daveplump

For some time now I have had a big drain on the battery. I finally got around to measuring the drain by disconnecting the battery negative and connecting the ammeter in series from the cable to the body ground in the rear hatch (door open). The reading fluctuates between 20 and 45 milli-amps. your thoughts on this will be appreciated. The battery is a new Walmart H8 (very heavy!) shows 12.4 volts; car running shows 13.4 volts across the postive-negative posts.
Thanks, Dave

Mrmerlin

is the 13.4 at idle ?
what do you get at 1500 rpm?

if its not over 13.5 then you have a bad diode in the alternator and the battery will go dead slowly as it wont be getting enough juice to keep it charged.

For the big drain roll down the windows and close the doors make sure that rear red warning light work then look into the door to see if it goes out after the door closes.

Have you worked on any of the interior lights if so then you may have one mis wired and this can cause the problem as well as having a pin switch not working as it should remove the pinb switches and clean them and inspect the wires

Alan

20-45mA is fine - most 928's are in this range. It could be you are not measuring it correctly - sometimes a bigger drain goes away when you disconnect the ground strap.

Connect your ammeter between batt -ve and another ground point before you remove the ground strap - you may find a larger current - do this after running the car.

Alan

Podguy

There is no reason you can not get the battery drain down to nothing. My 85 sits for months and fires right up without a charge. Well it did until I dropped a bolt down into the oil pan.

First place to look if you do not have an add on alarm systems is the rear window and mirror heater relay. These can stick and cause a constant drain even with the key off. Next place is the fuse panel. Push on it when the amp meter is in place and see if the drain fluctuates. The fuse panel can have some mild cross connections that cause a drain.

After that it is an issue of elimination. Only eliminate one thing at a time. This should be understood but I am always surprised my the number of people who do not follow this simple rule. It is the same when trouble shooting - always go back to the last thing you did.

To eliminate circuits you can try pulling different relays. Relays can wear with time and set up a mild current drain. I had this happen with a BMW starter that drained the battery through the solenoid when it was in a particular position.

Before you begin clean all the grounds in the car. A bad ground can create a back current drain and cause some really weird behavior. On the 928 there are two circuits - one that coming from the starter with the main cable to the battery. This can be disconnected to eliminate a lot of looking. The second circuit is the main accessory circuit to the fuse panel. The power for this one varies with model years I think. On the older cars it is the red wire on the battery terminal. If this come loose the car will not start.

You might also consider removing the fuse panel and cleaning all the fuses and relays followed by some dielectric grease on the connectors. The plugs are color coded and there is a chart for fuses and relays for each year on Dave's website http://www.928gt.com/

What ever you do - do not pull the gray wires off the back. You will never get the board back together. However inspect all the connections carefully. If you see any that have turned green/blue remove the wires one at a time and clean them. I have never found anything readily available that will clean the connections chemically - but I have seen chemicals in industry that clean copper to a bright shine.

Good luck - with patience you can get the car to be stable electrically.

Alan

Your battery is probably about 70Ah capacity - at 45mA this wil give you 3 weeks before the battery is appoximaly 30% depleted - a quite acceptable range.

For you to notice an issue - I suspect it must be quite a lot worse than this.

This points to either a bigger drain than this - or the battery is not getting recharged propperly

Alan

Landseer

I've solved most current draws on the early cars via removing, polishing and remounting the door and hatch area switches for the interior lights.

The hatch unit is mounted deep under the female reciever. It is reported to short on the body and I think there was a tech service bulletin on it.

If the draw continues, and a factory alarm is fitted, I've found that the electrodes mounted to the backs of the lock cylinders have corroded or been compromised. If somebody foolishly adds graphite to the locks, for instance, it can eventually wind up fouling the circuit, which is not isolated from the lock tumblers very well. A good, fast way to make the draw go away is to bridge the alarm at the Z plug on the CE panel, for which simple instructions exist. I just started an 84 that sat with battery installed for 8 months. Alarm bridged, and in this case no radio connected, no draw whatsoever.

Of course other shorts are possible, especially the harness that handles the overhead and hatch and door lights, the little red door safety lights, as mentioned above. A short in a fixture, against the body, can burn that harness randomly from end to end, or actually, from fixture to panel. Ones i've seen like this show signs of melts at the CE panel, and you'll see that when removign for cleaning and fuse replacement.

Just trying to help here, each of us has some different perspective and has seen different things, what I'm talking about is what I've seen across 7 or 8 cars, others have a bigger data set.

Podguy

The engine wiring harness can become crusty as well. The one on Erik's Euro parts car was completely corroded. It was held together with corrosion. When it was moved the harness shorted and caught the wiring on fire.

Alan

MY POINT IS 4THAT 5mA IS NOT A BIG PROBLEM... however you slice it...

There is no way that if you have a clock, ECU memory, alarm and radio presets to retain that you van get to 0mA leakage - this is a fantasy unless you removed all this stuff... 15-20mA is about the best you will ever get on a S4 car - early cars will be quite similar.

There is either a bigger drain or a charging issue - get the right measurement that show specific problems first - or you are just guessing in the dark - this can be a lot of fun (?) but typically wastes a who lot of time too...

Do appropriate diagnostics first before you start on resolution...

Alan