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911 Carrera Battery Draw
by Steve Grosekemper
In these days of
computer controlled automobiles parasitic battery draws have
become an increasing concern. Parasitic battery draw is power
that is drawn from the battery when the car is sitting in
a non-operational state. These draws are usually due to memory
features in computers, electronic control devices, and after-market
consumer electronics. It has become commonplace for cars left
for extended periods of time to have dead batteries. This
is an understandable situation and one that can be avoided.
Charge the battery or drive the car!
One situation that can be very frustrating and difficult to repair is an intermittent battery draw. The car that has been starting flawlessly for months can, without any warning, be found to have a
dead battery. Of course, when you take it in to your mechanic, he tells you that there is nothing wrong with the car. This has been a popular story in our shop this year, and this is what we have
found:
Starting with 1987 911s, 1985 1/2 944s, and 1985 928s, Porsche made a wiring change to make your car more convenient to live with. They made the power window relay come on with the ignition key, just
like they had done since the introduction of that option. However, the simple operation of turning it off has changed. Where on previous models you simply turned the key off, now a signal from the
interior light ground switch turns the relay off. Here is the reason why: You have just pulled into your garage and turned the car off. You pull the keys out and notice the windows are still down, so
instead of reinserting the key, you just hit the window button, and up they go! Then as you open your door, the relay is switched off, and the draw from the relay is gone. This is a really convenient
feature, except when the interior light ground switch fails and leaves the relay on. With this relay on, your battery reserve life may be reduced from weeks to only hours.
Fortunately this problem is easily repaired and even more easily diagnosed. First, make sure your interior lights are in the "door on position" and open the door. Do this several times and make sure
the interior light comes on every time. Second, turn the car on and then off. Open the door while operating the window. The window should stop the instant you open the door, and the interior light
comes on.
If your system is not working properly,
the problem is most likely a bad or dirty door switch. These
switches are located at the front of the door frame, next
to the door limiting strap.
Due to their design the majority of switch failures will be in 911 cars. The other models use a self-cleaning pointed ground contact switch, which is considerably more reliable.
The most common cause for failure in 911 series cars will be a damaged or missing dust boot (901.615.616.20-$1.45). If these are damaged or missing, replace them now! Remove the switch by unscrewing
it with an 11mm socket. Clean the contacts with a wire brush or replace the switch (964.613.601.00-$3.45). This is a great place to use some electrical contact oil, as this failure is almost always
caused by corrosion from dirt and moisture.
After repair of the door contact switches, go back and retest the power window/interior light operation several times. With this repair complete, you will be able to start your car with confidence,
even after long periods of down time. Or you could just DRIVE IT!
GOOD LUCK!
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