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Something that's been in the back of my mind for years that I've never been able to work out...
Like Clark's Garage says, the 2-pin switch in the radiator of the early model 944 triggers the low-speed (driver side) fan.
Turning on the AC puts both fans on high speed.
But...Clark's garage says (paraphrase) "if the radiator switch senses a high temperature condition, a contact closes bypassing the low speed resistor causing the driver side fan to come on high speed".
Now, I KNOW this is how the car works - my car with fully stock wiring configuration operates this way - at some point without the AC switch engaged, the fan goes to high speed.
But looking over the wiring diagrams there isn't anything that suggests HOW this works...There is only the two-pin thermo switch controlling the resistor side of the power to the DS fan for low speed. No connection showing a high speed activation route...
HOW does this work?
(my intent is to bypass the resistor so that the fans run high speed whenever they are needed for max cooling effect...will be 90-110 degrees around here for the next 3 months so the tiny 944 radiator can use all the help it can get...I just want to verify the actual function)
Spencer, you are correct. The only thing the two teminal radiator fan switch does is complete the ground circuit the first fan and grounds the cooling fan relay coil teminal 86. With the key on the cooling fan relay teminal 85 becomes hot. That cooling fan relay is a DPST. So whenever the key is on and radiator fan switch is calling for cooling, the coolant fan relay becomes energized closing the 30 and 87 teminals bypassing the resistor. The A and B teminals also close a sends power to the second fan. With the key off the cooling fan relay won't energize and power for the first fan has the resistor in the circuit for low speed. The second fan won't get power ether. If you think about it you could bypass the resistor altogether because it only used for radiator anti boilover after the car is turned off. So yes the early car is only high speed with the key on. I guess the computer of that era couldn't have fan control and this is what Porsche come up with???
For what it's worth, I installed a relay in place of my fan switch, triggered by my ECU. When the fans come on, they only run at high speed. That's when the relay basically acts as a short between the 2 fan switch terminals.
When I ran with the big Volvo fan (and later, the ebay fan I sold you, Michael) I had it hooked up with its own relay + battery wire, triggered by the high-speed fan "signal". This way the fan would come on when engine was hot enough, but it would NOT run with the radiator hot+key off.
Wiring the new way isn't an issue...
What I am interested in, is how the stock early 944 wiring turns on the high speed fan when AC switch is off.
I'd really have to dig into the wiring diagrams but I recall that the DME has a controller for the AC compressor clutch, maybe there is a connection somewhere that would allow the DME (seeing the engine coolant temp via the blue-top sensor) to energize the fan-controlling portion of the AC wiring?
What I am interested in, is how the stock early 944 wiring turns on the high speed fan when AC switch is off.
The cooling fan relay gets a ground signal from ether the radiator fan switch or the air conditioning relay. The cooling fan relay bypasses the resistor when energized.The cooling fan relay can only be energized with the key on and a ground signal.
The cooling fan relay gets a ground signal from ether the radiator fan switch or the air conditioning relay. The cooling fan relay bypasses the resistor when energized.The cooling fan relay can only be energized with the key on and a ground signal.
Marc, I know that - maybe you're not catching the full question.
What I am saying is...my driver side fan will go to high speed when the engine is very hot but AC is off.
This would be normal in a late model 944 with the 3-pin fan switch with both a high and low speed contact..but I have the early type 2-pin thermoswitch, so there is only one "on" position for that switch, for low speed.
It goes to low speed at about the first mark on the gauge when the thermoswitch in the radiator closes, like the diagrams show it should.
Going by the known wiring arrangement the driver side fan should NOT ever go high-speed unless commanded via the AC-relay "resistor bypass".
I have a few early car wiring diagrams. I'm going to study them tomorrow and see if I can get you a answer. Does your wiring diagram show the cooling fan relay teminal 86 connect to the radiator fan switch? Mine does. But if yours doesn't that would explain a lot.
Spencer, I'm not sure if this is what you are asking.
This how the fans run at high speed without the AC on.
The orange is ground signal to the cooling fan relay.
The green is the power to the cooling fan relay. It gets its power from the ignition switch key on.
