Just picked up this 1987 Trans Am a few weeks ago, and I'm sorting out all the bugs in it. Most of them are pretty easy to correct, but there's one that I can't seem to wrap my head around, and a few mechanics can't do it either. Ever since we purchased the car, both fans (my car has two) will instantly start running with the key in IGNITION or the ON position. They don't shut off until the engine is off. I've been told that '87 was the only year GM placed two fans under the hood of the f-body cars, and all the schematics I've been able to find seem to reflect that - they all show only one fan on the electrical diagrams, even the ones from the factory repair manual. If anybody could provide a link to a schematic that showed both fans, it would be greatly appreciated. Has anyone else experienced this problem with their '87? What are some good places to start looking? All the relays and relevant wiring looks to be untouched, and not all hacked up.

Other symptoms that may or may not be related are a high idle, a slightly rich-smelling exhaust, and a lower coolant temp.

 

The secondary cooling fan switch, on the passenger's side of the engine, is likely bad and causing the secondary fan relay to kick on. The primary fan relay will be commanded on, via the ECM, if the secondary fan relay is also on.

A simple test would be to disconnect the wire from the switch sensor. If disconnecting the secondary cooling fan switch wire stops both fans, then the switch has failed and needs replaced.

There are a few other possibilities but the secondary cooling fan switch would be my first check.

 

Okay, thank you for the info. I had read a little bit about that switch in other conversations, but it wasn't really clear to me before how it operated. You're talking about disconnecting and testing at the switch connecter located between cylinders 6 and 8, right?

 

Correct. It's near the knock sensor (I believe it's the same location on 1987) so make sure you got the correct one. Simply disconnect the one wire. Turn key to On, Engine Off. Both fans should now be off.

The secondary cooling fan switch is simply and On/Off switch that provides a ground, via the engine, when the engine coolant temperature exceeds approximately 238F. When the secondary cooling fan switch is closed (grounded) from meeting the temperature, it provides a ground to the control side of the secondary fan relay, when the key is On, and powers the secondary fan. There is another wire that goes to the ECM so that the ECM knows if the secondary fan relay has been powered. If so, the ECM then commands the primary fan on.

The secondary cooling fan switch can fail and either allow or not allow a ground path regardless of the temperature. I suspect that is your issue but doing the test is necessary part of diagnosing since there can be different issues.

 

Cool, that makes sense. Do you know if it’s easier to reach the sensor from the top or the bottom of the car?