Help! I think I fried something!

[jbscheff]

I was reconnecting the positive cable to the battery and accidently touched the wrench to the body (stupid!). After the zap, nothing seems to work. I checked the fusebox in the front and by the engine and all look intact. Maybe since it's late at night, or because I'm frustrated, but I'm lost. Where should I start?

The car is a 1973T with an '81 3.0.

[RWebb]

disconnect the battery and go to bed

write down if there was a big spark for tomorrow's review

write down if the neg. cable was connected at the time

[maxman]

You might have killed the diodes in the alternator.

So you're saying with the car turned off, nothing works?

Do you own a voltage meter?

We'll get you through this, but your might need you're alternator replaced/repaired.

[jbscheff]

I've got a voltmeter. Battery still shows 12 volts. Negative cable was already connected, guess I did this backwards. There was a spark on contact and a pop. Clock was running until the "incident", then stopped. No radio, no lights, no ignition when turning the key.

[maxman]

How do the fuses look? It's pretty easy to see visually if they are blown.

[Walt Fricke]

What you did is not uncommon. Usually all that happens is some of the lead on the positive terminal lug end gets melted off.

The big battery cable goes back to the starter. The other, smaller, cables which are also attached into the terminal lug go to to various places (ignition switch being one), and then get fused (usually) to run things.

There is no reason to expect that this damaged anything. Except, of course, for the odd coincidence that nothing works. But this short circuit, which drew a lot of amps quickly, did not draw any of those electrons through the working parts of the electrical system. It all went from battery to ground.

With key on, check to see if you have voltage to the cigarette lighter. And check at the fuse box. But the common points here are the battery ground, and the lower amperage wires which connect to the chassis.

You can test both ends of this equation with your ohmmeter. My guess is that the heat of this incident introduced some resistance in one or both of these. Easy to remove the ground strap at both of its ends, clean things up, and reinstall. If that doesn't help, see about the wires going into the positive battery terminal lug. Squeeze the lug with pliers to see if that changes anything. See if you have 12V in (both?) these wires. If need be, poke the voltmeter probe through the insulation just beyond the terminal.

These cars were not built with a "fusible link" as a sort of one time, high amp fuse.

All after you get sleep.

[Eddyvdw]

It's morning here in Europe so I'll throw in some fresh thinking.

Basically you accidentally reversed the current. This means that the current first goes to all the electrical stuff before reaching the fuses. The only thing what comes to mind which can't handle a reversed current are diodes (or semi conductors, not in a MT73 though). There are diodes in the alternator and the voltage regulator (I think...).

Good luck!

Ed

[Walt Fricke]

He didn't hook the battery up backward, so (especially for things whose circuits were not switched on, and were fuse protected anyway) how did this reverse the current? Ground didn't drop below ground, did it?

I suppose this could have caused a current spike, which acts like a reverse current somehow? Alternator diodes could be affected, and the voltage regulators are all solid state for a '80 motor, and conceivably that stuff could be affected. In racing we all use (because the kits have instructions) a high wattage resister to ground the alternator when the emergency kill switch is turned off with the motor running. This to protect the alternator diodes.

But no effect on these diodes should disable the headlights and so on. Or the starter. The car will start and run just fine with a bad VR - until the battery dies, of course. Had that happen.

[Eddyvdw]

Good point. My line of thinking was that If you put the + on the body and the current finds a ground somewhere, the current would be reverse.

[69911e]

Are you running 2 batteries or have things been changed?
I would first examine the ground cable as this is what is most stressed shorting the pos terminal to chassis ground. If 2 batteries, you have 2 gnd cables and the right side battery pos has a jumper to the left battery by the fuse box.
Measure the battery terminal gnd (not on clamp) to chassis voltage differential. It should be zero. If not zero= bad gnd.

[john walker's workshop]

you may have damaged the battery internally. try another battery.

[maxman]

Originally Posted by Walt Fricke

He didn't hook the battery up backward, so (especially for things whose circuits were not switched on, and were fuse protected anyway) how did this reverse the current? Ground didn't drop below ground, did it?

I suppose this could have caused a current spike, which acts like a reverse current somehow? Alternator diodes could be affected, and the voltage regulators are all solid state for a '80 motor, and conceivably that stuff could be affected. In racing we all use (because the kits have instructions) a high wattage resister to ground the alternator when the emergency kill switch is turned off with the motor running. This to protect the alternator diodes.

But no effect on these diodes should disable the headlights and so on. Or the starter. The car will start and run just fine with a bad VR - until the battery dies, of course. Had that happen.

I was thinking about that last night too. A wrench between the + terminal the frame is just a big load. The wrench turns into a heater. It should not have reversed the current load. If anything it should have created a huge voltage drop, not a spike?

I looked through the '73 diagram last night, and there is no master fuse of any sort. I'm thinking damaged battery terminal?

[Joe Bob]

Test the battery, charge it. Put a meter on the ground strap, see if you have continuity. Check condition of inside of the terminals. Clean terminals.

Try again. Connect + first, - last. I use marine style wing-nut connectors. The Optima I use has dual connectors allowing wing-nuts.

[RWebb]

as above - take a look at all the fuses tho just to be sure

you can also connect batteries able to supply on low amps to test things - I have use those 9 V batteries (like in a smoke alarm) to go thru & test stuff - lamp bulbs will glow but be dim

[jbscheff]

thanks for the help. I cleaned up the connections and especially the ground connection. Everything works now, whew!
This just shows the importance of connecting things in the right sequence.

[Traveller]

Originally Posted by jbscheff

This just shows the importance of connecting things in the right sequence.

Negative OFF first and negative ON last.

You can't short things that way. Oh boy...

[Joe Bob]

Gud fer ya, Bubba!