This is for a 91 3.1L firebird. Looking the wiring diagram, the fan relay is turned on and off by an ECM output. The ECM gets a temp signal from the coolant temp sensor. The trigger wire for the fan relay also shows that it's connected to a fan switch located in the head. So why does the ECM control the fan and also a switch? Or am I missing something?

 

The ECM controls the fan when the AC is on. (The AC pressure switch signals the ECM to start the fan.)
The temp sensor is also to enable the closed loop and for fuel control.

-Ozzy

 

So during normal operation the fan switch is responsible for kicking the fan on? Not the ECM?

 

Yes but the ECM will also activate the fan based on AC Pressure Switch request.

If you have a diagram you'll see that "ECM pin D12 (Coolant Fan Request) is the signal for the AC, but there is no input from the fan switch. The fan switch activates the fan relay directly.

Note: ECM pin E16 (Coolant Temp Input) is not for fan control.

-Ozzy

 

Ok, this makes more sense now. Thanks. I'm helping my friend diagnose a problem. His fan won't turn on so I told him the first thing he should try is to turn the AC on and see if the fan comes on. If it does then the fan switch should be the culprit.