96 Formula - Fans work WITH A/C but not without

[Racr_X]

1996 Formula Firebird - OBDII

Trying to figure this one out. Both my fans work in slow and high speed mode with the A/C turned on. The car cools just fine.

If you don't use the A/C and leave the car sitting, it will overheat and the fans will never come on. Any idea what the problem could be?

Thanks in advance guys !

[Injuneer]

Have you checked the coolant temperature sensor that feeds the PCM - the one in the water pump housing?

The stock programming for the fans is relatively hot.... 1st fan on (or "low" on later models) at 226degF and the 2nd fan (or "high") at 235degF.

[MN6WS6]

Sounds like a thermostat problem to me. The slow speed is the same speed that the fans use when the thermostat tells them to turn on. High speed is only used with the AC. The problem is, I don't know which thermostat tells them to turn on (there are two, one in the radiator or water pump, and one on the block itself). Maybe somebody else can chime in on that one.

[Injuneer]

Sounds like a thermostat problem to me. The slow speed is the same speed that the fans use when the thermostat tells them to turn on. High speed is only used with the AC. The problem is, I don't know which thermostat tells them to turn on (there are two, one in the radiator or water pump, and one on the block itself). Maybe somebody else can chime in on that one.

The fans are controlled by the A/C position switch via the PCM, and by the PCM, using the coolant temp sensor in the water pump housing. The PCM uses both speeds in response to high coolant temps, as indicated by the temp limits in the post above.

The "thermostat" in the engine block has nothing to do with turning the fans on. It is simply a mechanical device that blocks coolant flow until it reaches the minimum operating temp for the engine (180degF for a stocker).

There is also a temperature sensor in the drivers side head, but that sensor is connected only to the dash gauge.