brianswyatt

My battery has been draining to zero after just a few days of not driving. I put the car 1990 c4 cab, on a battery tender but don't like the idea that my car is not behaving like it should and I would like to be able to take it for longer trips and not worry about it being dead. I hooked up a multimeter today after work and this is what I found. After unhooking the negative terminal and putting the leads of the multimeter on the loop for the battery and one on the battery terminal it was reading 20.6 (amps if I am reading it correctly which would explain the battery). Using 20.6 as a base, I started pulling the under hood fuses. I noticed no drops on most fuses but to my surprise, I had drops on 6 different fuses. I am going to list the fuse and the drop. I then plugged the fuse in and brought the drain back to 20.6 and pulled the next one - recorded that drop, and so on. So all drops were from a base of 20.6.

fuse: 11 (Inside lights - clock) 6.9
fuse: 12 (Diagnosis central lock rear window washer) 19.1
fuse: 13 (Sunroof electrical top) 20.1
fuse: 16 (antilock system / limited slip diff) 19.8
fuse: 35 (DME) 19.3
fuse: 38 (Radio -booster) 19.2

The number at the end is what the amp draw went down to after pulling that fuse.

I stopped here before looking for interior fuses to pull as I thought something more may be at play to have so many losses over such a wide range of circuits. The car does have an aftermarket radio, but would this account for all the stuff? I hate to try to track all these down if the solution is something like a gauge that all these things runs through. Any advise on where to go next would really be helpful.

Jacke2c

I have been on here a while and the symptoms that you describe are those of the door switches not working when you open and close the doors. When you shut off the ignition after a drive the computer and several items remain "electrically hot" Once you open the door and the interior light comes on, it signals the system to turn off all power. Our door switches are notorious for being intermittent. A good cleaning of both door contacts fixed mine although you have to be careful taking them out since the factory did not leave a lot of slack wire to play with. I hope this helps.

Vegas993

An easy test for the door switches is to roll the window down while in the car. Shut off the ignition, get out and then reach in and try the window switch. It shouldn't work, if it does, your door switch is bad.

wallra

make sure your meter is on ma scale. you should see no more the 50ma. any more that that will kill the battery. how old is the battery. when I got my car it was going dead in a week and the po had put in a new autozone battery I changed the battery and never had a nother problem I let it set all winter with no charger and it starts right up. some times the tenders bowel the battery.

dlpalumbo

I believe you're reading mA. Your clock has most of the draw ~ 19 mA. I was having discharge in 2 weeks with around 17 mA draw. So your draw may not be unusual depending on accessories you have (eg radio). Your battery may be weak though.

My understanding is the clock is a nexus for a bunch of circuitry and has been the cause of many problems in the past. Search the forum. On my last visit to the shop, they removed a resistor (I think) from the clock to reduce the draw. IIRC they said they said there was circuitry for the cabriolet that was hot, but unused that was causing the draw.

I haven't yet found reference online to this particular fix.

Found it here, post #22. "Also the removing of the cab. top warning light in the clock if you don't have a cabriolet to lessen current draw."

Hope this helps.

Dan

JasonAndreas

If you don't have a cab then you can just pull the bulb for the warning lamp, see HERE. If you have a cabriolet then you need to disconnected terminal 30 and reconnect (the blue wire) onto terminal 15.

Vandit

Has your Porsche been consuming sushi containing uncooked salmon?


But for real, after you turn off the ignition, you CANNOT break the circuit on the battery terminal when testing for drain. Breaking the circuit kills any potential drains. I can keep the circuit closed by holding my multi-meter points strategically as I pull the terminal off the negative and use the multi-meter to keep the circuit intact.

Also, are your dome lights coming on when you open the door after you turn off the ignition? If they're not, and you can still use the power windows after exiting the shut down car, then your windows are killing your battery as a result of sticking door switches.

ToreB

Measuring the current in a 964 or 993 needs a bit of preparation.
Normal current draw in a 993 with ignition off is 25 to about 50 mA. (0.025A) An aftermarket alarm can introduce an additional 5-20mA. Theoretically, a fresh 75Ah battery would keep a 25mA current alive for about 100 days. Taking into account the needed power for cranking the engine, self-discharge and battery age, one could expect the battery to support at least a month of standing still and locked.

Normal cuplrits:
Door switches. Turning off the interior lights masks this fault. The door switches are to connect to ground when the doors are open, and are often corroded. The switches are connected to the lock/alarm control unit, and do not only control the interior lights, but also other functions such as keeping the ability to operate the power windows and sun roof.

The Climate Control Unit (CCU) is supposed to stay on for up to 20 minutes after ignition off. Sometimes the internal power circuit fails to turn off the unit, causing a drain of about 200mA.

Sometimes the On-Board-Computer in the tachometer (if fitted) does not turn off when the doors are locked. This will cause a drain of about 50-100mA.

Test current draw like this:
Use an amperemeter that can withstand about 10A. (in case the interior fans start at ignition on)
Cold engine and CCU A/C off, minimum temp and fan speed 0. (no cooling/interior fans shall run, and possibly damage the amperemeter with over 25A current)
Open bonnet and depress lock mechanism to turn off lights. Put a rag over the lock to avoid slamming the bonnet into a closed lock. This applies also to the engine lid. Open a window to avoid problems getting into the car if any problems.
Close and lock doors.
Disconnect positive battery lead and connect an amperemeter in series.
The brief battery disconnect shall cause all units in the car to briefly wake up, but go to sleep.

After 5-10 seconds you should measure a current of about 25-50mA.
If not, pull fuses one by one to determine the circuit, and any problematic unit.

Turn on the ignition briefly for one second. Current draw should rise to 0.5-1A. Many units will briefly activate, but go to sleep again and reduce current to about 25mA again.
On the 964, the two interior fans never completely stop at ignition on, despite setting CCU fan speed to 0. This will cause a higher current draw at ignition on.

If the current does not go down to normal 25-50mA again, check the following:
-Try to operate the electric windows. If they work with doors closed and ignition off, one or both switches have failed.
-CCU active? The CCU fan on the back runs when it is active, but this can be masked by a worn-out and inoperative CCU fan.
-OBC backlight on? Could be difficult to spot if the display is not activated


Cheers,
Tore

Harry Apps

There is a TSB for diagnosing current draw; see here:

http://www.pcarworkshop.com/index.ph...C_Parts_Guides

Mechanical TSB #31 - note the clock mod for cabs.

brianswyatt

Thanks to all who assisted here. Turns out the battery had a dead cell. I had it tested not long ago and they said it was good. Keeping it on a tender, it always looked full. Took it in again, and the parts store said "yep, this thing is DEAD". Replaced it and whammo, good to go. Had a Porsche tech here in town also check to make sure the draw was in spec, and he confirmed I was good to go. Thanks for all of your help, as I now know much, much more about my car which I intend to have forever. Thanks again to the Rennlist community.