Cooling Fan Troubleshooting — Early Cars without A/C

Cooling Fan Operation

Early cars 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.

Early circuit without A/C: (B) battery feed · (F3) fuse 3 · (TS) thermofan switch · (R) slow-speed resistor · (K) cooling-fan relay contact (shorts out R for fast speed) · (M) fan motor.

Cooling Fan Troubleshooting

Identify the symptom you are seeing, then jump to the likely cause below.

Note: This guide assumes a single failure is causing the fault. Multiple simultaneous failures can occur, but covering every combination is impractical — diagnose one cause at a time.

Blown fuse

The fan will not run. Auxiliary Fuse Panel: Fuse 3 — Cooling Fan.

Bad fan motor

The fan will not run. Disconnect the connector at the fan and take resistance readings. Infinite resistance or a ground indicates a bad fan motor.

Bad slow-speed resistor

The fan still runs in fast speed but not in slow speed. Locate the slow-speed resistor on the firewall behind the tachometer; resistance should be less than 1 ohm.

Bad cooling fan relay

The fan will not run in fast speed. See Testing the Cooling Fan Relay.

Bad thermofan switch

The fan runs continuously with the ignition OFF. Disconnect the thermofan switch connector; if the fan stops, the switch is bad. See Testing the Thermofan Switch.

Testing the Cooling Fan Relay

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

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.

A successful test confirms that the power supply to the cooling fan via the slow-speed resistor is good.

Test 2 — Fast Speed

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

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

Testing the Thermofan Switch

Tools needed:

• Thermometer (rated to at least 100 °C212 °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 °C198 °F, the contact should close (zero resistance on multimeter).

Repairing 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.

Close-up of the failure mode: (1) a hairline solder crack — a ring fracture around the pin; (2) a good joint for contrast.