I thought I had this problem figured out, but anyways.....I have a 1990 vert. My cooling fans are not turning on when the temp gets hot. I don't have digital gauges but I would say up to 250 or more if I let it. However the fans will turn on like they are supposed to when you have the A/C on. I have already replaced the coolant temp sensor in front of the intake(2 times), the collant temp switch in the right head, and the ECM. I am not getting any codes. The fans also work in test mode by shorting pins a to b.

I also read the wires going from the ECM to the sensor and shot for voltage at the sensor and got 5 volts. I got the voltage at the switch, and shot the wires going to the relays.

When I let the car heat up and check the sensor for ohms it drops down to about 180 ohms when hot and about 4.5k ohms when cold so I know that is working.

Any other components related to this system that would cause my fans to not come on when they are supposed to?

 

The ECM grounds one coil wire on the fan relay to turn the fan on. I would check continuity from the pin on the ECM to the fan relay coil wire, it may be open circuit.

 

I checked that wire doing end to end continuity. It was good. Then I kept the relay unplugged and started the car and let warm up to where the fans should come on checked to see if the ECM was giving that ground and it never does. So I know something is not makeing the ECM switch over inside. after I knew the car was warm enough and the fans should have kicked on I turned the A/C on and got a good ground at the same wire on the relay.

 

The ECM then must be defective, or the coolant sensor at the front of the block is not sending info to the ECM. The socket on the sensor fails, so you should try measuring its resistance at the ECM pins (ECM unplugged) to prove that the sensor is connected to the ECM. Further proof can be had by using a scan tool which reads the coolant temp through the ECM and the coolant sensor.

 

When I checked the readings for the sensor I was going from the ECM. I agree with you that is the best way to see if it is hooked up. Where can I get a scan tool from? and how much are they. Does auto zone loan them out or rent?

Also does anyone know how many relays there are for a 1990 cooling fan. I know the 2 mounted on the rad, but I see one behind the battery also. Does anyone know were that goes to or what it is for?

 

My helms is vague on this to say the least but it makes mention of the VSS (vehicle speed sensor) controlling certain aspects of fans. It doesn't say or show how it's wired to ECM or how it effects it other than it turns fans on under certain conditions when vehicle speed is under 12 mph or under 55mph and off under certain conditions when vehicle speed is over 12mph or 55mph.

I wonder if you could have a problem with the VSS end of the scheme??? Sorry I offer no solution...just something else to check in to since you've seemingly checked everything else. Not trying to offer a problem without a solution, just pointing out that vehicle speed and engine speed plays a role.

Here's a couple of places it makes mention of it..I appoligize for poor quality of image. I dont have a scanner so I just took digital pics out of my Helms 1990 Corvette GM servive manual. Hope this helps.

 

My 90 has two near battery as well as on the radiator. One is an upshift relay (mine is auto trans) The other is AC blower motor relay.

 

I agree with you about the VSS. I thought the same exact thing when I found out the ECM didn't work. I just don't know where to go with it though. I can't find any good schematics that show the wires for it and where they go. I guess I might have to do get a diagnostics done or something. This one has me stumped.

Thanks for the thought!

 

If all else fails you could call Gordon Kilabrew and see what he advises.

Or, you could wire a manual switch. I wrote a tech tip that touches on this. If you do it my way then the AC pressure switch and others still work as normal. It just gives you full control of when they are on and off otherwise. It's a ground switch only. Please never wire a 12v hot switch to activate fans. Too much current draw to run safely without a relay. Just use a ground switch to same point ECM grounds relays.

Here's the Tech tip...enjoy.
https://www.corvetteforum.com/techti...=282&TopicID=2

 

thanks for the suggestion!

 

You have some strange symptoms.

Unplugging the CTS plug should not cause the fans to come on. With the plug disconnected, the ECM would see 5 volts and think the engine temperature to be
-40 F degrees.

If you short the connector, the ECM would think the engine temp. to be over 266 degrees F and turn the fans on.

You say you've replaced the coolant temp. switch in the right side head twice.

Are you saying the sensor between spark plug 6 and 8
passenger side head? If so, that is a sender used for the temperature gage in the cluster.

Do you have a temperature switch in the driver side head between spark plug 1 and 2 ? The manual shows a temperature switch which shorts out at a certain temperature and turns on Fan 2 (Fan on passenger side). It is a backup if the ECM fan circuitry should fail.

Since you've replaced the CTS, ECM and verified the wiring the only thing left would be the Mem Cal.

The Mem Cal contains a Eprom mounted on a small circuit board and some circuitry to allow the engine to run in case of ECM failure.

The Eprom contains the informaton when the ECM should turn the fans on or off.

 

I checked between the 1 and 3 spark plugs and there is just a plug there. I don't have that switch.

Yes I replaced the switch on the pass. side once, and the CTS 2 times.

The Eprom that you are talking about, is it the stick you take out of the ECM? I haven't tried that yet.

 

Yes it's located behind a small metal plate on the ECM and has two eject handles.

When you swap out a ECM you remove the Mem Cal and install it in the new/rebuilt ECM. Nothing else needs to be changed.

If you purchase a new Mem Cal, take the original one with you to the dealer. There is a sticker on the Eprom with some letters on it they need to match up.

The Eprom contains engine calibration information specific to your vehicle.

Cost for the Mem Cal should be less than $100.00.

You can purchase software and make changes to or examine the information in the Eprom from Tuner Cat. The base program is like $70.00 and you need the definition file for your ECM which is another $20.00. Everything is done by e-mail.

You'd need a Eprom programmer to read the Eprom and a cable setup to connect to the Eprom since it's soldered in. Many people unsolder the Eprom and install a socket.

 

Thanks for the info! I guess its a trip to the dealer after all.