Electrical mystery

[ronskydivepops]

My 71 Gt has a wiring issue that doesn't happen all the time. When I'm driving along with the headlights on(low beam) and the radio on and turn on the turn signal the ammmeter gauge goes from showing a charge to showing a discharge. The radio turns off and then turns back on when I disengage the turn signal. I have an in line fuse on the output of the generator because of prior wiring issues from the previous owner. The fuse blows and I'm wondering why. Also my horn doesn't work after the fuse blows. I've rewired the harness in the car as the previous owner had splices hidden everywhere. I rewired per OGTS wiring specs. Everything works fine when I start it up and try everything electrical. Yet, I don't think that I've tried the radio, lights, and turn signal at the same time until this happened!! This is the second time this has happened in about three weeks time. The gauge always shows a very slight charge when it's running. The radio has one wire run directly to the fuse panel for the power feed and has a inline fuse on that wire which doesn't blow out. After the fuse blows off the alternator the gauge keeps showing a major discharge and the radio will keep turning on and off if I turn on the turn signal. Any clues??

[GTJIM]

Somewhere in that mess of wires there is deteriorated insulation that is either allowing the "juice" from one circuit to leak into another or a short to the body of the car. A real bitch to find and can take lots of careful searching. With the fuses replaced disconnect the batter and connect the battery leads together (without the battery!). Now switch everything on into the condition that caused the fuse to blow and check out the circuits, one by one for continuity and shorts to ground. Good Luck!

HTH

[ronskydivepops]

Hey GTJim. This may be a dumb question, but that's how I get some good answers sometimes! Should I have the engine running and then disconnect the battery cables to check for shorts and continuity??

[ronskydivepops]

Here's another part of the mystery. This only happens when I have the headlights on. I can drive, use the signals and radio all day long with no problems. Could there be some problem with the micro-switches in the headlight circuits? I've heard all kinds of things about these!!

[Al Olmo]

I wonder if your headlight rely is malfunctioning? If you have an extra one it could be a good idea to replace it.

[ronskydivepops]

How would I know if the headlight relay is at fault? When I flip them open they turn on fine, and the high and low beams work fine, as does the high beam indicator on the dash!!

[GTJIM]

If you have the engine running then disconnect the battery .... the engine won't be running! The idea is to try and have the electrical circuits in the same switched on condition in which the fuse blows - without the fuse blowing - hence the disconnected battery before switching everything into that condition. The leads are connected without the battery to complete the circuits so they can be checked. The key should be switched on to the running position for instance - but since there is no battery in the circuits the fuse won't blow and you will be able to check out the circuits with a ohm meter.

[yellaopelgt]

continuity... then try switching things off and on ,high beam low beam ect... to see witch circuit is the culprit. my guess its in the headlights somewhere. as you said it only does it when they are on.at least that is where I would start...

[jlthunder]

OK, Here are my 0.02. It sounds like you have a poor wire somewhere. What I believe is happening is when you have a load(everything on including lights) on the system, the radio goes on and off due to low voltage. Lights can function with low voltage, although not as bright, but an electronic piece of equipment needs a specific voltage. If it is not met, then it won't work.

One suggestion is to check your voltage at the alt and another place, like the cigarette lighter. When the car is running, you should get 13-14 volts at both places. The cigarette lighter might be a little lower. Start turning on appliances, turn signal, wipers, lights, etc and watch the voltage. If you drop to 12 volts at both places, your alt can't keep up. If the alt stays within spec but the volts drop at the cigarette lighter, you have exceeded the capacity of the wireing harness at one point.

Remember, volts x amps = watts.(Correct me if I'm wrong) If the voltage drops, the amperage increases to maintain the watts which can cause a fuse to blow. If the harness was rewired with too thin a wire, this can happen. The wire can also heat up and melt or worse, catch fire

Keeps us in the loop.

Jeff

[yellaopelgt]

what size alt. are you running?

[Bo Mows]

What i would do in a case like this is to eliminate the possiblity of a ground. Disconnect the Neg lead on the battery, reproduce the condition when the fuses blow and touch a 12 volt test light between the Neg cable and the Neg post on the battery. The car not running. If it glows, there is a short (ground). Begin turning off the sources that are on and see if the light goes out. If so, there's where you should start. If it does not go out, then you have a soild ground in the wiring and I would do the continuity check with the battery cables attached to each other. (Off the battery of course). If it does not glow then i would put money on the low votage output.

Bo

[guyopel]

With the radio going on and off with turn signals indicates a back feed from a high reistance in the ground circuit . That is a majior clue in finding your problems .. HTH Yours is not a simple problem and is hard to diagnose with a test light .Your best tool would be a ohm meter ..

[namba209]

Ron, first off, check to see which fuse lug the radio is powered by. If it's one of the three next to the lugs in the panel, those are the light circuits. if it's on one of the four on the other side of the lugs, then it's on the accessory circuits. It appears you have bad insulation on one of the turn signal wires and it is shorting out against worn insulation on the headlight circuits. Not too uncommon in the front harness. Another area to check is the major ground points for the front harness in front of the radiator support hiding outboard of the brake MC. Two screws in the sheetmetal for the all the front lite grounds are there. Check for cleanliness and tightness. HTH.