I have a 1991 Mercury Cougar LS, V6 with about 110,000 miles on it. Recently, I've started having a problem with the battery going dead and requiring jump starts. I figured the problem was either in the battery or the alternator and since the battery was old and past time for a replacement, I went ahead and replaced it with a brand new, fully charged battery. The terminals were both cleaned to ensure good contact, etc.

The engine cranked much faster and better with the new battery installed and I figured I had the problem resolved. After the car sat a day and a half after having the new battery installed, the new battery was also completely dead however. It wouldn't even begin to crank the engine. Since the engine had run only a total of about 10 minutes after having the new battery installed, and since there were no lights, accessories, etc. that had been left turned on, I figured I had to have a dead short to ground somewhere in the electrical system (as opposed to having a bad alternator which wasn't charging, since the car hadn't run long enough for charging to be an issue).

I took an ohmmeter, disconnected the battery and put the ohmmeter across the disconnected hot car lead to the electrical system and the car ground. With the ohmmeter set to Rx1000 it swung to the pin on the right but with it set to Rx1, it showed only a few ohms and didn't go all the way to the pin. To me, this didn't indicate a dead short as I expected and I seem to recall hearing somewhere that it is normal for a car electrical system to show a very slight draw on the battery even when inoperative.

I had a similar problem a few years ago and the problem turned out to be a dead short to ground in the passive seat belt system. That's disconnected now so I know it isn't the problem.

I thought the alternator might be a likely culprit this time but the ohmmeter showed no difference in readings with the alternator unplugged. A test of each alternator lead to ground didn't show any dead shorts either.

I went ahead and did my ohmmeter test with each of the fusible links, circuit breakers and small fuses disconnected one by one. This is how I found the seat belt system short a few years ago. I never saw any difference in the ohmmeter readings this time with any of these items removed so I am totally stumped. The only fusible link I couldn't disconnect was the 80 amp one for the ignition system which I feel is not likely the problem anyway.

Does anyone have any ideas on what might be going on here? If I have a dead short to ground, why isn't it showing up on my ohmmeter tests? And, if I don't have a dead short, what could be discharging my battery?

Thanks in advance.

 

Here's how to diagnose a battery drain

Battery drain