my car over heats when is in idle and the fan does not turn on to cool it off, the only way the fan turns on is if the A/C is on.... i have checked everything the radiater works all the wires work relay works, just put in new coolant switches, flushed the coolant, my brother believes the thermostat is working cuz coolant is running through the engine.... what could be the problem??

 

we need more info, what year is your car? what motor? TBI or TPI? single fan or dual fans? are there any mods done to it? does it over heat with the A/C on? what does the temp gage read when it over heats? is it boiling the coolant in the recovery bottle? did you replace the radiator cap?

 

its a 90 rs 305 TBI with single electric fan, no mods. the only way to cool the engine down is to drive it without stop for alittle while or when i turn the A/C on the fan kicks in and cools the engine down. the temp gage on the dash hits red which i believe is 260. the coolant is running fine. i'm not sure about the radiator cap, could you explain that to me?

 

Well if the fan isn't kicking on unless ac is on then i would expect theirs a simple solution to fixing that for now, i'd say just jump across the 2 sides of the relay and run it like that for now until you have time to figure out the problem.

As for the problem i wouldn't count out the relay, as relays tend to test good under no load but once a load is placed on them they fail (had this problem myself). An easy solution is to swap the fuel pump relay beisde your fan relay with your fan relay (jumpered across the bottom with a paper clip) or such so your fuel pump will turn on without cranking it numerous times due to gm's oil pressure switch.

As for the thermostat considering they cost about 3$ it's safe to say i'd replace it considering it's only 2 bolts......... and a hose clamp in your way.

Lastly i wouldn't count on gm's gauges to be working properly either, i realy realy doubt that you are running at over 260* as you state above because, imo if you realy were things would start failing on you (coolant boiling out wierd places, headgasket failure and junk like that.

 

radiator caps have a life span, its about 2~3 years. when they get old the gaskets won't seal properly, the spring starts to get weak & they won't hold pressure like they should.

try this, unplug the fan switch & then start it up & ground the wire for the fan switch, the fan should come on. if it does then everything should be ok & it will show you have a problem with the switch. if it doesn't come on then post back.

like flaming-ford said, your gage may not be working right.

 

We just checked the relay again, this appears to be working correctly. The one question I have is that there is a grn/wht wire going from the relay to the coolant switch. We tested the wire and we have resistance so the wire is good. When we ground out the wire the fan turns on. I am guessing that the switch is designed this way too. When the coolant reaches a certain temp the switch grounds out to complete the circuit and the fan should turn on. Does this sound correct? If so, let me ask a silly question. When we installed the new switch we wrapped the threads with teflon tape, could this possibly prevent the switch from grounding against the engine body and not allowing the circuit to complete?

 

thats not a silly question, its a good question, its only silly if you don't know & don't ask it.
your right about how the fan switch works, & yes the teflon tape could be the problem.