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1985 Temperature Sensors

renegadeslave

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Sep 28, 2013
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1985 Black Corvette Coupe
Hello everyone!

My Vette has recently had some overheating issues and I have diagnosed two problems- One, my main cooling fan sounds horrible. I'm installing a replacement motor tomorrow. Two, the main cooling fan was not getting the signal to come on.

I have found the temperature sensor between cylinders 1 & 3 for the temp gauge. I found the sensor between cylinders 6 & 8 for the auxiliary fan. Where is the sensor that provides information to the ECM? I've read on here that it's on the front of the intake manifold, but I haven't been able to locate it.

I've found the relay- works fine. That's how I know the fan sounds like it's about to die. Seems the issue is either the last temperature sensor or the ECM itself. ;shrug
 

renegadeslave

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1985 Black Corvette Coupe
Welp, I believe I found the ECM temperature sensor. It seems to be reading the temperature correctly (644 ohms when the engine was approximately 130*). Checked all the circuits and grounds and determined the ECM was not putting out the correct signal. Replaced the ECM. The main cooling fan still will not turn on when it's supposed to. However, when the ECM temperature sensor was unplugged while the engine was running, the main cooling fan ran constantly. That leads me to believe the ECM temperature sensor is the problem, but that doesn't make sense according to the schematics and explanation in the service manual.

Oh, and the check engine light did not come on until the car was running with the sensor unplugged.

Any suggestions?
 

vetteoz

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1990
However, when the ECM temperature sensor was unplugged while the engine was running, the main cooling fan ran constantly.
That leads me to believe the ECM temperature sensor is the problem, but that doesn't make sense according to the schematics
The CTS may be indicating correct at one ( cold ) temp as you tested but not at a higher temp ?
Have you read the resistance at a higher temp and seen how it relates to the chart ?

Could also do the FSM CTS diagnostics; it may be a wiring problem.

CTS diagnostics

http://www.chevythunder.com/cts_diagnostic_pg2.htm
 

renegadeslave

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Messages
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1985 Black Corvette Coupe
Thanks for the suggestions. It is reading the correct resistance at a higher engine temperature. I've checked all the circuits and grounds. I've gotten the fan to come on by jumping the pin (D2, I think) to ground. So, it all points to the ECM not controlling the fan like it should. My next thought is that the PROM is malfunctioning. I got a remanufactured ECM and had to pull the old PROM out to put in the "new" ECM. Could the PROM be the problem? I'm not having any other issues that I know of.
 

BigJimZ16

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Portage, Indiana
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2010 Grand Sport Vert JS Blue
Thanks for the suggestions. It is reading the correct resistance at a higher engine temperature. I've checked all the circuits and grounds. I've gotten the fan to come on by jumping the pin (D2, I think) to ground. So, it all points to the ECM not controlling the fan like it should. My next thought is that the PROM is malfunctioning. I got a remanufactured ECM and had to pull the old PROM out to put in the "new" ECM. Could the PROM be the problem? I'm not having any other issues that I know of.


Bad PROM, doubtful. Bad ECM probably not, most likely a wiring or connection problem at the ECM. Do you have a factory service manual for trouble-shooting procedures?
 

renegadeslave

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Messages
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1985 Black Corvette Coupe
Yep, I have done all the troubleshooting suggested in the service manual. I have traced the wires I could and saw no problems. Pin D3 to ground activates the relay and the fan with no problem. The ECM is getting the correct signal from the sensor (correct resistance measured at cold and hot engine temperatures at the ECM) and the engine is running fine. Also, the Check Engine light is not on. It did light once the sensor was disconnected. The fan also ran continuously with the sensor disconnected. Is that normal?
 

vetteoz

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The fan also ran continuously with the sensor disconnected. Is that normal?
:thumb

The CTS is one of the " essential " sensors required for ECM operation.
Without that signal, the ECM goes into " limp home " mode where it essentially runs the engine off a preset program without sensor input.
Part of limp home is the fan running full time
 

renegadeslave

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Messages
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1985 Black Corvette Coupe
So that shows that the ECM is getting a signal back to the fan. :hb

The only anomaly I found was that D3 is a brown wire instead of a black/white, and there is a slight measure of resistance, about 1 ohm. I'm thinking if that was the problem, I would have a lot of other issues....
 

vetteoz

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So that shows that the ECM is getting a signal back to the fan. :hb
That shows the ECM is capable of running the fan ,( all of the control circuit is good from the ECM to relay,the question is why does it not do it when it is supposed to.

