My son bought a 1985 trans am, 5.0 HO,4 barrel carb and 5 speed manual from my sister.The problem he's having is the fan will only work with the defroster on.I know where the sender is that activates the relay, which my sister replaced ( on passenger side of head)so I'm going to unplug it and ground it to see if it will turn on the fan.If it doesn't, I will replace the relay. The problem is I don't know which relay it is, theirs three on the driver side, and two of them have the same number on them.

 

Pop the plug off of both and short the two large leads on the plug together. Fan will spin when you've chosen the correct relay. Other one should be for the fuel pump.

The front one 'should' be the fan relay, but they could have been swapped around at some point, note the non-factory fasteners.

The relay must work for the fan to engage with defrost. The HVAC control head grounds the same relay lead. Your issue is either in the wiring to the switch in the head, or the switch itself.

 

or... you're not allowing the coolant temp to reach the existing fan switch's set point (which is considered too high by some).

 

I unplugged the relay and jumped the plug, fan ran.cleaned the plug and inside the relay, plugged it in,turned heater to defrost and air-condition,fan turned on.Then let engine run till it got past 230 and it started to over flow the expansion tank,fan did not come on.I cut the wire half way down to switch in the head and grounded it, fan turned on.So from where I cut the wire and grounded it,the wire to the relay works. rso my thinking is, the plug to the switch is bad or they put a bad switch or the wrong switch in. Do they make a aftermarket switch that will turn on sooner than 220?

 

Yeah, those switches are famous for failing due to lousy design. Do a search for fan switch on the cooling board and you'll get a ton of info about the alternatives to the stock switch that you have. Also many other more reliable systems are available as well. Depends on how much work you feel like doing
https://www.thirdgen.org/forums/sear...archid=9051899
heres a bunch

 

I'm going to replace the switch in the head, and the plug. Can I run another wire of the replacement plug wire and run it to a toggle switch, just in case the switch fails again? I know the toggle switch will just be a ground not power.

 

splicing into the switch ground lead for a toggle is not a problem.

the fan switch connectors are getting scarce. it's the same connector used on the knock sensor. apparently still available from Rock Auto.

 

Has anyone used the jet fan switch, that Summitt sells?I see it starts the fan at 195 and turns the fan of at 180.

 

I have a jet switch in my gta. I put it in about 15 years back and it still works fine. The fan comes on around 195 and turns off around 175/180 .

 

Common sensors for the thirdgen are

SW555
SW505
SW500
SW502

They can use the PT308 or PT770 pigtail

I like SW555 210 on temp off at 205, on my 84 the factory temp gauge was so in accurate would read about 250 before the factory switch would come on.

All these part numbers are from the manufacturer wells. I hope this helps someone.