Battery drain

The battery drain on my '88 S4 5 Speed with factory alarm follows.
All doors closed, key out, alarm off = 49ma.
All doors closed, key out, alarm on = 48ma.
Doors open, all interior lights off = ~500ma.
I don't exactly remember the drain with the key in the ignition and doors closed but I think it was ~500ma. For some reason the factory alarm makes the drain 1ma less when on. As Ed says YMMV. Yes, my alarm works OK. A battery lasts longer when kept fully charged so I recommend a battery maintainer if you are going to let the car sit for a week or more. Don't forget to take the key out of the ignition.

Earl Gillstrom '88 S4 5 Speed


ok guys, jumped the negative battery cable, and I'm showing 4.96 amps. draw on the battery. That's wrong right? Shouldn't it be 2.20 amps or so. Anyway, pulled every fuse and every relay with no change. Am I doing this wrong ? Please help my fingers are sore as hell. There is no power through the fuses when the key is off, how would this show a difference in draw when I pull the correct fuse. Help guys, mechanical I am, electrical I'm not...


A typical battery can furnish perhaps 30 to 40 Amp/hours before being too low to crank the engine.
30/0.11=273 273/24=11.4
So your car should be good for about 11 days. Two days is VERY short.
A more normal and desirable drain is around 20 - 30 mA, which should make a good battery last for a month or more.

Some possibilities:
- Battery is failing.
- Bad connection on a battery or ground cable.
- The battery isn't being fully charged. (I just found that one of my battery maintainers no longer fully charges a battery.)
- The drain is higher than 110 mA. (Not likely, since I know that you are competent in technical matters.) Measuring between the ground cable and the chassis should measure total power output.
- There is an intermittent drain that you didn't catch.

Remember that some audio gear has two power inputs - one from the 30 bus (constant battery power) and one from the 15 bus (ignition switched). The 30 bus power is the culprit in battery drain-down problems.

Also remember that quite a few 928 electrical devices aren't fused.

My first guess is that the battery has reduced capacity.

Wally Plumley
928 Specialists


 From the 928 Technical Bulletins:

When checking constant current draw on a battery, proceed as follows:
1. Remove ignition key from ignition switch.
2. Open rear hatch and disconnect two-pin connector for rear hatch switch.
3. Lock doors.
4. Disconnect battery ground cable from body.
5. Connect ammeter between ground cable and body. Reading: After approximately 15 seconds maximum 30 milliamps.

Important: Several components such as temperature sensors continue to draw current after engine is shut off. Therefore, this test should only be performed after engine Is turned off for at least 45 minutes.

Wally Plumley
928 Specialists



The current draw from the battery at rest (i.e., nothing on, no doors open, etc.) should be milliamps, not amps. Hopefully, this is just a misunderstanding of the meter settings....

It is critical that the test be made with no door open, and the hatch firmly shut. This means that you have to run the meter wires out the hatch, look thru the rear glass, or be inside the car. You also must wait long enough to ensure that the timed interior lights are off.

Some of the fuses have power all the time, some only when the ignition is on, some only when the lights are on, etc. Possible sources of current draw include:
- Interior lights on due to a faulty door/hatch switch. The interior lights should come on when a door opens and go off when, or shortly after, it closes. If this happens, the interior lights are usually not the problem.
- Audio amp that is on all the time - an incorrectly wired aftermarket unit.
- Improperly installed aftermarket alarm system.
- Faulty diodes in the alternator. You check this by checking the current draw with the alternator wires connected and disconnected. CAUTION - the red wires are hooked directly to the battery.
- Glove box light always on.

There are others, but this will be a start.

Wally Plumley
928 Specialists


It shouldn't be anywhere near that. The reading should be under 100 milliamps, at best. You're not doing this with the ignition turned on, are you? The key shouldn't even be in the ignition when you're doing the test, and the doors and hatch must be closed.




First, thank you for the input and ideas. I've made some measurements and I need to understand the wire routing from the battery and from the jump post to make some sense of them.

First, when I got my meter reading on a 'finer' scale, I read 135 ma w everything in place and all doors/hatch etc closed. I can see a significant increase if I open the door or the glove box. The battery was only at 12.7 volts so that's another issue I've got to deal with (new battery most likely or new charger).