Does the main fan run when the A/c is turned on?
 

renegadeslave

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1985 Black Corvette Coupe
Yep, the main fan runs when the A/C is on.

The sensor I have doesn't look like the replacement sensor that's coming up on Advance's website. Mine has a pigtail with a white connector. Hmm.
 

vetteoz

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Mine has a pigtail with a white connector. Hmm.
Not a stock CTS then.
The TPI CTS ( same unit is used for the MAT sensor under the plenum on a TPI ) is a 2 wire unit with a female plug ;No wires attached

See
85-92 TPI TBI Coolant Temperature Sensor


Image1-3.jpg
 

renegadeslave

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Sep 28, 2013
Messages
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1985 Black Corvette Coupe
Thanks for the pictures. I have ordered a stock CTS and a new connector. Hoping that is the problem though it doesn't seem likely if the other sensor was giving the correct resistance. I'll be getting the parts on Monday and will post an update!
 

renegadeslave

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Sep 28, 2013
Messages
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Location
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1985 Black Corvette Coupe
Just got around to replacing the CTS yesterday. And... the main fan still does not turn on with the engine running. However... it will run if the engine is the appropriate temperature with the ignition in the "ON" position (but the engine itself not running). The fan does still come on with the engine running and the A/C on.

Any guesses?
 

WhalePirot

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1984 White Z-51/ZF6-40/Shinoda body
From what I've read, it sounds normal, now. Early '85s were like '84s, with two sensors, one in each head. The right sensor triggered the fan by grounding the relay (as does the A/C-ON), getting power with the IGN-ON, engine running or not. If the sensor 'sees' a temperature below its target value, the circuit remains open; the fan stays off. This conserves fuel and power. I believe the fan is shutoff above 35-40 mph, but that may be a later model (move advanced ECM) feature; airflow being sufficient for cooling above that speed.

Once upon a time, finding lower trigger temperature sensors, to drop the seemingly high operating temperatures, was a problem, but no ECM reprogramming was required on the early C4s.

Verify proper ops by starting a cool engine, idling to warmup, watching for fan startup at design temperature of the new CTS. :w
 
Last edited:

renegadeslave

Member
Joined
Sep 28, 2013
Messages
9
Location
Maryland
Corvette
1985 Black Corvette Coupe
From what I've read, it sounds normal, now. Early '85s were like '84s, with two sensors, one in each head. The right sensor triggered the fan by grounding the relay (as does the A/C-ON), getting power with the IGN-ON, engine running or not. If the sensor 'sees' a temperature below its target value, the circuit remains open; the fan stays off. This conserves fuel and power. I believe the fan is shutoff above 35-40 mph, but that may be a later model (move advanced ECM) feature; airflow being sufficient for cooling above that speed.

Once upon a time, finding lower trigger temperature sensors, to drop the seemingly high operating temperatures, was a problem, but no ECM reprogramming was required on the early C4s.

Verify proper ops by starting a cool engine, idling to warmup, watching for fan startup at design temperature of the new CTS. :w

Actually, there are three sensors. One on the driver's side of the head that sends the signal to the dash temp gauge. One on the passenger that sends the signal to the auxiliary fan. And the one on the front sends the signal to the ECM. That's the one I replaced. I know it's normal for the fan to run with the ignition in the "ON" position; however, the engine can be the appropriate temperature and running and the fan will not be on. The fan turns on for a split second if I turn the key to "OFF" (after the engine is running). If I turn the key to "OFF" and the put it back to "ON" without going all the way to start, the fan comes on. As soon as I start the car... the fan goes off.

Yes, I have started a cool engine and idled to warmup. The aux fan starts appropriately, but the main cooling fan does not come on.

Thanks for your help and my apologies for not explaining that better the first time!
 

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