Electrical question - spark when negative lead connected

[submerge]

I pulled my fan and housing to replace it with a restored unit today and after I finished instillation when I reconnected the negative lead to the battery there was a fairly significant spark and all the electric went out. The leads also got warm, the battery was connected for about 1 minute before I pulled the negative off. I pulled the fan, removed alternator, bench tested the ohms and it was -.2 or .2 (i think that tells me the diodes or internal fuse is still good?) reinstalled, double checked all the connections and then checked again. Everything is connected correctly, exactly as it was. There are no blown fuses, nothing is melted, nothing smells burnt, the terminals on the battery are clean.

After I re-installed I still get the spark. I have an open circuit I guess? I would assume it has to do with removing and re-installation of the alternator, but it is exactly the same.

1977 911S - stock 2.7 - rebuild about 1500 miles ago - new wiring harness and alternator at that time.

How do i chase this down? Remove all the fuses one by one and connect the battery until the spark stops? Is there even a fuse to the alternator?

appreciate any thoughts!

thanks

[SC-targa]

Try the fuse for your clock. That draws a small amount of current

[Porboynz]

If the battery leads got warm you were pulling some serious current. Try wiring a 55W headlight bulb in series with the negative earth lead and see if it glows bright. If it does leave it connected and start disconnecting things, probably the alternator as that's where you were working. When the light goes out you found your short. You are sure the battery is correctly connected right? Earth strap to negative, never hurts to double check.

[T77911S]

the 55w is overkill.
just use a regular test light if you want to do that.
remove the power connection at the starter first if you suspect the alt, but tape the lead end so it wont short.
connect the TL between the batt and neg cable.
(first check the light from pos to neg on the battery, that is max brightness).
if not lit or dim, connect the starter cable.

I would also remove power to the fan you were working on. if you have a serious issue NOW, chances are you did something to cause the problem.

[submerge]

Small slice in the boot over the positive lead was allowing an arch to a rivet on the air deflector, which grounded it out. I put some electrical tap inside the rubber boot and a small piece over the top of the boot and everything came back to life. Fewww.

[Norm01]

Originally Posted by submerge

Small slice in the boot over the positive lead was allowing an arch to a rivet on the air deflector, which grounded it out. I put some electrical tap inside the rubber boot and a small piece over the top of the boot and everything came back to life. Fewww.

If I recall the leads are on studs to the alternator, I would loosen the nut holding the positive lead on and rotate it so the wire cannot rub against the rivet and wear through your tape.