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Tracking down a short circuit.

Guest dstaton

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Guest dstaton

Hello Folks,

My 1962 Starfire is nearing mechanical solvency. Last necessity is the electric. As some of you may remember, I received the car an electrical mess. It had both a 4 wire and a 3 wire voltage regulators, two horn relays, redundant wiring and half completed corrections or repairs. Generally, I have been able to remove added circuits and found the original wiring present and serviceable.

I was able to start the car some time ago, and turned my attention to the dash lighting, vacuum assists, blower. Symptoms included very dim dash lights, visible only in complete darkness, a horn button that drives only one tone and tentatively, turn signals that flash the bulb but not the dash on one side but flash the other side just fine, and a Hot light but no Gen on Acc position.

I replaced crumbling sockets in the dash panel, replaced bulbs and checked for grounding at their sockets, restored the signal switch, restored and eventually replaced the light switch. The ignition switch tests functional. Finally, I noticed the DelcoRemy S10 was dead and had it rebuilt. And I replaced the voltage regulator. The starter and solenoid are new-rebuilt.

Finally, (yeah I now know I have worked last things first) after checking for shorts, I read a full 11.78v between the ground strap and the negative terminal of the battery. After pulling all the fuses and reading a persistent full leak, I pulled everything off the horn relay except the starter lead and the battery feed. The test meter still reads .05. Before I continue, I would love some advice on the significance of this read, and any other guidance you can offer. Do I have the right sequence of checks now? Battery to horn relay to fuse panel to load, right?

Just when I start to feel like I am beginning to understand the electric system – the mystery dance... Any advice or commiseration would be excellent.





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Guest dstaton

Whoa, this is algebra. Looks like I'm digging in the right hole.

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[TD=class: alt1, bgcolor: #F5F5F5]That sounds very feasible.. Of course a lot of variables come into play on top of that measurement. Aside from temp (which you have and will assume the block was at ambient) is battery voltage at the starter during cranking, cranking RPM (a stiff tight motor will slow the starter, which in turn will draw more amps) etc

You can back peddle all your numbers using basic ohms law. Measure the DC resistance of the starter, measure the resistance of the soleniod and add them together (it's in parallel so 1/starter + 1/soleniod = 1/total), devide that number into your cranking voltage at the starter and you now have the amps you're drawing.. You need a good meter though as the ohms will be 0.something ohms.

ie, .05 on starter, .8 on the soleniod = (1/.05) + (1/.8) or (20)+(1.25)=1/21.25= .047ohms. If there is 12 volts at the starter during cranking, 255 amps are being used. If only 9 volts during cranking, only 191 amps are being used. Next, see how much voltage is at the battery during cranking, divide the difference by your ohms and you now know how many amps is being lost in the cables and connections. (for those interested, multiply your volts times your amps to get watts, ie the starter above would use over 3000W at 12V and only 1700 at 9V, that's 1/2 the cranking power with a 3V drop)




My apologies. My dad always said you messed up if you ever leave nobody with anything to say.

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Wire a 12 volt light between the battery terminal and the ground strap. Disconnect and or shake wires everywhere.

When the light goes out you have removed the short. Have fun.

When my 61 Starfire first came to live with us I had a similar problem.

When the drivers door was opened the oil pressure warning light would come on, with the key off.

If your short occurs with the key off that points you toward those circuits which are active ,such as parking lights and courtesy lights.

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  • 2 weeks later...

Any idea how my alternator could cause the bulb to light? When I pull it out of the circuit the short goes away, and when I add the alternator it lights the bulb. Alternator tests fine after a complete rebuild. I took it to be tested twice...

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