Cooling fans

[AmericanThunder]

I was working on a friends 1996 Trans Am this afternoon.
He suspected his cooling fans weren't working and sure enough when jumpers acrosss the ALDL they weren't. Fuses checked ok, as did all 3 relays, (it uses a 3rd relay to run the two fans in series to give low speed operation). All the circuits checked out, power where is was supposed to and grounds where it was supposed to be. Lastly i disconnected the wiring from the fans and ran them directly from my jumper box. Both fans worked.
Very puzzling. After studying the circuit diagram I noticed that the ground for the circuit is via the PCM, which means the PCM must be using internal relays to switch the fans on. This means that something inside the PCM has failed.
Makes you wonder whether other components will be degrading to the same extent?
I have sourced him a manual fan switch which will give him fan control back, just without automatic switching on/off.
This is the best short term solution whilst he decides what to do with the car.
A fun afternoon full of circuit diagrams, fuses, relays, multimeters and bits of wire, and once again my ramps came in handy!

 

[TransAmDan]

I don't have experience with the 96 PCM. i know '98 onwards the PCM checks the temperature sensor on the head, if its over a threshold it turns Fan 1 on, over the next temperature threshold then the Fan 2 comes on. Could it be the temperature sensor, however usually when it fails the car thinks you are -40deg C, so would be running pig rich.
Perhaps take control away from the PCM, and wire in a thermostat switch.

 

[AmericanThunder]

I think The PCM controls are the same from 96-02.
It could be a sensor but the guage reads fine. The car is not in the best of condition. It's had many years of service without investment of any kind as the owner is not mechanical inclined and he recognises that fact. He wants the car to run another year, so a manual fan switch is the easiest and cheapest option.
Maybe next year he will swap to a Corvette C5.