1988 Iroc. Just swapped engine and have everything hooked up but fans will not switch on no matter how hot it gets. Checked the wires and grounds all are good, checked both relays both are hot, fans will not turn on even when the air conditioner is engaged. Running out of ideas, need some. Thanks

 

I had the same problem after my recent swap. Everything was hooked up as it was before when they worked. I just wired up a 20A toggle switch directly to the fan and mounted it behind my hatch release switch. Not an ideal fix, but cheap and easier than chasing wires and sensors.

 

need more info, tpi or carb? dual or single fans? senders from original engine or new?
assuming your car is stock. you say the relays are hot. when you ground the wire from the ecm to the coil side of the relay do the fans come on?

 

Did you apply power & ground using 'jumper' wires to be sure the fans are OK ?? Stuff does go "shelf dead" so you should double check !!

Don't trust your temp gauge. Have you got it hot enough to bubble back into the overflow ?? If it starts to bubble into the overflow and the fans don't kick on - that's when you KNOW there's a problem !

Run it from cold till you feel the thermostat open (squeeze/feel upper hose) Once the T-Stat opens it's only just reaching operating temps. Stock fans don't come on till around 235 and coolant will not usually bubble into the overflow till about 245/250. Run it till it bubbles to see if the fan kicks on.

The ECM and the sensor on the pass side of the block are what will turn on the fans. ( when you turn on AC the signal still gets sent thru the ECM. )

Find the fan relay on the radiator support then turn the key to RUN ( don't start ) Place a 'jumper' wire from ground to the Green/White wire's PIN @ the fan relay. This will act as the "switched ground" from the ECM to the relay and should turn on the Primary fan; regardless of engine temp. If the fan works than you will need to figure out why the ECM isn't sending a signal. ( Bad ECM, Bad cooling temp switch in the manifold, bad connections, ect...)

If the fan doesn't work when the relay is grounded at the Green/White wire then the relay could be bad, there might be a break somewhere in the Power or Ground, something could be unplugged, or the fan motor could be dead.

P.S. The same switched-ground 'jumper' wire test can be performed on the heavy duty fan relay. It's usually mounted beween the battery and the overflow.