Trouble shooting an electrical short. - Defender Source
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  #1  
Old August 6th, 2012, 03:41 PM
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Trouble shooting an electrical short.

The battery is running down in my truck and I've narrowed it down to a couple of culprits (Oil pressure & Fuel cut off switch). I haven't disconnected everything to ID these two, but I have eliminated quite a few. At some point the MOD painted over the wires in the engine bay where they enter the bulkhead so I'm having trouble tracing those by color. Specifically the engine wiring harness. If anyone could tell me the colors of the wires that leave that connector I would be grateful. Specifically the two sets of small wires that appear to enter the plug at the same location.
Some other information about this short. Ignition is off, and I'm measuring 7.4 volts at both the fuel cut off and oil pressure terminals when disconnected from the switches. When the oil pressure wire is left connected, I measure 4.37 volts at the fuel cut off wire. When the fuel cut off wire is left connected and I measure the pressure switch wire I get 1 volt. The battery is and measuring 12.77 volts.
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Old August 6th, 2012, 08:54 PM
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Back in the Good Old Days I bought a '57 Austin Healey 100-6 for $300. Back in the '50s the wiring had fabric coating, and this stuff did a good job of absorbing all the stuff floating around in the engine compartment. So I found myself with a car that had a lot of black cloth-covered wires in the engine compartment. And a battery that kept discharging.

I discovered that by carefully using my pocket knife to scrape the black fabric on the wires I could reveal enough color to identify the wires and sort out the SPOTs. After a day or two of tracing wires and correcting wrong connections, I had a reliable daily driver.

If I were in your position, I would start scraping off some of that paint to see what is underneath.
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Old August 6th, 2012, 11:59 PM
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What year 90? I can scan the wiring diagrams if you need them
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Old August 7th, 2012, 11:24 AM
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John I thought of that as well but was apprehensive as I don't want to remove the tracer. Jason thanks as well, I have the schematic but that raising questions as well. The schematic shows a white with black tracer going to the fuel cut off but mine looks like it all white. The oil pressure looks correct as white with a slate tracer.
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Old August 8th, 2012, 12:42 AM
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Quote:
Originally Posted by Baldewin
John I thought of that as well but was apprehensive as I don't want to remove the tracer. Jason thanks as well, I have the schematic but that raising questions as well. The schematic shows a white with black tracer going to the fuel cut off but mine looks like it all white. The oil pressure looks correct as white with a slate tracer.
Try a 3m pad (the green synthetic wool types) and some engine degreaser or solvent

I'm sorting a few oddities that don't seem to match up to the diagrams myself
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Old August 11th, 2012, 08:27 PM
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Jason, thanks for the suggestion. I did just that and have traced the wires back but not finding it. Somewhere 12 volts are escaping and I'm just can't source it yet.
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Old August 12th, 2012, 06:44 PM
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Ok, I've located the short and now need to figure out how or why it developed. The wires on the engine wiring harness had melted and fused together. The specific wires would be the two large brown wires between the starter and the alternator along with the alternator warning light wire. I'm not sure if it was the heat of the engine, heat from current flow or a combination of both. Any ideas on why would be great, thanks.
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Old August 12th, 2012, 07:37 PM
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If you think it's current I would say go to town with your meter.

I dont have a lot of experience with electrical except what I've learned as I go along, but the wires I've come across that are burned due to current will be noticeably different. Look for discoloration in the middle of the wire and opposite side of the burn. Basically more burn from the inside out vs outside in

If it was heat from the engine bay I would assume the insulation would show damage but not as much into the wire itself or as deep/extensive as heat from too much current
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Old August 12th, 2012, 09:03 PM
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Taking a closer second look I would say it's current related as the two larger wires show the bulk of the damage. Very little to no melting of the alternator warning wire other than where it was consumed by the two larger wires.

Next step I'm guessing is to test the output voltage of the alternator!
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