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How do I get my '85 Cooling Fan to Turn ON @ 200F OFF @ 185F Automatically & Manually


Well-known member
Feb 20, 2016
Owego, NY
1995 Red Corvette (C4, L98)
How do I get my '95 Cooling Fan to Turn ON @ 200F OFF @ 185F Automatically & Manually?

I have a 1985 Corvette with a Single Fan.

The ECM is set to turn the Fan on at 239 Degrees. I think that's too Hot. Might be good for emissions or something but it's too Hot for Johnny.

Is there something I have to change. I know there's a sensor or switch on both sides of the engine. Do I need to switch one or both of those? Do I need to add a sensor/switch to the block (if there's a place for one)? How do I bypass the ECM? I guess my goal is to get the Fan to kick on at 200 and off at 185.

How do I set up a switch for manual operation?


John :pat
Im headed out to work in a few,the easiest way is to install a toggle switch to ground out the fan relay.I did that on my 85.If ya run it too cold the CEL will set with a EGR code.Found that out when I tried a lower thermostat.
Theres the relay location,splice into the Green/White wire,thats the Grd. Run that wire inside the car & to 1 side of the toggle switch. Run another wire from the other post on the toggle switch to a good ground inside the car,your done.
these are nice for spliceing ,no striping wires etc
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Im headed out to work in a few,the easiest way is to install a toggle switch to ground out the fan relay.I did that on my 85.If ya run it too cold the CEL will set with a EGR code.Found that out when I tried a lower thermostat.

Hi Wayback! Thanks for the reply. I'm working on the Fan Project today and I'm not sure where to go with it for automatic operation but I like the Manual Switch Operation too so Thanks for that info and the warning tip. It would be cool to be able to cool at one's whim. What Thermostat was it that you tried? I'm not sure about that either. I have a Hi-Flo 195 but I think maybe a 180 might work, although I'll have to eventually get the Prom re-programmed because the ECM is set to see 195 for air/fuel ration 14:1, or it will stay in Open Loop. So I've read.

I've been doing tons of work on my car the past two months. It's been in down-time all that time... and then some. Long horrible story. I'm just about finished and it's time for me to sew up the cooling system and I want to do it ALL.

I can also go with a somewhat semi-automatic operation which involves a manual thermostat with a knob and a probe you stick in the radiator through the top radiator hose. Wicked Cool! if you got a nice place for the T-Stat and are crafty about dressing up wires and such. Which I feel I have a forte for.

I guess there's a number of ways to do things... maybe that's the problem which influenced this post... too many options.

Let me know about that Stat you were using when you got the code set by the ECM.


John V. Brennan :pat
U.S. Coast Guard Veteran (retired/disabled)
1985 Corvette, Base (with some wicked cool awesome mods), Bright Red, Big Fat Tires on '95 Vette Rims

PS. anytime you want to confrontate about these '85's of ours... I'm here for you and I have both the Haynes and the Chilton's Manuals, and several good books, and other resources... and sometimes I get around to reading/watching them.
The fan-on for an '85 with a single fan is controlled by two devices: the ECM, which turns the fan on when A/C pressure goes below 180-psi, and the cooling fan switch (screwed into the back of the right-hand cylinder head) which closes when the ECT is above 238°F.

The only way to get the fan to turn on/off automatically at a lower coolant temperature is to change the cooling fan switch. In the old days, both GM and the aftermarket sold "lower-temperature" fan switches. I doubt GM still sells one but, CAC sponsor, Zip Products, sells a lower temp fan switch for your engine. Find it at: 1984-1991 Corvette Low Temp Cooling Fan Switch

It turns that fan on at 200 and off at 185.
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Thanks for the pics and info. Very Helpful. I want to replace that Factory Fan Switch but all that AIR Pump piping is still there atop the manifold, and in the way of my hand tool ratchet. I just removed the AIR Pump System and installed the Bypass Pulley. I'm not sure how to take the pipes out though. I was thinking about using Mr. Crowbar and bending things back and forth till they break off, but I'm not sure that'll work so well and I don't want to wreck anything else in the process. I don't have a torch, or air compressor, and I'm handicapped with only about 25% use of my right hand, 80% of my left, can't walk really, except bounce off the walls and conveniently located objects, chairs and such, and sticks, and whatever...

Oh, Hey! The IAT to confirm, I hope, is underneath the Throttle Body, passenger side, kinda' large, with an odd looking connector which I haven't tried to remove/pry off yet. Right?!?!

I did the Coolant Temperature Sensor next to "whatever it is"... holy crap! TIGHT! Broke my favorite ratchet with the hinged head... Had it for years... put a very large, 2.5 ft., heavy duty, gas/water pipe on it (didn't have a torch to heat it up with neither), finally it cracked loose, along with my ratchet, but still had to use the pipe to loosen it some more. The sensor was rotten. I had terrible gas fumes, well, the car did. Guy from Corvette Central said, on YouTube, that a bad CTS could cause that problem. It needed to go anyway.

Might do an adjustable cooling fan thermostat thing.

I've gone over my entire cooling system inch by inch. Just a few more inches to go. Got some really cool Blue Goodyear/Continental Hoses, complete kit, even comes with the hoses for the Z51 Heavy duty cooling option, from RockAuto. Not doing the heater pipe to water pump hose till I do the water pump (if I ever do it). Did the Throttle Body Bypass. What do you think about electric water pumps?

I replaced quite a few sensors/switches, etc... since with RockAuto, eBay, Amazon, they were all mostly somewhat inexpensive to very cheap. Ha! You wouldn't believe all the cool crap I got for my car! I've got tons of mod stuff, accessories, and a complete roadside break down/pit stop kit just to name a few. Been collecting the "stuff" all the time I couldn't drive the car. I love it!

Johnny Goddavette :pat

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