When there's a ground signal (fan switch closed) and power (key on) to cooling fan relay the contacts close. The blue which is non switch battery power flow round the resistor allowing full voltage to the DS fan.
Also the second set of contacts in the cooling fan relay close turning on the PS fan
There's no low speed fan operation when the key is on. My 1983 operates in this exact way. Is the early 1985 different? I don't know. The only way the DS fan could run at low speed with the key on if the orange was omitted in the diagram below
I really don't know what else to say about this other that your car may have something different. Marc
In thinking about it some more and reviewing the diagram your explanation seems sound.
In further thinking, I think my DS fan bearing may be shot making it only look like low speed...
IN further review I believe the root of my confusion is due to Clark's Garage description lacking some clarity...the original wording made it seem like there was a secondary activation temperature for the thermoswitch to go to high speed.
As it is written:
Cooling Fan Operation of Pre-1985.5 944s with Air Conditioning
On early 944s equipped with air conditioning, there are two electric cooling fans. The driver's side cooling fan is supplied via a parallel circuit. One leg of the parallel circuit has a resistor which causes the fan to run in slow speed. The other leg of the parallel circuit consists of a contact in the cooling fan relay. The passenger's side fan is supplied via a contact in the cooling fan relay.
When the coolant temperature reaches a preset value (normally 92 °C or 198 °F), the temperature switch (normally referred to as the thermofan switch) closes and cause the driver's side fan to run. We'll refer to it as the primary fan. Since the resistor in series with the fan is still in the circuit at this point, the fan runs in slow speed. The fan will run as long the temperature remains above the setpoint, regardless of whether the ignition is turned on. This is a common problem area which will be discussed later.
When the ignition is on and the air conditioning is turned on, the air conditioning relay picks up. This causes the cooling fan relay to energize, closing two contacts. One contact causes the secondary cooling fan to run. The other is a contact that is in parallel with the "slow speed" resistor for the primary fan motor. This shorts out the resistor and causes the primary fan to run in fast speed. The second fan has no "slow speed" resistor so when it starts, it always runs in fast speed. Also, when the air conditioning is turned on (with the ignition on), the primary fan will run regardless of coolant temperature.
When the ignition is turned on, if a high temperature condition is sensed by the thermofan switch, a contact closes in the thermofan switch circuit which energizes the cooling fan relay and causes both fans to run in fast speed.
As it would be better written:
Cooling Fan Operation of Pre-1985.5 944s with Air Conditioning
On early 944s equipped with air conditioning, there are two electric cooling fans. The driver's side cooling fan is supplied via a parallel circuit. One leg of the parallel circuit has a resistor which causes the fan to run in slow speed. The other leg of the parallel circuit consists of a contact in the cooling fan relay. The passenger's side fan is supplied via a contact in the cooling fan relay.
When the coolant temperature reaches a preset value (normally 92 °C or 198 °F), the temperature switch (normally referred to as the thermofan switch) closes and cause the driver's side fan to run. We'll refer to it as the primary fan. The fan will run as long the temperature remains above the setpoint, regardless of whether the ignition is turned on. This is a common problem area which will be discussed later. When the switch is closed with the ignition is OFF, the fan runs in slow speed since the resistor in series with the fan is still in the circuit.
When the ignition is turned on and the coolant temperature reaches a preset value (normally 92 °C or 198 °F), the temperature switch (normally referred to as the thermofan switch) closes which energizes the cooling fan relay, bypassing the slow-speed resistor and causing both fans to run in fast speed.
When the ignition is on and the air conditioning is turned on, the air conditioning relay picks up. This causes the cooling fan relay to energize (regardless of the radiator thermofan switch being closed or open), closing two contacts. One contact causes the secondary cooling fan to run. The other is a contact that is in parallel with the "slow speed" resistor for the primary fan motor. This shorts out the resistor and causes the primary fan to run in fast speed. The second fan has no "slow speed" resistor so when it starts, it always runs in fast speed. Also, when the air conditioning is turned on (with the ignition on), the primary fan will run regardless of coolant temperature.
07-07-2017, 08:05 PM
Early 944 fan wiring mystery...
