Don Terrill’s Speed-Talk

I've apparently got a short somewhere. 6 months ago it would drain the battery dead in 3-4 days if it wasn't driven...it has progressed to killing a fully charged battery in less than 24 hours. It doesn't appear to be on a fused ckt...I pulled all the fuses, no change. Everything still works, lights, radio, etc. I have physically inspected all th wiring but am seeing nothing obvious. Nothing has changed on it in over a year...I barely drive it anymore. Any pointers to help isolate this?

Use a meter and measure the current draw.
Pulling fuses is "guesswork".

It might be just a dud battery.


If not then a good chance it's due to a modification.........like an aftermarket alarm.
A fault in an alternator can do it too but you'll need to measure the current draw first.


If you're not driving it every day, it's often best to disconnect the battery.

If you have a (let's say) 72 amp-hour battery that's getting drained in 24 hours, that's 3 amps. That's a lot of current but it's not enough to be a dead short that would blow a fuse.

The first dead-simple thing to do is, starting with the battery known to be charged, disconnect the negative terminal, come back in a couple of days, reconnect the negative terminal and see what happens. If the battery is discharging on its own without being connected to anything, you know what that means.

If it really is an electrical drain, disconnect the positive terminal and connect a multimeter set on measuring milliamps (current) between the positive terminal of the battery and the (disconnected) cable. This should be zero or a small number of milliamps if you have something like a clock, alarm system, radio, etc that is on constant power. If it is anywhere close to an amp, you indeed have a current drain.

Chances are, the cable on the positive side splits to go to several different places on the vehicle. If you know you have a current drain, check each one, as above, individually.

I hate electrical troubleshooting. If the root of the problem really is an intermittent short ... good luck.

We bought a new '77 AMC Matador, 4-dr car years ago for our family.

After several years it got to draining the battery.

I had to go to work out of town and told my wife to call the AMC place, get it towed in and fixed!

Anyway, I went to pick it up the next Sat. morning after I got back home.

I was talking to the Service Writer and the Mechanic that worked on it was standing right by us two.

I asked what was going on with it and the Mechanic popped up and said that back then, they were known for doing this but AMC didn't do anything about it. He went on to say that it had a faulty dizzy modular in it that caused it. Fixed it and it was a great old car!

One other thing was that the 258", 6-banger, would draw moisture so bad that the next morning on a dewy night, the block would be dripping wet!

My old '48 Dodge 5 pass coupe's flathead 6 did the same thing! At times with the plugs being on top of the flathead, I would have to take a rag and dry them off before it would start!! Boy, back then, the hippies loved that old coupe! it had a Hi-Drive 3-speed stick in it! A fluid coupling with a clutch assembly on it!

pdq67

A bad diode in the alternator will do it.

Another old trick is to disconnect the positive at the battery then check
between that lead and the negative post on the battery with an ohmeter.
Should be more than 250 ohms, if less, start disconnecting stuff until the
reading goes above 250.

Note that you are checking the entire wiring system without the battery.

Some newer stuff will screw you because the computer could turn on
loads at random or pre-selected times. That won't show up on a static
test as above.

Yet another old remedy is too look carefully at all electrical devices and if any
have "Lucas" on them, immediately beat it to death with a BFH.

You don't have a short, you have a draw. You would go about looking for a short by replacing the fuse that keeps blowing with a light bulb and seeing what you have to unplug or what wiring harness you have to move in order to make the light go out.

To add to peejay's solution , if you are working by yourself and unplugging ,wriggling and generally bothering the electrics from one end of the vehicle to the other , put a test light [multi meter]on a long length of two core and the other ends connected to the battery supply terminal and the main take off cable {if more than one, test each individually} this way you can take the light [multi meter] with you no matter where you are on the job .

Cheers.