My '88 5spd is configured as:

Jump Post: has one heavy line in, connecting to two 'connectors' w 2 red wires and one w 1 red wire.

Battery Pos Post: has one heavy line, two 'connectors' w 2 red wires each, and one I added for the stereo amp/bass unit.

-I was able to verify that the stereo amp/bass is drawing change when it was disconnected

-I've been told that the heavy line from the battery goes to the starter, then the alternator, then to the jump post. With only the heavy line on the battery and all lines on the jump post, I draw 114 ma....remove the 2 smaller connectors at the jump post and this goes to 0. So this would seem to say that the alternator diodes are not leaking and that all current is flowing to the circuits fed by the 2 red connectors. These seem to go to the same place since the current is the same w either/both hooked up. (told it's the fuse panel...anyone know if these feed anything else in addition?).

-the 2 connectors at the battery (w 2 red wires each) seem to draw around 70 ma. These wires go into a 'sheath' that disappears under the passenger side rear panel...anyone know what they feed?

I still need to make some more detailed measurements on a per fuse basis and pull the head unit schematic out to be absolutely certain I've disconnected it. The XM tuner only draws about 3 ma so that's minimal. As Earl mentioned earlier, the car used to draw 45 ma. Since then I've put in the new stereo system and nothing else electrically.. wish I'd measured the drain after installation so I'd know what is was then.

The measurements are a bit confusing and I suspect some feed through/ "sneak paths" since the currents don't add up out of the wires at the battery positive.

That is: everything connected 135ma heavy cable only at battery positive, all on jump post 114 ma only 2 smaller connectors at battery (no heavy cable), all OR only heavy on jump post 70 ma

So, does anyone know the routing and/or what's fed from:
the 2 connectors w 2 reds each on the battery positive?
the 2 connectors (one w 1 red, one w 2 reds) on the jump post ?

Thanks again for all your help. I've pretty much concluded I need a new battery but I'd like to understand this higher current drain.




Go to Page 97-301 in the Factory Workshop Manual.

In Zone C82, you will see:
- The 35.0 black battery cable from the battery positive to the starter 30 terminal. (Numbers with wires are cross-section in square mm.)
- The 6.0 and 4.0 red wires running from the battery positive to U11 and U12 to feed the injection, ignition and fuel pump relays.
- The 4.0 red wires that go to Zones (not terminals) C35 and M33. If you go to Zone C35 on Page 97-291, you won't find the other end of the wire! Oops!
They made a mistake - look in Zone N34 on the right side of the page, and you will find the red 4.0 wire feeding the mis-named "blower final stage" - oops!. They mean "cooling fan final stage". Apparently, Hans had had a brewski for lunch that day. If you look at Zone M33, you will find the other 4.0 red wire, feeding the other fan thru the "blower final stage".

Back to page 97-301, this time to Zone B84, where you will see the starter 30 terminal, with:
- The 35.0 black battery cable.
- A 16.0 red cable running to the alternator output terminal (B+). From the  B+ alternator terminal, a 16.0 red wire runs to the "wire connector", B+ which is apparently the Jump Start Terminal. From this wire connector, three 4.0 red wires run to the 30 terminals on top of the Central Electric Panel.
- A 10.0 red wire running to Zone M76. In Zone M76 you will find the 10.0 red wire running to the connector at the ABS unit in the left front fender.

Hope that this helps!

Of course, it doesn't narrow your search too much. It means that the drain is on the 30 bus in the Central Electric Panel, which basically means anything that is constantly-powered...

One interesting thing is that (if my quick 'n' dirty conversion is correct) 16.0 wire is just a bit heavier than 12 gage in AWG sizes. That means that when we jump-start the 928 using the Jump Start Terminal, we are using a 12-gage wire that runs from the terminal to the alternator, and from there to the starter. Pretty small, eh?

Have fun, Jim!

Wally Plumley
928 Specialists

Hello all,

I know that the 928 has a high, let's call it, 'idle' - battery drain, but what is normal for a 91 S4?

I live in The Netherlands, where the climate can be described as rain almost every week, all year long, with a few dry periods a month. Not so nice, but the winters are therefore not that strong either. So usually I can drive the 928 at least once a month, even in winter periods. Usually a lot more. (I do try to prevent driving it in those rains.) So I have not the problems some of the winter-storage US-owners have.

