Charging system problem - Experts, help keep Porsche practical! (photo)

[Early_S_Man]

Andy,

I doubt if worn brushes is your problem ... never have seen brushes wear out before 50K miles!

Your 'inadequate output' problem sounds like one or more failed open rectifier diodes. An open-failure diode reduces output current by 1/3 ... I have seen two open failed diodes for two field windings/phases reduce output current capacity to 1/3 of its' rated capacity, and the truck's battery [Datsun 620] slowly went down over a period of weeks, ocassionally needing to be charged with external charger! A new diode array fixed the problem for around $15 in 1978!

[KobaltBlau]

Warren,

Is the diode array in the alternator even on the external regulator cars? Any idea if the diodes would be replaceable, I would bet that this is an SEV Marchal unit; it is certainly original.

Thanks!

[Grady Clay]

Andy,

Diodes are actually pretty rugged but three things will kill them;
heat, over current, and over voltage. Vibration can also be an
issue but I think a 911 engine would fall apart first.

Here are examples of the diodes.



They are incredibly inexpensive. You can probably get “automotive”
ones for next to nothing (Porsche speaking).
You can also find higher quality, larger current capacity and
slightly higher voltage rating. Warren may be able to spec
these.

Be aware they come in two different polarities; one where
the cathode is the body and one where the anode is the
body. IIRC a SEV Marchael/Motorola use both.

The body of the diode presses into the heat-sink. The
electrical current also flows through the heat-sink. A
socket works well to press the diode out and in the heat
sink. When pressing in, use a socket that just barely
fits through the hole in the heat sink – you want to press
only on the metal housing and not the glass seal. The
outer knurling on the diode body is 0.500” (English
measure, not metric; just like wheels and seat belt bolts).
If you find the diode not tight in the heat-sink, you will
probably need to replace the heat-sink. That interference
fit is necessary for heat transfer and electrical connection.

The Factory Workshop Manual Vol. II (pp. SL1-SL13) has
good testing and repair procedure for the SEV/Motorola
alternator. I’ll post that if we need.

Best,
Grady