My friend had a no-start (low battery) on his 86 F150 4x4 4.9 so he hooked jumper cables from his Crown Victoria and went inside the house for 30 min.
When he came back to the truck he realized he had hooked up the cables wrong. The Crown Vic battery and truck battery were both dead. And several wires were burned up on the truck. I was able to get his Crown Vic repaired by replacing a fuse and charging the battery.
I didn't have time to try diagnosing the truck that day. Since then, he taped up all the burnt wires and replace the alternator with a reman unit. But it still has a short to ground somewhere when he hooks up the battery cables.

I guess one thing I need is a wiring diagram for this circuit and of course any ideas on what circuit is likely to have a short to ground after having the jumper cables connected in reverse polarity.
I have not started trying to diagnose this yet but I'm about to get started now. I'll post any details I find.
Thanks for any help!
Mike

 

Mike, don't know where to even begin except to say a lot of damage may have been done that you will just have to work out one at a time.
The EEC may have been burnt and it could just keep going on and on.
Once a battery has been flattend like that it damages the plates so much that often the battery will never recover to full usefullness.
Also the diodes in the alternator will conduct and over heat.
Some one has to pay attention next time.
Lucky there was not a big bonn fire out of it and lose two vehichles.
Good luck.

 

That's what I told him. And the vehicles were right next to his house too. And he lives 17 miles from the nearest fire dept, so it could even have burned his house down.

 

Check the voltage on the truck's battery. Pay special attention to the polarity markings on the battery terminals and the ones on your meter. It's very possible to recharge a dead battery the wrong way, and then it's polarity is permanently reversed.

They have this problem on electric vehicles sometimes, where you have a whole bunch of batteries in series. If one has a problem and goes dead, the current passing through it from the other batteries will reverse it's polarity.

 

Never thought about that! I have seen this happen on emergency lighting battery banks though! You have to wonder how many repair guys know about that kind of trouble!

 

By the time I got a chance to work on it he already had someone tape up all the burned wires and install a new alternator. But it still acted like a dead short when the battery was connected. I tested battery polarity (which was still correct) but after charging the battery it load tested bad. I installed a known good battery and the starter engaged when I connected the cables. So I replaced the starter solenoid and it fired up and ran fine. Alternator is charging too. So he's happy for now but I told him not to count on those taped up wires for long. His income is a limited disability check so he said he would pay me to get all the wires replaced when he can afford it.

Anyway, till then, thanks for the help everyone!

 

Your REAL lucky! I've seen batteries explode from that and spray acid all over the place.

 

Yep, I've seen that too and learned to keep my face out of the way when connecting or disconnecting a battery, anytime. Thanks!