_Vette_

This morning I got into my vette and started it. (It was around 68 degrees out, the engine was obviously cool.) When it started, the fan came on. Its never done this before. Now the fan never shuts off unless the engine is off. I guess its good that its keeping it cool but its kind of unneeded until it warms up. Any ideas what the problem may be? Or should I get the manual bypass switch?

toptechx6

First thing I would do is check for fault codes, several DTC's turn on the cooling fan by default, some may not illuminate the check engine light.

rodj

And if that checks out , then look at the fan temp switch in the head or the A/c pressure switch.
Both , if faulty , can turn the main fan on independent of the ECM
( computer) which normally controls the fan

kimmer

If the fans are always on you are in limp mode, like was said check for codes, something is wrong.

Cliff Harris

Could be a bad coolant temperature sensor telling the computer the engine is hot. A short to ground on the sensor wire would also do that.

rodj

If the CTS is faulty engine will run like cr*p ; generally pig rich

Calderone

it's happening the same to me ! in my case i think has something to do with relays

garryowen

not sure if your 85 is the same as my 84 but, the pressure switch in the back of the A C compressor was bad and keeping fan on.

as soon as key was turned on and no matter if A C was on or not.

dvoeltz

I just had my 94 to the shop for service and discovered the AC is always on. Traced it down to the controller in the dash or the module that it plugs into. I have to pull the dash and unplag the controller. If that turns off the fan/AC, then it is bad. If not, then it is the other part.

You can also try unplugging the AC wiring under the hood. Open the hood and walk around to the passenger side. Near the firewall under the hoses is the electrical plug that controls the AC. Unplug it and see if the fan turns off. If so, you might have the same problem I have. If it is cool, just leave the AC unplugged until you can figure out which component is bad.

BTW - For those of you who listen to "Under the Hood Show", it was Russ Evans from the show who did the work on the car and discovered this problem. I had it in to have the fuel system cleaned. I didn't even know the fan wasn't supposed to run all the time.

Dave in Pierre, SD

garryowen

dave buy running all the time, do you mean your compressor was on or cycling also?

i had u-pluged the pressure cut out wire on the back of the compressor and my cooling fan then stooped.

is this the wire you unplugged or the A C clutch wires?

would like to know just a bit more. will check mine to see if i have picked the wrong switch, causing cooing fan to stay on.

_Vette_

That does happen. anything over quarter throttle it will stutter, run choppy, or die. But once the car warms up to around 185 it runs perfectly fine. The fans did shut off finally. The battery died because I left my driving lights on Charged it up and the fans work like normal.

dvoeltz

The wire the mechanic told me to unplug is on the passenger side of the engine compartment near the fender under a couple of coolant lines. By unplugging, it stopped the coolant fan from running constantly. The Tech told me the fan would still run when the water temp needed it to do so. But the fan runs constantly when the AC is on. So this will help save the life of the cooling fan until I get the problem fixed. By unplugging this set of wires, the AC compressor is also turned off.

Dave in Pierre, SD

ken019

where is the a/c pressure switch?

Cliff Harris

Depends on the year. In 1984 it's on the back of the compressor.

On my car it's on the A/C high pressure line (small connector):

DaveP85C4

This discussion seems to be for C68 (electronic controls) A/C. The C60 manual controls handle fan request differently than C68. C68 has an independent switch on the high pressure line that specifically tells the ECM to turn on the fans if pressure is high enough to require condenser airflow. This circuit and switch is called "fan request".

The small switch in this pic is the fan request switch.
The larger switch in this pic is the high pressure cut-out protection for the compressor and system.

If you UNPLUG the small switch, the cooling fan will run.

Throughout this thread, there is the belief that if you unplug the "hi pressure" switch, the fan will stop. This is incorrect on two counts:
A: There are two switches on the high side. Fan request, and Hi-pressure cut-out.Unplugging the hi-press may turn off the fan indirectly because the compressor will stop and the pressure will drop, turning off the "fan request". Unplugging the fan request, turns ON the fan.
B: The fan request switch OPENS to turn the fan ON. The fan request switch CLOSES to turn the fan OFF.

