944 Low Voltage Until You Rev to 2500 RPM

Another archive from an old Rennlist post, as this one keeps coming up again and again.

If your voltage stays low when you start the car (alternator not charging) and then jumps up to a normal charging voltage when you hit 2500 or 3000 RPMs, it’s the alarm.  Yes, the alarm.

Somewhere in Germany in the 80s, some engineer said to himself “ve shall make ze alarm control ze DME relay AND ze alternator field current.” No one knows why.  But it happened.  And if your alarm is broken or removed, you or someone before you probably only bridged the wires to allow the DME relay to be energized (pins 1 and 4 on the alarm box connector), started the car and said “good enough.”  Well, it wasn’t.  Now go bridge pins 7 and 8 and your alternator will get +12v to the blue D+ (field) wire when you start the car, which will allow the alternator to start working at low RPMs.  Without that, you need to get the RPMs up high enough to “auto excite” the alternator.  The better condition your alternator is in, the lower that number will be.

Here’s the diagrams to prove it:

You can see the have you bridge ping 1+4 and 8+7, same as what Clark’s Garage says, but their graphic must be an earlier version of the FSM, as it doesn’t include the “BRIDGE ONLY FOR VERSION WITHOUT ALARM SYSTEM”.

So, tracking down what the 4+1 bridge does, I found that pin 4 ends up at terminal 86 on the DME relay, and pin 1 ends up at terminal 15 on the ignition coil. So you’re going nowhere without this one.

Ping 8 traces down to A40 on the sheet to another sheet in location H43. Here’s what we’ve got there:

Pin 7 traces into the central electrics on another sheet, and then goes to yet another. I don’t think it’s necessary to trace this one any further, because it obviously is what feeds the field current once the car is done with the starting sequence.

To sum it up – you can bypass an alarm with 1+4 and have you car run, but you will have to rev it up to 3k RPM to start the alternator if you don’t have 8+7 bridged as well. Revving you car up to 3K RPM, while “normal” to many of you, is simply broken. Fix it or don’t fix it. It’s up to you. But make no mistake, it’s broken.

 

Note: if you have an early car, make sure your alternator light bulb in the dash works.  Porsche didn’t put a resistor in parallel with it until 85.5, so a blown bulb in an early car will cause this as well.

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