Hi, thanks for checking out my post.

My 1990 runs very hot when in traffic. Water temp is climbs up to 260 and stays buried there. The cooling fan on the passenger side of the radiator never comes on. The one on the driver side only seems to come on when I turn the AC on. Even when the engine is idling hot at 260, neither cooling fan will come on. ONLY the one on the driver side when I turn the AC on.

Not sure where to go from here. Anyone else have similar symptoms on a 1990 C4? Any help woul be MUCH appreciated. THANKS!!!

 

260 is very hot, you need to find out why the fans are not coming on.
Do you have a scan tool?

 

I've heard this thing happnin' before - no coolant fan unless the A/C is on.

You search this forum with those keywords, you will get hits; probably the solution too...

 

I know of at least one cooling problem that has stumped a couple of pros.

 

Have you checked the relays on the DS of the shroud to make sure they work?

 

There's two, just swap them and see if it makes the primary fan come on and not the passenger side, if so that could be the problem.

 

the fan issue needs to be addressed..BUT...junk the the fins is the #1 cause of the C4 overheating. the radiators need to be removed and blasted with air & a pressure washer every couple years. dirt, rocks, sand and everything else goes flying out. one very easy way to test if yours needs a cleanin is to get the car up to 240ish and spray the radiator from under the nose with a garden hose for a little bit. if the temps drop like a sinker you need to clean the radiator.

 

Hey Guys,

No, I do not have a scan tool. I don't even know what that is.

I am going to do some more research here, and try some troubleshooting today. I have the Haynes manual, plus I am going to try the idea that someone mentioned here of swapping the relays. Will post my results. Thanks again guys!!!

 

A Zone, or Advance A will scan for free. I would do that before buyin' tons of parts.

In the meantime, turn on the heat. (and take note if no heat, you got blockage at the core).

 

After you check relays, check sensor/switch in engine heads and wiring. You can go to a lower temp switch for about $15 to help you run cooler.

 

There is no temp switch in a '90. Both fans are computer controlled. There is nothing in either head relating to the fans. It is possible the ECT sensor is bad, but that should cause driveability problems also. More bogus advice and he isn't even here. BAD duck!

RACE ON!!!

 

Are you getting senile. The same switch/sensor is used in the 1990 as used in my 1989. Stick to your antique cars old man and you won't look like such a fool!

A little more info for the OP. If no thermal switch was previously installed in your 1990 the sensor hole may have a plug in it. You can remove the plug and put a thermal switch in wired to the relay. You can get the switch and a wiring diagram from Mid America. It is a simple job and over rides the ECM. The switch is 15.00 and turns the fan on at about 200* and off about 185* your main fan will remain under the control of the ECM. This helped cool down my 1989.

 

My autozone doesn't have an OBD1 scanner. Only scans on 96 and above. (something like that).

 

260F will cause a head gasket failure. Check your radiator it might be blocked. I would not drive it when it gets that hot. I guess you should check those fans. It could be the main fan needs to be replaced.

 

This is a great thread. Certainly hit the spot with my recent issues of moving to Texas with outside ambient temps in the low 100's.

While getting my emissions test done yesterday...during the 45 minute idle my '91 also overheated.
I got it home...but not much else...and I felt lucky to get her this far. So today I replaced the radiator cap and put a 160 degree thermostat in it hoping it will help.

I am very interested in the "fan switch" previously mentioned. My fans come on, but not until the car is well over 200 degrees (closer to 240).

Is this an easy install? I have the factory manuals (including the electrical supplement), and have found the referenced page in the electrical supp regarding the fan switch - but at times feel these books are a bit over my head.

Thanks again for the timely thread and advice!

SOF Vette

 

There is a plug between Sparkplugs 1 and 3 on the drivers side on the 1990 and 1991 cars.
You can remove that plug and install a sensor/switch and wiring harness to take fan control from the ECM. This is a simple procedure. Go to Mid America Corvette web site and look under cooling systems.

 

Well, since my test drive was a FAILURE!!!

Mid America here I come

Thank you Duck!

SOF Vette

 

that's actually not a bad idea since only one water jacket in the 90-91 is used for the temp gauge. this whole post has me thinking that a 10 buck 180 stat and a new radiator from corvette recycling for 99 would solve this problem. there's a reason this car is overheating and my money is on a radiator that hasn't been removed and cleaned out in awhile. as old as the 1990 is now just buying a new one while they are cheap is the way i would go.....did go

 

The '90 (as all our Vettes) left the factory without any cooling problems and it's better to get it back to what it once was. On the '90, the ECM drives both fan relays (before '90, it only grounded the Main Relay) based on inputs from the Engine Coolant Temp Sensor and/or the a/c pressure switch. The first thing you need to do is to verify that you actually have an overheat situation. Plug in a scan tool and compare the Engine Coolant Temp Sensor reading to your Display. If they're within 5 degrees of one another, they're probably ok. If not, you may have a faulty Display. Otherwise, while observing the fans and with the Scanner, the Main Fan should come on with a reading of 226 degrees and that should cool it back down to something in the teens. If the system is clean, you shouldn't see the secondary coming on at all, but to test it, you can partially block air flow by placing the nose up against the garage wall and when the Coolant Sensor hits 228, the Secondary should come on. If the fans are working within specs, you've got something blocking air or coolant flow - debris; maybe a broken shroud; or since it's a '90, you might want to make sure that it isn't blowing exhaust through the head gasket at #7 and if the coolant looks like crap or the heater core is plugged up, you'll probably want to look into that.

To the test the a/c side, it's better to use a manifold gage set, but the scanner will at least give the voltage from the a/c pressure sensor and each volt is about 100 psi. System design is maintain a high side pressure of about 200 psi (2 volts from the a/c sensor) and with the a/c running, the main fan should be on at about this number; the secondary around 228 psi (or 2.2 volts). Note, if it's much below 65 degrees, you might not achieve this pressure, so wait until you have at least 70 degrees to test it. As a side note, overriding ECM control of the fans with the a/c running is an easy way to get into trouble - should the fan(s) fail to come on, your high side will rise to the point that it can blow up - hopefully, the a/c sensor signal will shut it down before that happens, but why risk it?

 

Hi,

I have followed all of the troubleshooting procedures in the Haynes manual (my vette is a 1990). I cannot isolate the problem! Here is a summary:

I grounded terminals A-B in the troubleshooting plug under the dash. Both fans come on.

I checked for voltage at the various terminals in the relay connectors. All okay.

Replaced both relays. No change, fans still dont work.

I installed fused jumpers in the relay connectors, and the fans both come on when jumped. Okay.

I replaced the cooling fan temperature switch. No change.

I grounded the cooling fan temperature switch. Neither fan comes con. ***NOTE, when I grounded this switch, the coolant temp indicator buried itself at the high limit. I believe there is another sensor in the intake for the gauge. Not sure why this happened. Anyone else see this???

I am starting to think that the problem is in the ECM. I am considering installing manual switches in the dash.

Any thoughts or guidance would be appreciated fellows. THANKS!!!