Random battery discharge

davidthompson@p · 12-18-2007, 07:33 AM

The battery on my '86 3.2 SC (74 RS lookalike) will sometimes discharge completely in 3 or 4 hours. It will be fine for 3 or 4 weeks then I get in, try and start then find the battery dead.
Is this likely to be the alternator? The car has covered 161000 miles.

Any help appreciated!

Bench-Racing · 12-18-2007, 09:53 AM

This sort of mystery culprit is sometimes the underhood (front bonnet) dome light. Check switch, hood striker, and plunger alignment. Might be faulty or out of alignment so it turns on intermittently even when the hood is shut. Just a thought.
Regards,
Charles

Zeke · 10:29 AM

There was a recent thread about discharge. Look for posts by "Will Ferch." One interior type lamps shouldn't do a complete discharge in 3 hours to my way on thinking. Stranger things have happened, though.

Here is the thread.

davidthompson@p · 12-18-2007, 11:47 PM

Thanks for these replies. There are no interior lights, or obvious other potential opportunities for discharge - 250kgs were stripped out when the car was converted - and the last time this happened, the car started instantly after a two week rest, then I drove it for about 40 miles, left it outside my garage for a couple of hours to cool then when I went to put it away, it was dead. I charged it overnight and it's fine. Could this simply be that the alternator is not charging? Could it charge intermittently? Any ideas how I might check this?

MBEngineering · 12-19-2007, 09:27 AM

HI david.
which alternator/regulator have you on the car??, if you have no way of testing the charging rate run the car down to quick-fit or some locale garage/Porsche specialist to check it out for you. check the earth strap's on the battery and the gearbox for a good connection as the car looks very clean and if new paint is on there the earth points could have a bad connection on them?

regards mike

911pcars · 12-19-2007, 10:21 AM

Intermittent electrical issues can be problematic.

The easier part is determining whether the alternator or battery is the culprit. If it's either one, replace it (please confirm first). However, if there's a parasitic current drain in the system, you have to isolate the offending circuit, then inspect the complete path. If you don't have any interior lights or there are disconnected accessories, you may have an errant source voltage connector creating a partial short directly to ground or maybe there's a short through adjacent wires in the harness; not enough to blow a fuse, but enough current drain to discharge the battery.

If it's a parasitic drain, I suggest looking in the archives for the basic procedure to get you started.

In the meantime, install a cutoff switch on the battery so it doesn't discharge while it sits.

Sherwood