cooling fan diagnosis

[fatboyreyn]

my 86 coupe cooling fan comes on 2 seconds after the motor is started, whether it's hot or cold. the service manual says the fan comes on when collant temp is above a certain degree, AND when a/c pressure is high, AND when mph is below 40. My question is do all three of these have to be met for fan to come on, or any ONE of these at a time. If the latter is true, the fan runs all the time under 40 mph? Changed fan relay with a known working one, and no difference. Any input would be appreciated.

 

[jmccloud]

Cooling fan control

It's a little more complicated than one simple 'AND' or 'OR' situation.
If the speed goes over 40, the fan power cuts off. Regardless of other conditions.
If the speed is under 40 and either the coolant temp is up 'OR' the A/C is on, then the fan is powered.
To further complicate the picture, different years have different decision points (temps, pressures, etc.) and different years have one fan or two fans and the two fans may be wired to run in series (slow) or parallel (fast) depending on either coolant temp or A/C head pressure.
Worse yet, a previous owner may have 'tweeked' the set points or made extensive modifications to the fan control system, changed the thermostat temperature, etc.
Your '86 is different from my '95 but for example, here's how mine works; if the A/C is on, the fans run 'slow' and when the head pressure rises, the fans run fast. Same is true for coolant temp. and the fans come on 'slow' at about 215 and on 'high' at about 230. I'm told that the fan power is removed above 40mph but have never rigged up a way to verify it.
Check the Helms manual for your exact year to discover how your year Corvette left the factory.
Good luck.
PS my '95 does not but I once had a '91 Buick that ran the fans for a few seconds after startup. Perhaps it was some self-diagnostic. Also, if the ALDL connector is used, the fans run.

 

[fatboyreyn]

thanks for the info. I guess I can assume there is something amiss with my fan. I'm sure it's not supposed to run all the time, which mine does. I will seek out a helms manual. Thanks again jmmcloud

 

[vigman]

A couple of things to check

I assume the fans STOP when the car is shut off, which means it's a control thing ( low current ).

2nd is yours an auto climate control or manual?

3rd do you have an "AUX" fan ( look in front of the radiator from under the car).

The MAIN coolant fan relay CONTROL lines are a Dark Blue ( Pin D ) hot in run, and a Dark green/White ( pin F ) and is grounded when A/C is called OR the engine temp is to high... In my 88 there is no dual speed for the cooling fans.

Also check on the A/C lines ( the high side, small ones ) there should be a connector on the pipe, with a Dark green and a black this is the high pressure side sensor. make sure it's connected and not JUMPED ( spliced )

The switch is OPEN @ 190 psi (or below) and closes @ 240 psi ( read A/C running ).

My .02

Vig~

 

[fatboyreyn]

coolant fan

I assume the fans STOP when the car is shut off, which means it's a control thing ( low current ).

2nd is yours an auto climate control or manual?

3rd do you have an "AUX" fan ( look in front of the radiator from under the car).

The MAIN coolant fan relay CONTROL lines are a Dark Blue ( Pin D ) hot in run, and a Dark green/White ( pin F ) and is grounded when A/C is called OR the engine temp is to high... In my 88 there is no dual speed for the cooling fans.

Also check on the A/C lines ( the high side, small ones ) there should be a connector on the pipe, with a Dark green and a black this is the high pressure side sensor. make sure it's connected and not JUMPED ( spliced )

The switch is OPEN @ 190 psi (or below) and closes @ 240 psi ( read A/C running ).

My .02

Vig~

1st the fan does stop when car is shut off. 2nd my 86 coupe has climate control, 3rd, I do have an aux. fan in front of radiator,which comes on when a/c is used. I did check all THREE a/c line sensors. Funny thing is I checked my GM service manual, checked all the wires and relays, made it throw a code 14 and 15, like the troubleshooting section of manual says, and everything checks out OK. I cleared codes, and GUESS WHAT? now the fan doesn't come on when car is started!!!! Go FIgure???????:confused I checked codes again, and the fan comes on when code reader is inserted and key is on, so I can assume the ECM is working and will turn fan on at correct temperature. Thanks for all your help buddy Where I live it's no longer hot out, so the car doesn't get very hot anymore. I have a 165 thermostat, and the car runs around 170, unless I am hammering it. I think I had a wire grounded somewhere, and all the poking around I did moved the wire:eyerole, or the little Corvette gremlin struck again!!
Thanks again for your help and advice!!!!!!!!

 

[speedmaster4]

aux fan on 86 is controlled by the sensor on the side of the left(drivers) cyl head, between spark plugs for #1 and #3...if the green wire from the sensor finds its way up to the exhaust manifold, the insulation melts, grounds the aux fan relay, and the aux fan runs whenever the ign switch is 'on'...bump in the road may move the wire off the manifold and all is ok ''until''