off and I didnít realize it until I got home..)

the new one was down around .4 or .5 ohms. The fuse is 10amps, Iím not sure why the fuse didnít blow. When I pulled the pump out though, I checked continuity across the ground and red wire on the fuel relay and it was good to go, no more continuity
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Old 07-13-2018, 08:05 PM
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Originally Posted by Bharl View Post
Iíll try running the pump. I suppose I can check the draw by hooking it to the battery and seeing what kind of amperage itís drawing.
If you haven't removed the FP yet, you can check the current draw by removing the FP fuse, then plugging your amp meter/multimeter into the fuse socket. Turn on the ignition (not the engine) and read the current. There's no way your high resistance FP could cause you to burn up the org wire to your FP relay.

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