Cooling Fan Operation and Troubleshooting

Introduction

When troubleshooting cooling fan problems, confusion often arises because the fan circuit differs between early (pre-1985.5) and later 944s. Some wiring diagrams in circulation are of poor quality or outright incorrect, and even the factory drawings can be difficult to follow. The descriptions here are based on the factory electrical diagrams and practical troubleshooting experience.

Cooling Fan Operation — Pre-1985.5 944s With Air Conditioning

On pre-1985.5 944s equipped with air conditioning, there are two electric cooling fans. The driver's side fan is supplied via a parallel circuit: one leg contains a resistor (slow speed), the other leg contains a contact in the cooling fan relay (fast speed). The passenger's side fan is supplied via a contact in the cooling fan relay.

When coolant temperature reaches approximately 92 °C (198 °F), the thermofan switch closes and causes the driver's side fan to run in slow speed (the slow-speed resistor remains in circuit). The fan will run as long as temperature remains above the setpoint, regardless of whether the ignition is on — this is a common problem source.

When the ignition is on and the A/C is turned on, the A/C relay picks up, which energises the cooling fan relay. This closes two contacts: one starts the passenger's side fan (always fast speed); the other shorts out the slow-speed resistor for the driver's side fan, running it in fast speed. The driver's side fan also runs regardless of coolant temperature whenever the A/C is on.

When the ignition is on and a high-temperature condition is sensed by the thermofan switch, the relay is energised and both fans run in fast speed.

Cooling Fan Troubleshooting — Pre-1985.5 944s With A/C

Symptom	Possible Cause	Testing / Repair
Driver's side fan runs continuously (ignition OFF)	Bad thermofan switch	Disconnect thermofan switch connector. If fan stops, the switch is bad. With ignition off, both switch and relay would have to fail simultaneously to cause this.
Both fans run continuously (ignition ON)	Bad thermofan switch	Disconnect thermofan switch connector. If fans stop, switch is bad.
Both fans run continuously (ignition ON)	Bad A/C relay	Remove A/C relay. If fans stop, A/C relay is most likely bad.
One or both fans will not run	Bad fuse(s)	Auxiliary Fuse Panel: Fuse 3 — Driver's Side Cooling Fan; Fuse 5 — Passenger's Side Cooling Fan
One or both fans will not run	Bad fan motor	Disconnect connector at fan and take resistance readings. Infinite resistance or a ground indicates a bad motor. It is highly unlikely for both motors to fail simultaneously.
One or both fans will not run	Bad cooling fan relay	With a bad relay, the driver's side fan should still run in slow speed on high coolant temperature. See relay testing procedure.
One or both fans will not run	Bad A/C relay	A bad A/C relay will only prevent fans from running when the ignition is on and A/C is on. Both fans should still run in fast speed on high coolant temperature.
Fan runs but not in slow speed	Bad slow-speed resistor	Fan will still run in fast speed. Locate slow-speed fan resistors on the firewall behind the tachometer. Resistance should be less than 1 ohm. Infinite resistance = open; resistance higher than 1 ohm = partial failure.

Testing the Cooling Fan Relay — Pre-1985.5 944 With A/C

Tools needed:

• 12 VDC power supply and leads
• Multimeter with alligator clip leads

Procedure:

1. Remove the Cooling Fan Relay.
2. Take resistance readings with the relay de-energised:
   • Terminals "A" to "B" — should be infinite (open circuit)
   • Terminals "30" to "87" — should be infinite (open circuit)
3. Connect a 12 VDC power supply across terminals "85" and "86". You should hear the relay click.
4. With power supply connected, take resistance readings:
   • Terminals "A" to "B" — should be zero (short circuit)
   • Terminals "30" to "87" — should be zero (short circuit)
5. If any reading above is bad, the relay is bad and should be replaced.

Testing the A/C Relay — Pre-1985.5 944

Tools needed:

• 12 VDC power supply and leads
• Multimeter with alligator clip leads

Procedure:

1. Remove the Air Conditioning Relay.
2. Take resistance readings with the relay de-energised:
   • Terminals "A" to "B" — should be infinite (open circuit)
   • Terminals "30" to "87" — should be infinite (open circuit)
3. Connect a 12 VDC power supply across terminals "85" (positive) and "86" (negative). You must connect positive to terminal 85 — there is a diode in the circuit that will prevent the relay from working if you reverse polarity.
4. With power supply connected, take resistance readings:
   • Terminals "A" to "B" — should be zero (short circuit)
   • Terminals "30" to "87" — should be zero (short circuit)
5. If any reading above is bad, the relay is bad and should be replaced.

General Circuit Testing — Pre-1985.5 944 With A/C

Tools needed:

• Multimeter
• Electrical jumpers

Test 1 — Slow Speed Operation (Primary Fan)

1. Disconnect the thermofan switch connector (below the upper radiator hose connection at the radiator).
2. Jumper the contacts in the electrical connector.
3. The driver's side fan should run in slow speed.

A successful test confirms that power supply to the driver's side fan via the slow-speed resistor is intact.

Test 2 — Fast Speed Operation (Both Fans)

1. With the thermofan switch connector still jumpered, turn the ignition on.
2. Both cooling fans should run in fast speed.

A successful test confirms that power supply to both fans is good and that both fans and the cooling fan relay are working properly.

Test 3 — A/C Mode Fast Speed (Both Fans)

1. Remove the jumper from the thermofan switch connector.
2. With the ignition on, turn on the A/C switch.
3. Both cooling fans should run in fast speed.

A successful test confirms that the power supply, both fans, the A/C relay, and the cooling fan relay are all working properly.

