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parasitic draw

F

fommil

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Joined
25 Aug 2019
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7
Hi all, I have a Carrera coupe from 1994 (993). I got it 4 months ago. Loving it!

The battery has gone completely flat on me a couple of times in the last month. I suspect a parasitic draw.

I have investigated the power draw to be 0.14 amps while resting.

If I remove fuse 12 (emergency flashers, rear window wiper, onboard computer) it goes down to 0.1 amps.

What I wanted to ask was:

1. does 0.04 amps sounds reasonable for the onboard computer? (which is very minimal in this model)
2. does anyone have a clue where the 0.1 amps draw could be coming from? I tried every fuse in the fuse box, even the really big ones.

I'm investigating if there are more fuses, or unfused things.

There are a couple of things worth noting:

1. the alarm system was installed after sales by Porsche
2. a previous owner has installed a speaker upgrade. I can see a large board related to this in the passenger door cabinet, it reads "infinity tweeter" and is full of copper coils.
3. there was a black and red cable pair attached directly to the battery's positive pin. I am still investigating what it is (I suspect the speakers) but removing it made no change to the power draw.
4. https://www.bergvillfx.com/porsche-964-993-battery-current-measurements/ lists some known issues... the bonnet light seems to be disconnected and I can confirm that the glove box light goes off just before I close the glove box.
 
Hi

I cannot be of much help but I can tell you that these cars are really hard on batteries, and they should really be kept on a modern battery conditioner.

As far as the parasitic draw there should not be any wires attached directly to the battery. What makes you think that they are speaker wires? Disconnect them and see what happens.

Best of luck

Berni
 
berni29 said:
Hi

I cannot be of much help but I can tell you that these cars are really hard on batteries, and they should really be kept on a modern battery conditioner.

As far as the parasitic draw there should not be any wires attached directly to the battery. What makes you think that they are speaker wires? Disconnect them and see what happens.

Best of luck

Berni
Click to expand...

Personally unless it's a daily driver as above I'd just leave on a conditioner :thumb:
 
My carrera last two,three weeks before the battery is flat if I'm not using it the alarm/immobiliser runs it down, I either use a ctek type charger or take one of the leads off 👍👍
 
If the current draw is only 0.14 then a fully charged good condition battery should still start the car after 3 weeks of not being used .

In short you are about 0.11 of a discharge on what would be considered standard .

Unfortunately that fuse is more then just what you have mentioned with clock , tiptronic control unit and central locking .. all term 30 also off of that fuse .

I would start by having the battery and charging checked .. is the battery old .. do you get over 13 volts engine running all lights and electrical loads on when tested at the battery ... less than 13 and the alternator is very tired .

How often do you use it .. does it sit idle for long periods of time then a short journey .. etc .

Wires direct to the battery can be a ctek charger lead or in this case something aftermarket like a stereo system ... if no difference when disconnected then you can ignore these .

Glove box , bonnet lights tend to be 0.2 or higher .
 
Hi
Open your front bonnet and check the light is warm to the touch, with age the switch gets sticky and the light remains on taking out the battery...........just an idea
 
The normal current draw in the 964 and 993 is about 25mA. (0,025A)
You describe a typical scenario of failing door switches, (activated relay) or failing switch for the bonnet/engine compartment light. A bulb draw about 100mA.
Did you do the test as described on my current draw test page?
Cheers,
Tore
 
Thanks everybody for your hints and tips!

I have read the maintenance manual and discovered two more fuse /relay boxes that I need to check. FYI there is one on the left side of the engine, and another tucked away in the bonnet beside the ABS. The weather is bad today so I cannot investigate any further: I'll report back when I've found the rogue circuit.

@ToreB I haven't done the electric window test yet... I am going through all the fuses first. But if that comes up negative I'll try your full checklist (is it covering circuits that don't have fuses in the main box?). BTW I think your larger troubleshooting guide has diagnosed a wheezing sound in my dashboard, so thanks for that!
 
Another possibility is the battery in the siren that keeps the alarm blaring even if the main battery is disconnected. After 25 years this will be kaput so instead of charging, then cutting off draw when full, it continuously draws charge from the main battery. Solution - Replace siren unit or get a new alarm system.

Actually... just get a CTEK like everyone else says. Then you can sort out all the other problems at your leisure without getting a flat battery.

Happy Christmas!
 
Tobesetc said:
Another possibility is the battery in the siren
Click to expand...

That's a very plausible explanation!

The alarm was added as an after market upgrade, so it doesn't have a fuse box entry (hence I wasn't able to test for it with my multimeter). When the battery is disconnected, the siren stops, further evidence that there is no siren battery (or it is dead). I have still not figured out how the alarm system was installed, but I'd like a way to test this postulate.

There are two wires that are attached directly to the positive terminal. Further experimentation has led me to conclude that it is related to the anti-theft because the engine doesn't start up without them connected. The 100ma drain still happens even if they are disconnected. So maybe it's the immobiliser, and the siren is wired directly into the power supply somewhere else.
 
ToreB said:
Did you do the test as described on my current draw test page?
Click to expand...

I have done the checks and it has unfortunately not revealed any more information. Electric windows don't operate and there is no visible light in the dashboard (unless I turn on the engine).

Incase you missed it in my original message: I've already eliminated the possibility of anything that has a fuse or relay in the main / engine cabinet fuse boxes. I've checked the amp draw with each fuse and relay removed and the draw is still there. I suspect after-market upgrades (i.e. anti-theft and audio), or perhaps a component that is unfused or has a dedicated fuse.
 
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