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This is a long posting, sorry in advance and thanks for any help.

Would like some advice on possible battery drain and use of a battery disconnect switch to bypass the problem. I’m driving a 1990 740tic with 136K. Essentially problem-free until last fall when I had two no start instances. So I replaced the battery (AutoZone since the battery still had some warranty left on it). Everything seems OK until about a month ago. Again had a no start (engine would turn over, but no start). I normally use my 740 about 2-3 times a week but when I do it’s a 70-mile round trip to work. The car was in my garage (under house) for about a week when I tried to start it.

So I wondered if I had a battery drain. Using a Fluke digital voltmeter I measured 13 volts after using a battery charger (engine off). After 12 hrs, reading was 12.84 volts. After another 10 hrs, reading was 12.70 volts. After another 10 hrs, it was 12.64. After starting up, reading was 14.22 at idle and 13.76 with headlights and blower fan on hi.

But after sitting for about a week another no start. So I took it to my mechanic. He indicated that the charging system was OK and his test for voltage drain only indicated a 22 milli-amp at battery. When he started to isolate the drain by disconnecting fuses, he found that when he disconnected fuse #5 (interior, trunk lites, radio,...), the drain went down to 11 milli-amps. The only other fuses to have any effect on the initial drain but not to same degree was # 1 (fuel pump) and 2 (central locking, hazard flashers…).

Now he had the car in his lot for two nights and was able to start each morning (we’re just north of New York City, so each nite has been in the 20’s). So his feeling is that if I use the car everyday I should have a problem. And if I were to not use it for an extended period of time, I should pull the #5 fuse or disconnect the battery.

So with this all said, I’m thinking about getting a battery disconnect switch. I figure I can also use this as an extra bit of anti-thief prevention too. In looking thru an old J.C. Whitney catalog, I found a WirthCo Battery Disconnect Switch. It looks like a guillotine switch where the switch is clamped to a battery post and the cable is in turn clamped to the switch. Very simple concept, lift the blade of the switch to cut the current.

Any opinions on the battery drain and possible use of this type of disconnect switch. Any potential harm to electronics with the battery disconnected for possible 5-day periods. I’m hoping 1990 cars/bricks have relatively simple electronics compare to current cars.

thanks