However, the last 6 weeks I've been busy and it was relatively unusually snowy. Not so bad you couldn't drive, but since the government started doing some serious over-the top spraying of salt, I kept the car in.

Yesterday I wanted to start, and although the battery wasn't completely 'dead' (so no no automatic configuring the car was done/needed, as when the battery is really exchanged), it was too empty to start. Is it normal this already happens in 6 weeks?

The battery itself is less than a year old, and the build-in indicator only indicated I should charge it. And when I then took it for a spin on the German AutoBahn, it still ran as should.




Hi Armin,
since I'm living in the Netherlands I think its best that I respond.
Obviously conditions are very similar.
I have a 928s4 with aftermarket alarm and radio. In normal use I drive the car at least once a week, summer or winter time, but normally a few times a week.

Drain should be about 20-30mA, and I seem to recall something like that, maybe even 50mA. One time my car was almost without power, and I found the car clicking relays but nothing close to starting. It was on a parking yard at the A'dam airport, where it has been for about 3 weeks. The only explanation I had was that the alarm system has "false alarmed" very frequently causing the power to drop too quickly.

The battery recharged and ran fine from then onwards. Still use that one 2 years later.
5 years ago I replaced the 1 year old Varta battery which was suddenly completely dead by a Berga 65AH. A 65AH should be able to hold about 4 weeks or so based on 50mA drain. It needs 30AH to be able to start the engine still, so 35AH/0.05A/24h=29 days. 6 weeks would require a battery maintainer or recharging.

Another thing to keep in mind is that the lights normally do not switch off or sound alarms in a 928 when they are left on. I installed the 928 chime unit which provides the convenient "lights on alarm"

So its time to check the drain and prepare to recharge the battery when not using the car for 6 weeks. Always disconnect the battery if you're using a powerful fast-charger (or even with any charger!).

Hope that helps.
Kind regards
1988 928s4 cherry red
The Netherlands (Venlo)


Actually, the "normal" idle battery drain isn't really high on a 928. Even the later cars, with much more electronics, should have a normal drain of 20 - 40 mA (key removed, after several minutes).

Batteries vary greatly in ampacity, but even an old or slightly under-sized battery in reasonable condition should be able to crank most 928s after furnishing twenty Amp/hours. This means that under worst-case conditions (40 mA drain, twenty A/H ampacity) for a normal car, it should crank after three weeks.

Best real-world conditions (20mA drain, forty A/H ampacity) would be twelve weeks.

There are many things that can impact these numbers. Some of these include:
- Aftermarket alarm and stereo installations.
- Tired battery
- Dirty battery connections, including things like the engine-to-chassis ground cable.
- A battery that isn't fully charged as a result of short-trip driving, especially with lights, wipers, HVAC on.
- Poor connections in the electrical system.
- A tired starter.

Six weeks is a long time - have you considered that it may be that your car is punishing you for not driving more often?

A battery maintainer (not a trickle charger) is one possible answer, if the car is parked in a secure location with mains power.

Wally Plumley
928 Specialists


Many of the electrical devices and circuits on the 928 are not fused, so pulling fuses doesn't get everything.

Some common drains include:
Interior light circuit - it is common to find the interior lights all turned off, but you can't turn off the red lights on the end of the door.
Be very suspicious if the interior lights don't work normally - that is, on when a door is opened, off when the door is closed.
Aftermarket stereo systems, especially with a powered amp.
Aftermarket alarm systems.
Glove box light.
Stuck relay
Worn ignition switch or switch link.
Bad diodes in the alternator.
Faulty wire harness, especially the ones across the front of the car and at the jump start terminal.

To check the alternator, hook an ammeter in series with the battery ground and check the drain (better if the doors and hatch are closed). Disconnect the alternator (all wires) and see if the drain changes. WARNING!!! The heavy wires on the alternator are hooked directly to the battery positive!
Do NOT let them short to ground!!

The engine to chassis ground is on the bottom of the engine, right side. A faulty ground can cause several problems, but not a current drain.

Wally Plumley
928 Specialists
30,000 parts online!


German outline from the manual: