Why do my radiator fans run only when AC is on?

Thanks for the reminder. I forgot to include that I swapped the good starter relay with the fan relays. The fans still only run with the AC on. The boiling coolant and temperature gauge at mid-range still confounds me. When I turn on the fans by turning on the AC, the boiling stops but the temperature gauge still remains at mid-range.

Thanks for the temp sender details. Since I replaced it with a new one I didn't OHM check it, but I will. The temp gauge has no numbers so I am wondering what mid-range is as a number. 180F? I'm planning on changing the thermostat today and will submit feedback.

I just came back in from installing a new thermostat. There was no thermostat in there when I removed the cover. I thought maybe that could be the reason since the designed water routing was not being effected. But with the new one installed the fans still don't turn on. I double checked the relays by switching them with the known working one in the starter.

One note on the fans. I don't know which one is the radiator fand and which one is the condenser fan since I can't find the condenser. Anyway when I turn on the AC, only the one on the passenger side starts. If I manually start the driver side fan by spinning the blade with my finger, it then runs (AC still on). It does not run when manually spun if the AC is off. I'm guessing that the start winding is bad.

Would that cause the no fan problem with AC off?

The only thing that I can think of now is the ECM not sending the run signal. A rebuilt ECM is about $450, ouch!

I think that your detailed info directed me to the problem. I see now that the condenser is in front of the radiator and both are of same dimension. Looking down from the top I did not see the two and was figuring that it was all radiator. OK so knowing which fan is which really helps. It was the radiator fan that I could start with my fingers when AC was on. And figuring that it has a bad starting winding I tested the radiator fan circuit by unplugging the 12V connector at the fan housing and tested the supply for 12V. While warming up: 0V as expected. When boiling bubbles in the overflow tank: over 12V! At that point I manually spun the fan and it ran without the AC on!

Fan motor as Docfixit suggested! Thank you Docfixit!

I'm going out right now and getting a new fan motor.

Do you think that I can remove the radiator fan cage without removing the radiator?

Thanks for the diagram. I was able to remove the radiator fan shroud from the top. I moved the front section of the upper radiator toward the passenger side and completely removed the resonator box, part of which sits on top of the radiator and shroud. The four 10mm mounting bolts were then accessible for removal and the shroud was removed upwardly.

With a special 20% discount coupon I was able to get a new fan motor at Advanced Auto for about $50. Replacing the motor in the shroud on the workbench went easily. I used wire nuts to splice the new motor wiring to the existing in the shroud.

Finally, the fan is now operational and working properly.

As an added benefit I also found the thermostat missing and now have a new one installed. The temperature gauge stays mid-range.

Thank you Docfixit for the details that enabled me to figure this out and fix it.

Thank you, I will.
Also from above, "I moved the front section of the upper radiator toward the passenger side" makes no sense and should read "I moved the front section of the upper radiator hose toward the passenger side." My fatigue at that point was showing!

Hi guys. Having this issue myself. I'm just having trouble locating senor for gauge. I replaced the ect senor on thermostat housing. Just can't locate the other sensor?