Don't believe me? Unplug the small switch with the A/C off, engine running, and the fans not on. The fan will run. Jumper the two terminal to each other, and the fan will turn off.

crowz

I did this in 1999 and its used by most as reference for the a/c part of the fan. At least from 86 back as it was for my 86 coupe yeaaaars ago. Noticed the pics were taken with the mavica camera I had then. Saved the pics on a floppy disk as you took the pictures. Seems so ancient now.

This is from my site :

This covers all the wonderful things one can and sometimes has to do to keep cool air coming out of the dash

First we'll cover the dreaded "A/C works fine till I stop". This is a normal problem that effects most 84 and newer vettes. The a/c will cool perfectly till you come to a stop and then it blows hot air. What happens is a small switch located in the lowside line goes bad. This pressure switch causes the cooling fan to cut on when the a/c is used. When the switch goes bad the cooling fan won't come on except for when the motor gets hot enough. As long as the car is moving there is enough air passing over the a/c's radiator to allow it work. When the car comes to a stop no air is passing over the a/c's radiator. I.E. it blows hot air. The fix is actually very easy and anyone can do it. The switch has a Schrader valve under it so the freon doesn't escape while the switch is being changed. The switch cost me around $16.00. I had to get it from the dealership. None of the parts places carried the switch. To test to see if this is the problem, wait till the car is cold. Usually 130 to 160 degrees. That way the temp of the motor wont cause the cool fans to come on. Now crank the car and turn on the a/c. The cooling fan should come on within 30 seconds. If it does not then this will cure your problem. If it does then the switch isn't bad. Another test if to unplug the switch. As soon as the switch is unplugged the fan should come on. Also this is a cheap fix, while your waiting for the new switch to arrive Just leave the plug off the switch and the cooling fan will run all the time, as long as the engine is running.



This shows the general area of the switch.



Here we can see 2 switches the small one is the fan switch and the large one is the low side cutoff for the a/c to cycle.



The one I'm pointing to is the one to change. Remove the plastic plug from the switch and unscrew it counter clockwise. I.E. turn it to the left Try to do this fairly fast to let as little of the freon out as possible. The simple screw the new switch in place and again do it fairly quickly. The switch doesn't have to be TORQUED down so don't get carried away. Schrader valves don't like to be tightened onto hard Then plug the harness back onto the switch. Now crank the car and turn on the a/c. The cooling fan should come on within 20 seconds of the a/c coming on. If it does then all is well.

crowz

Also back in the day we used to unplug the a/c pressure switch when doing runs at the track to keep the fan on all the time.

DaveP85C4

Uh, the large one next to the small one is NOT the "low side cut-off.." How can it be? It is installed in the HIGH SIDE just like the fan request switch. The large one is the High-Pressure cut-off. It is for protection against over-pressure in the system. Under normal conditions, it never activates.

The Clutch Cycling Switch can be seen on the LOW SIDE line in the bottom left corner of the pic.

Cliff Harris

That is incorrect. The cooling fan cycling switch is the one next to the high pressure cutoff switch.

The switch on the low side is the safety switch that cuts off the compressor if the refrigerant is too low.

Hot Rod Roy

Cliff, please re-evaluate your answer. There must be some misunderstanding here. Dave is talking about the Clutch Cycling Switch (aka "pressure cycling switch"), not the Cooling Fan Cycling Switch.

It's true that the low side pressure cycling switch will not allow the clutch to engage if the refrigerant ambient pressure is too low (insufficient refrigerant), but if there is enough refrigerant pressure in the system to close this switch, the clutch will engage. The compressor will then operate, which will increase the high side pressure and reduce the low side pressure. When the low side pressure is reduced enough to open the low side switch, the clutch de-activates and the low side pressure will begin to rise. When the low side pressure rises enough to close the low side switch, the compressor clutch engages again, repeating the cycle.

The cooling fan switch on the high side actuates (opens or closes, depending on the year of Corvette) only when the high side pressure rises above the ambient refrigerant pressure, which indicates that the compressor is operating, and therefor the radiator fan(s) should be turned on.

The other high side switch is only a safety switch, as Dave explained.

(Ambient refrigerant pressure is the refrigerant pressure at room temperature.)