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Makis Makis is offline
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Join Date: Mar 2002
Location: London, UK
Posts: 414
Hi,

A number of people have complained in the past that their porkers
radiator fans are on all the time. This happened to me today again. I
decided to get the books out and study this phenomenon and try to
undestand it once and for all.

Principle of operation of the Fans
The 944 post 85/2 is equiped with two Fans. The Fans are controlled
by a control unit that is capable to drive the fans at two speeds.

The control unit takes input from the following sources.
1. Two termo-switches monitoring the coolant temperature, one is
activated at 92C and the other one at 102C.
2. One AC Low Pressure or refrigerand temperature switch.
3. AC switch.
4. One Intake pipe cooling switch (16V models).


From the wiring diagram I can deduce the following action.
1. Thermo-Switch (92C) on - Forces low speed fan operation. Note,
both fans are on not just one.
2. Thermo-Switch (102C) on - Forces high speed fans operation.
3. AC Low Pressure switch on - Forces high speed fans operation.
4. AC switch on - Causes high speed fans operation (probably)
5. One Intake pipe cooling switch (16V models)on at 42C - same as AC
switch on.

Control Unit pinout
30M1 - 12V Fan1 power
30M2 - 12V Fan2 power
V1 - Slow Fan1 speed
V2 - Slow Fan2 Speed
M1 - Fast Fan1 Speed
M2 - Fast Fan1 Speed
TS - 92C Thermal Switch grounded on switch on
TF - 102C Thermal Switch grounded on switch on, also AC low pressure
switch grounded on switch on.
X - 12V Control unit power.
31 - Ground
AC - 12V if either AC switch or Intake pipe cooling switch (16V
models) is on.

Debugging procedure
I believe the fans should be on if TS or TF are grounded and AC is
set to 12V. So if the fans are on after switching engine off and
ignition on to the following:
1. Remove cotrol unit
2. TS grounded? If yes, low speed fans selected.
3. TF grounded? If yes, high speed fans selected. (note this
overwrites 2 above)
4. AC at 12? If yes, high speed fans selected.
5. If none of 2,3, 4 Fans should be stoped.
6. If 5 but fans running then control unit likely to be broken.

Hope this info helps if you are affected by this phenomenon.

I think in a hot summer day if the fans are running more than you
would expect blame porsche. But they may have a reason for doing so.
If on a cold winter day this is still the case then suspect something
wrong with your car.

Makis
Old 09-12-2002, 02:26 PM
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