Jfrahm

Hi all,
Sometimes I jump in my 1992 968 and find the battery dead. It seems to be random, although last time I did leave the keys in it. I had been messing with the alignment and had it up and down the ramps a number of times, and at some point I got distracted (and it snowed) so the car sat for a week or so.
The battery was replaced last summer after killing the previous battery due to numerous deep discharges.

I have not been able to detect a constant drain, it seems to be random. Most of the time the drain is reasonable and constant and the car can sit for several weeks and still start right up. Other times after a few days it's totally dead or nearly so.

These are my ideas at present:

Alternator? Can it draw excessive current in some rotational positions but not others? I changed the voltage regulator but I have not swapped alternators. I suppose a bad diode might only be in play in some rotor positions but this is speculation.

Delayed accessory power? Can this stick on? Can it be disabled?

Other ideas? I tracked down a glovebox light I think might have been coming on from time to time with the lid closed. Checked the trunk light, made sure it was off. Interior light is not staying on when this happens.

968gene

a failing radiator fan relay. after several years and several batteries I walked into my garage and although the car had not run in weeks, the cooling fans were running. after running for a few minutes they cycled off. I replaced the relay and never had to charge the battery again.

MB968

I don't drive my car very often, can sit for days in between. I ran into a no start due to low battery a time or two. Haven't found any issues. Bought a Battery Tender Jr. for less than $20. If I'm not going to drive it for a few days I just keep it plugged in. Seems like an easy way to insure I can drive it when I want. Also, one advantage for me, I keep the car locked. This way I don't have to worry about it starting.

If you have days in between driving your car, you may want to install a similar unit to keep the battery up.

Jfrahm

Yes, I use tender on on my 928 but the 968, I never really know when it'll be used again soon and when it's going to sit for a few days. The timeframe really does not require a tender IMO except for the real dead of winter and even then the battery should survive a few weeks. What bugs me is the unpredictable drain. It can sit for 2 weeks and be fine or sit for 1 week and be dead as a doornail (albeit rarely.)

The cooling fan relay is interesting, I have not heard the fan running and I think I might, the car is not too far from my kitchen, living room, etc. I can mark the fan and see if it turns by itself.

Thanks for the suggestions so far.
-Joel.

Eric_Oz_S2

Did you leave the hood open? The alarm has a constant drain, as does the light bulb under the hood (even if it is not on, but the hood is open). Disconnect the negative lead and measure the current through the lead. I tested mine and there is a noticeable difference in current with the hood open over closed.

The only way to test for sure is to monitor the current and disconnect one fuse at a time and monitor the current.

Jfrahm

Yes actually in this case the hood was open. The light was not on, but I will check on that current draw. Thanks!

Paul Waterloo

Clark's Garage describes the fuse method:

http://www.clarks-garage.com/shop-manual/elect-26.htm

Jfrahm

Thanks Paul,
I have been through that, my issue is that I have not been able to diagnose the problem when the drain is actually happening. 99% of the time, I have a normal drain but every now and then a large drain hammers the battery flat. Like if every now and then the delayed accessory power was staying energized, or one of the courtesy lights stayed on, or something like that. Most times I park it and it has a minimal drain of little consequence.

mbardeen

I'm chasing a similar situation on my car. My suspicions at the moment are either the cooling fans running when they're not supposed to, or the starter exciter wire grounding against a cable in the battery-starter-alternator wiring loom.

My cooling fan relay is new, so I've discounted that for the moment. I'm going replace the cables in the loom soon.

ernie9468

Same happen to me last year,very hard to detect, the problem was the fan relay.It will come on & off & unless you happen to walk on it when it's doing it it's almost impossible to tell the culprit.We took the relay apart clean all contact (this is nerve wrecking since you don't want to file those contact s to much) afterward we clean everything with (Quick dry electronic cleaner ) & redone some soldering which were bad in the rear of the relay,working just find up to now no more battery drain.Btw if you let it go it will get to the point where the fans will be on steadily,suggest you borrow a relay from someone & try it out first before you take yours apart.

ernie9468

Inside the the fan relay:front & rear

Jfrahm

I'm sure I have a spare fan relay and I will give this a try. I have not remembered to mark a fan blade to see if they are turning mysteriously.

Thanks,
-Joel.

ernie9468

Good idea this is the first thing I'll do mark a blade,like said very hard to trouble shoot since it's an on off situation.GL & take care.

Jfrahm

Updating this thread,
I have not had a random drain since this thread, but I have been careful not to leave the keys in the ignition. I did mark the fan blades and have not observed any fan movement with the car off.

I have not tested leaving the key in the ignition to see if there is a drain in that condition.

-Joel.

Der Treiber

Just throwing this out there, that the PO of my S2 complained that if he let it sit for more than a week or two, his battery was dead. I asked if he left it locked during that time, and the answer was "of course". But then a grin flashed as he realized the security system gets engaged when the door locks, and that might be what was draining it. I don't really know. Since I park it in a locked garage, I never lock the car, and have never had a dead battery, even after a month or two between drives. May have nothing to do w/ it, but...