Testing the Thermofan Switch — Pre-1985.5 944

Tools needed:

• Thermometer (rated to at least 100 °C / 200 °F)
• Multimeter

Procedure:

1. Connect a multimeter across the contacts on the thermofan switch. At room temperature, contacts should be open (infinite resistance).
2. Suspend the thermofan switch in a pan of water. Insert the thermometer to monitor temperature.
3. Heat the water. When temperature reaches 92 °C (198 °F), the contact should close (zero resistance on multimeter).

Cooling Fan Operation — Pre-1985.5 944s Without Air Conditioning

Pre-1985.5 944s without A/C use a single radiator cooling fan (rather than two). It is supplied by a parallel circuit similar to the A/C-equipped cars. If high coolant temperature exists with the ignition off, the thermofan switch closes and runs the fan in slow speed (via the slow-speed resistor). When the ignition is turned on, the cooling fan relay is energised, shorting out the slow-speed resistor. If the high-temperature condition still exists, the fan runs in fast speed.

Cooling Fan Troubleshooting — Pre-1985.5 944 Without A/C

Symptom	Possible Cause	Testing / Repair
Fan runs continuously (ignition OFF)	Bad thermofan switch	Disconnect thermofan switch connector. If fan stops, switch is bad.
Fan will not run	Bad fuse	Auxiliary Fuse Panel: Fuse 3 — Cooling Fan
Fan will not run	Bad fan motor	Disconnect connector at fan and take resistance readings. Infinite resistance or ground indicates a bad motor.
Fan will not run	Bad slow-speed resistor	Fan will still run in fast speed. Locate resistor on the firewall behind the tachometer. Resistance should be less than 1 ohm.
Fan will not run in fast speed	Bad cooling fan relay	See relay testing procedure.

Testing the Cooling Fan Relay — Pre-1985.5 944 Without A/C

Tools needed:

• 12 VDC power supply and leads
• Multimeter with alligator clip leads

Procedure:

1. Remove the cooling fan relay.
2. Take a resistance reading with the relay de-energised:
   • Terminals "30" to "87" — should be infinite (open circuit)
3. Connect a 12 VDC power supply across terminals "85" (positive) and "86" (negative). A diode in the circuit requires correct polarity.
4. With power supply connected, take a resistance reading:
   • Terminals "30" to "87" — should be zero (short circuit)
5. If any reading above is bad, the relay is bad and should be replaced.

General Circuit Testing — Pre-1985.5 944 Without A/C

Tools needed:

• Multimeter
• Electrical jumpers

Test 1 — Slow Speed

1. Disconnect the thermofan switch connector (below the upper radiator hose at the radiator).
2. Jumper the contacts in the connector.
3. The fan should run in slow speed.

Test 2 — Fast Speed

1. With the thermofan switch connector still jumpered, turn the ignition on.
2. The fan should run in fast speed.

Cooling Fan Operation — 1985.5 and Later 944s (All)

Late-model 944s use a similar slow-speed resistor arrangement, but there are two resistors — one for each fan motor. Both fans run (rather than just the driver's side) when the thermofan switch closes on high coolant temperature, even with the ignition off.

The thermofan switch on late cars is a dual-element switch: one contact closes at 92 °C for slow-speed operation, and a second contact closes at 102 °C to bypass the slow-speed resistors (fast speed). Slow-speed operation is available regardless of ignition switch position; fast-speed operation requires the ignition to be on.

Cooling Fan Troubleshooting — 1985.5 and Later 944s

Symptom	Possible Cause	Testing / Repair
Both fans run continuously (ignition OFF)	Bad thermofan switch	Disconnect thermofan switch connector. If fans stop, switch is bad.
Both fans run continuously (ignition OFF)	Bad cooling fan relay	Disconnect thermofan switch connector. If fans do NOT stop, the relay is most likely bad.
Fan(s) will not run	Bad fuse(s)	Fuse 10 — Cooling Fan 2; Fuse 15 — Cooling Fan 1. If neither fan runs, it is unlikely both fuses blew simultaneously.
Fan(s) will not run	Bad fan motor	Disconnect connector and take resistance readings. Infinite resistance or ground = bad motor.
Fan(s) will not run	Bad cooling fan relay	See relay testing procedure.
Fan(s) will not run in slow speed	Bad slow-speed resistor	Fans will still run in fast speed. Locate slow-speed resistors in the passenger's footwell behind the glove box (on the underside of the battery tray). Resistance should be less than 1 ohm.
Fan(s) will not run	Bad thermofan switch	Even with a bad thermofan switch, fans should still run in slow speed when A/C is on.
Fans will not run in fast speed	Bad cooling fan relay	See relay testing procedure.

Testing the Late-Model Cooling Fan Relay

Procedure:

1. Ensure the A/C switch is in the OFF position.
2. With the ignition off, disconnect the thermofan switch connector (below the upper hose connection at the radiator).
3. Jumper terminals 2 and 3 on the thermofan switch connector. Both fans should start in slow speed.

Note: The terminals on the thermofan switch connector are not numbered. Determine terminal numbers by the orientation of the tab on the plug connector — see diagrams below.

4. With the jumper still installed, turn the ignition ON. Fans should remain in slow speed.
5. Turn the ignition OFF and swap the jumper to terminals 1 and 3.
6. Turn the ignition ON. Both fans should run in fast speed.
7. Turn the ignition OFF and remove the jumper.
8. Turn the ignition ON and switch A/C ON. Both fans should run in slow speed.
9. If the relay fails any of the tests above, it is bad and should be replaced.

Repairing 944 Fan Relays

If the cooling fan relay is confirmed bad, inspect it before purchasing a replacement. Like many electronic components on the 944, the fan relay is susceptible to cracked solder joints. Open the relay and inspect the board with a magnifying glass — cracks will be approximately the width of a human hair. If any joint looks suspect, re-solder it.