Battery short

[69mi 10]

A while back... I drove to a friends house and was sitting in the car listening to the radio with the engine off. I started to mess with the light dimmer and lost all power. I put a volt meter to the bat and it was totally discharged. I swapped out the battery and when I connected the cables...bam... Full arc weld short! Shock and oww

 

I guessed it had something to do with the light switch and disconnected it and pulled the light switch off to look for burnt wires and terminals. Nothing looked smoked, so I left the light switch disconnected and reconnected the battery terminals. I got power back and got the car home. I carefully checked the light switch for signs of damage and found nothing, I put the switch back in after some troubleshooting and the car worked normally for the next year.

 

The other day I went to start the car, the battery was low and after the second crank attempt, I lost all power...this brought back bad memories of shock and oww. The battery was totally discharged again. I disconnected the light switch but still have a short when the cables are placed on the battery.

 

Now I'm thinking that it may be the ignition switch or the starter relay. With the battery disconnected I have continuity between positive side of starter relay and ground..I don't think it should be that way.

 

All original wire harness with a few mods... Bypassed resistor wire for pertronix, tapped into some blue black wires for power to radio and amp, tapped into ignition switch for alternate 12v power to instrument cluster circuit board.

 

Any ideas.....thanks

[Midlife 814]

You probably have an intermittent short caused by a power line's insulation being worn and touching the chassis. By fiddling with the wires on the first failure, you probably moved the wire enough so it didn't short out. I'd check the headlight harness starting at the starter solenoid and work your way under the radiator back to the firewall. I'd also look at the wires going to the alternator.

[69mi 10]

Thnx for the advice. I'll start there. Is there a way to check the short without connecting the battery cables and risking minor explosion?

[magician 13]

There's not a positive side to a relay. If your talking about where the battery cable connects to it, it should not ohm to ground. Replace solenoid first if it does.

[Midlife 814]

Thnx for the advice. I'll start there. Is there a way to check the short without connecting the battery cables and risking minor explosion?

Yes. Disconnect the negative battery cable from the battery. Now use a volt-ohm meter, using DC currents, set to 10 or 20 amps, and measure from the negative battery post to the negative battery cable. Normally, you should see 100 milliamps or so. A dead short will be more than 10 amps, so you may blow the fuse in the DVM.

[mustangmike6996 34]

Also,

 

WITH THE BATTERY DISCONNECTED, you can use the multimeter set on ohm/continuity/resistance and check positive cable to the engine ground. This is a fast check to see if there is a short but if its a raw wire in a small spot it might not be accurate. You can also check the voltage drops. I had a car do this to me from high resistance in a terminal.

 

With the battery connected and with the meter in Volts DC put one meter lead on the battery post and put one lead on the solenoid post, move to the other side of the soleniod then down to the starter. You should see 0.03V DC or less of voltage drop.

 

Do this with the negative side. same deal, one meter lead on the neg battery post and the other on the bare engine block. Should read 0.01v DC or less.

 

Essentially, what you are doing is seeing how much voltage is being consumed by the connections or cables. If you are higher than the limits that I mentioned, you can work your way around each component and see what is eating all of the voltage

 

As mentioned above... I had this happen to me. I had a 96 mustang GT that started fine one day then was no crank no start no interior power etc. I found that my positive terminal was dropping (consuming) 11 volts. replaced the terminal and cleaned the cable, never had an issue after that.

[mustangmike6996 34]

Did you ever get this issue figured out?

[69mi 10]

Not sure..the battery was low and The starter solenoid was stuck so I think that is what brought the battery to zero volts. I banged on the solenoid to disengage and everything seems normal again.

 

Afterwards, I did the checks as described above and those were within limits. So right now I don't think I can troubleshoot until the problems pops up again.

 

I talked to an auto electric dude and he speculated that the capacitors in my radio amp can cause mysterious electrical draws like that.

 

Anyway, thanks to all for the responses.