I use a trickle charger that's hooked up all the time. But Wally is right, if you pull the interior light relay, the battery will last a lot longer. In my case about 4 weeks instead of 2. I have given up chasing the current drains. I have had just about every relay pulled and still had a drain. I had the same problem with my '83S and with my '90GT. I did find once, that my rear hatch light switch was continuously making a ground (it wasn't pushed down far enough) so I removed it and isolated the wire. I also once found a corroded door switch that was constantly grounded. That helped too, but the battery still drained.
I was able to get my drain down from 1/4 Amps which drains the battery in about 2 weeks, to 0.013 (about 1/8 Amp) which makes it last about 3-4 weeks .
There is always a 1/8 Amp drain, sometimes a little less, but it sure seems like this problem is inherent to the 928.

Peter deJong
Well, I drive the car about every week since I think one should enjoy and not just admire.
The battery easily holds 3 weeks (when I'm traveling) and I do not switch off anything. No maintainers. The battery is not a superb quality, is about 5 years old now and still thrust worthy ... I think. I think the magic is in regular charge and discharge. Keeps them healthy.

Sometimes the glove compartment light stays on due to switch problems. You might want to check that.
I now have 1/8 Amp drain. Can not get it any lower, but a PO installed a good Alpine stereo unit in mine. The wiring never impressed me. Which wire are you talking about? I like to check that.
It is often the alarm. Especially the trigger at the radio.
Not super-easy to explain which wire.
It is brown/yellow, and located to the right of the stereo. Rather thin wire. (At least on a '91)

It's a very classical problem with aftermarket stereos. Drains battery within a week or so.

Johnny Bilquist
Don't know the colors but am guessing Johnny is talking about the always-on wire which is intended to power the memory and clock in your radio. Radio shop monkeys often tie the radio power leads together to all the power leads from the car. As a result the "clock power" routes to many other things in the vehicle which are on the same circuit as the radio.

No no no. I'm not talking about anything of that sort. It's the alarm system. The wire is supposed to be isolated at all times. If the alarm system is active, and the wire becomes grounded, the alarm is triggered.
However, if it is grounded all the time, the alarm does not trigger, but it drains a lot of current.

On my '90 GT, it took a lot of testing to determine that the problem was a leaky component in the cooling fan control amp. I eventually got it down to about 20 mA, which is the normal level. This problem only applies to '87 - up cars, though.

Common problems on the earlier cars include:
- Aftermarket alarm systems.
- Aftermarket stereo systems.
- Malfunctioning interior light systems.
- Malfunctioning glovebox lights.
- Faulty diodes in the alternator.
- Faulty SCRs in the fan control module. (S4-up)
There are other possibilities, but these seem to be the most common.

Important questions are always, "Do your interior lights all work exactly as designed? That is, do they all go on when a door is opened, and go off when it is closed? Same with the hatch?"

Wally Plumley
928 Specialists