My cooling fan will not kick on unless the A/C or the defroster is turned on. It will overheat if it just idles unless I turn on one of the above. I replaced one of the relays that controls the fan. It still does it however. I also tried replacing iwith the other relay (there are two of them at the very back on the driver side). What does each relay do? What else is keeping my fan from turning on unless I turn on the A/C or defrost?

 

Your fan temperature switch is probably shot or disconnected. It's mounted in the cylinder head on the passenger's side I believe. Replace it and you should be fine.

You can also run a manual fan switch if you prefer, or connect the relay to ignition power and the fan will run all the time (although I don't recommend that.)

 

Check your heavy duty cooling fan temp switch in the right head between nos. 6 and 8 cylinders it is probably bad or could be a faulty connection The relay controls the fan but the temp switch controls the relay. The fan is set up to come on with AC head pressure which is why it runs with the AC on, otherwise with out AC the fan is temp controlled.

 

Read my article:

http://www.thirdgen.org/techbb2/show...hreadid=170903

BTW, the primary fan (driver's side) on a dual fan setup is controlled exclusively by the ECM. The ECM turns the fan on with A/C (as long as vehicle speed is below 35-40 mph) or when coolant temp reaches 223 degrees (this is an EPROM constant).

The secondary is controlled by the ECM (only with A/C engagement -- it is not coolant temperature dependent) and thermal switch located in the passenger side head. The switch has a temperature setting of 243 degrees (unbelievable!).

 

Well, I worked on trying to figure out this problem all day, and still no luck! I had my original 305 exchanged about ten years ago for a 350 out of a '87 or '88 Chevrolet truck. I thought everything was basically the same as far as the electric fan and everything goes-it still has the original cooling equipment. However, there is a coolant sensor mounted on top of the thermostat housing connector that is wired into the harness (I know this is wired to the fan through the harness). There is also another coolat sensor connected into the intake manifold at the very front right beside the other one. I could not find a fan switch anywhere- I looked under the car, around the starter area, and above around the passenger side head. I replaced the coolant sensor mounted on the thermostat housing and this did nothing. The other one was old and brittle, so I replaced it with the other one I removed (only two years old). Shouldn't there be a switch somewhere on the car-how can a sensor act as a switch, or can it?

Another weird thing I've noticed: when I run the car, the fan won't kick on unless I turn on the A/C, as I mentioned before. After that, I can shut off the A/C and the fan will continue to run sometimes and sometimes it doesn't. If I remember correctly, when this first started and I was testing it, the fan even came on after it had stopped WIHOUT the A/C being turned on-it just seems it has to be WOKE up by the A/C or defrost. Once the ignition is cut off, it's like it resets and the fans must again start by turning on the A/C or defrost.

Any help is greatly appreciated-this is really frustrating,

 

i had an 89 with a 350 TBI dropped in and that SOB overheated on a cold day in january. all i did was make its own manual switch and had it installed, never over heated again. and i did that after messing with the relys and a new thrermostat.

 

my buddy had the same problem with 87 TA he just put a switch in his car so he could control when the fan came on it cost him about 5 bucks and colved his problem!