Howdy,
I recently picked up an 86 Corvette, a dream car of mine, and I've been going through it fixing little things. Both the oil temp and coolant temp gauges read "LO," which I assume is either a sending unit or a plug/wire issue. I've got a coolant temp sending unit/plug kit on its way.

After driving the car home the other day, I noticed that it was burping into the coolant reservoir after shutdown, either an indication of a failed radiator cap or a hot radiator. This worried me, since my sending unit hasn't arrived yet. I replaced the radiator cap and thermostat, despite knowing that this would make it harder to deduce exactly what the root cause is. Upon completion, I test drove the car in the rain and noticed that it felt quite warm upon return and while idling with the hood up I noticed that the radiator fan was not turning.

Assuming that my engine was indeed up to temperature (I believe it was), my question before digging into it today is this: Is the radiator fan turned on by the single-wire temperature probe that plugs into the radiator? I have read that it is based on an actual numeric temperature reading that prompts a command from the ECM, but if that is the case what is this other probe for? I have a green wire running from a probe on the passenger side of the radiator to the fan pigtail.

Additionally, I notice that someone has removed the AC lines on my car under some previous owner. I have also read that a bad AC pressure sensor can prevent radiator fan operation, could the fan be off because my AC system is open?

As I get a temp sending unit in I am hoping I can at least see what temperature the car is running at. I don't know anything about the old thermostat, but the new one opens at 195 degrees Farenheit. As soon as I have a minute I'll be poking around with a voltmeter and changing the fan relay to start with.

 

Welcome to the forums. 195 thermostat is gms choice,(emissions). I like 185 myself a and a few degrees advance. That system (I'm going off of memory) had two temp sensors around the thermostat. One was for the dash temp gauge and the other one was for the ECM. The only way to tell which one was which was to pull the wire (engine off key on)and ground ot test light the sensor wire and watch the dash gauge, it should go to the high side. The radiator sensor must be the fan temp sensor, they vary on temp settings to check the fans on, yours might be bad. if you ground that green wire (engine hot or cold key on engine off) the fans should check on. It shouldn't be boiling over. The A/C systems have a pressure switch in the high pressure line between the the compressure and the evaporator that detects high gas pressure and turn the fans on. If the fans fail to check on with high pressure it will blow the high pressure hose. If you look between the a/c compressor and the evaporator where the a/c line should be there should be a connector that goes into a pressure switch, like the connector on the dryer receiver. should be there somewhere if you jump wire that connector it should turn the fans on. Check the green wire to the radiator and see if it's working. If you ground that wire you should hear a solenoid check on.

 

After looking at a picture of the A/C system I'm not positive if the fans are controlled off the high press line or the dash switch. I see the receiver dryer is between the compressor and the radiator. the low pressure switch is on the evap. box and the high pressure switch is below it on the inner fender. The dash switches in those years controlled the a/c compressor and fans in the a/c and defrost positions and the high pressure disconnected the compressor when the pressure got to high. You should be an to turn the radiator fans on with the a/c switch unless the fuse was removed. That should be by the right side door.

 

Thanks for the detailed replies, Thomas.

Tonight I took some time to try a few things.
First, I started the car and engaged the A/C, but the fan did not turn on.
Then, I unplugged the coolant temp switch and grounded it to the frame with a jumper. No matter where I tried grounding it, I never got the fan to kick on.
After that, I unbolted the fan, unplugged it, and tried running it with a pair of jumpers to the battery. It spun up pretty freely, and the amp draw fried my jumper (predictable).
Then, I removed the cooling fan relay, and tried applying 12V jumpers to the two actuating pins in the center. I observed the relay clicking in my hand.

So, if it isn't the relay and it isn't the fan motor, it must be a failure to get a signal voltage to the relay, right? One thing I can think of is to check for reference voltage on the temp sensor, maybe that would tell me something? Even then, I wonder if that temperature signal runs back to the ECM and the ECM then sends voltage to actuate the relay.

I should acquire my replacement temperature sending unit and my new relay tomorrow. But if those don't fix it, where to go from here?

 

It's always good to check all the fuses first and wiring contacts. It might be time for a service manual to troubleshoot that wire that does nothing. YouTube is also a good source of information. There has to be a reason the temp wire is not working.

 

Helm Inc. is a good source for factory service manuals. Some aftermarket Corvette vendors have just a wiring diagram that you can order also. You might use the engine block as a grounding source rather than the frame.

BTW, welcome to the forum.

 

Thank you both, all good tips. I would like to note that the green wired probe in the radiator is not a temp sensor, but instead is the coolant level sensor.

I found a 10A fuse in the 3A C/Fan slot, so it's possible that something could have been damaged by an overdraw that the correct fuse would have prevented.
Using a voltmeter, I can get 11V at the fan plug and 11V across the two power wires that the relay closes. They are hot with the key off or on. When jumping those two wires as if the relay was closed, I was unable to get the fan to spin. However with an ammeter, though the pins are finicky, I was briefly able to read 3.2A on my meter.

When I jumped the fan to the battery the other day it spun but it obviously drew a heavy enough amperage to melt the jumper in my hand. I am beginning to wonder if maybe the problem is just a bad fan motor, and it kept overdrawing and blowing the 3A fuse which prompted someone to put the 10A fuse in its place. I think my next move will be to replace the fan and go from there.

If the fan did happen to draw 4-9 amps through that 3A circuit, do you guys have any ideas about what could have been damaged? I don't have my wiring diagrams yet.

 

I suppose it could have melted some other insulation causing it to short out to another circuit. The higher than normal current could also possibly damage the relay contacts.

 

The Fans will draw 20 t0 30 amps when running they have a fuse or fuseable link somewhere between them and 12 volts source. Using a small direct jumper wire between fans and battery will get hot. I would use the gm recommended fuse for the fan relay controller. The experts prefer a 180 degree engine thermostat with an engine fan switch of 200 to 205 on the engine left side cylinder head between spark plug 1 and 3, Witch is about 20 degrees higher than the thermostat so the fans don't run constantly. If you run the 195 thermostat you would want a temp fan switch 20 degrees above that, that would be the same as the stock switch 215- 220 degrees. So be aware not all fan temp switches are not the same. If a fan turns easy and doesen't overheat the lead wire It's ok most of the time. But they do go bad.

 

I did see one idea where you switch the dark green wires on the fan relays and let the ECM control the Aux fan and the temp switch control the main fan, not sure of the advantage.