Temperature Light

[reholmes]

My 1987 TPI engine runs at a constant and consistent 200 degrees, unfortunately the overheat light comes on at that temperature. Do I have a faulty sensor/sender unit? Would replacing it correct the issue?

 

[John Robinson]

Is the 200 degree reading from the analog or the digital reading? The first thing I think you should do is try to verify what the actual temp of the water is.

Have you had the system pressure checked?

Have you replaced the radiator cap?

Do you have to add water to the overflow tank?

When you turn off the engine do you hear gurgling or popping noises from engine?

Because of the age of our cars we have to look at things like a bad wire connection or a broken wire or a bad ground. In other words everything is suspect from the wires to the sensor. You may be right that it is the sensor for the dash light that is bad. However if it is good and the warning is coming on then you have to look at the wiring to see if it is shorted somewhere. Look at it this way if you turn on the water to wash your hands and nothing comes out of the faucet . You check with your neighbors and they have water and your all on the same water line. Now you know that the water is there. Next you find out the water is running at other faucets in the house. Now you know that the water is coming to the house so that water line is not broken from the source. Now you have isolated the problem of no water at that faucet to the line from the house feed to the faucet water line possible being broken. This same process of elimination is what has to be done when we have a problem with our cars. Since my car is a 93 I don't have any hands on knowledge of your model year car. Before long someone with a car closer to your year will be along and they will have a better idea of what may be your problem with your year C4.

 

[reholmes]

Is the 200 degree reading from the analog or the digital reading? The first thing I think you should do is try to verify what the actual temp of the water is.

Have you had the system pressure checked?

Have you replaced the radiator cap?

Do you have to add water to the overflow tank?

When you turn off the engine do you hear gurgling or popping noises from engine?

Because of the age of our cars we have to look at things like a bad wire connection or a broken wire or a bad ground. In other words everything is suspect from the wires to the sensor. You may be right that it is the sensor for the dash light that is bad. However if it is good and the warning is coming on then you have to look at the wiring to see if it is shorted somewhere. Look at it this way if you turn on the water to wash your hands and nothing comes out of the faucet . You check with your neighbors and they have water and your all on the same water line. Now you know that the water is there. Next you find out the water is running at other faucets in the house. Now you know that the water is coming to the house so that water line is not broken from the source. Now you have isolated the problem of no water at that faucet to the line from the house feed to the faucet water line possible being broken. This same process of elimination is what has to be done when we have a problem with our cars. Since my car is a 93 I don't have any hands on knowledge of your model year car. Before long someone with a car closer to your year will be along and they will have a better idea of what may be your problem with your year C4.

John,

Thanks for your response. Temperature readings are from a Stewart-Warner analog gauge. Cooling system functions flawlessly--running temperature is constant and consistent, and as I understand typical for an L-98 running 180 thermostat. I have disconnected the sensor/sender, front of engine under the TB, and get expected results--no gauge reading and constant light. I might add, this is essentially a new car: 4000+ miles, 1987 build with new engine, stored in climate controlled conditions for 25 years, thoroughly checked out one years ago, all components like new--the car is pristine. Not to say components can't deteriorate over time even if unused.

 

[WhalePirot]

Consider where the temp probes are located, as the water temp in different places varies plenty.

# 7 gets the coolant last and the temp drop across the radiator is easily 20*, +/-.

 

[John Robinson]

The sensor you have disconnected is the one the ECM uses to sense when the engine is warm enough to go from open to closed loop. Based on your additonal information you might check the wire circuit ground for the over heat light. Because these are plastic body cars the ground connections are probably more important than on a normal car. As I am sure you are aware but it is worth repeating. The ECM reads the ground wire values from the sensors to make adjustments and by the same token the instruments also work off the voltage on the ground wire. Normally you would expect the ECM to read the hot voltage but it does not. That is why when you read the manual in the electricla section it almost always first wants you to run a diagnostic check on the ground wire values. I am only guessing here but since the temp sensor you have disconnected is how the ECM know to go to closed loop it may be that by not having a signal from there it is interpriting that as an over heat and turning on the light. I am not sure on your 87 but on my 93 that ground wire and the TPS and the MAF and a couple of others all go to a common ground bundle in the wiring harness and then to the ECM.

Judging from the picture your car looks prestine. I wish mine looked that good. What a great find.

 

[reholmes]

Consider where the temp probes are located, as the water temp in different places varies plenty.

# 7 gets the coolant last and the temp drop across the radiator is easily 20*, +/-.

Thanks. Correct, but as the light and the gauge are coming from the same probe I have to assume that the light is coming on at what the gauge is reading-awkwardly stated :-[

 

[reholmes]

The sensor you have disconnected is the one the ECM uses to sense when the engine is warm enough to go from open to closed loop. Based on your additonal information you might check the wire circuit ground for the over heat light. Because these are plastic body cars the ground connections are probably more important than on a normal car. As I am sure you are aware but it is worth repeating. The ECM reads the ground wire values from the sensors to make adjustments and by the same token the instruments also work off the voltage on the ground wire. Normally you would expect the ECM to read the hot voltage but it does not. That is why when you read the manual in the electricla section it almost always first wants you to run a diagnostic check on the ground wire values. I am only guessing here but since the temp sensor you have disconnected is how the ECM know to go to closed loop it may be that by not having a signal from there it is interpriting that as an over heat and turning on the light. I am not sure on your 87 but on my 93 that ground wire and the TPS and the MAF and a couple of others all go to a common ground bundle in the wiring harness and then to the ECM.

Judging from the picture your car looks prestine. I wish mine looked that good. What a great find.

Good point--the curse of the floating ground. As a Mera owner, I should be sensitive to that.

 

[Vettelt193]

Another point is none of mine ever ran constant. In traffic the temp would swing up until the fans came on. On the highway I'd be much lower than 200 and that would vary based on the weather and speed. With the 180 degree stat in cool weather cruising 55 I would be right above 180. With a 160 stat temps would go as low as 165 but usually 175 in good weather, 185 highway in the heat. Again, swinging all the way up to 230 in traffic.

You have a Ferrari replica, correct? And that is sitting in a C3 chassis? It could be a setup issue. It could also be air bubbles in the cooling system. Chevy changed the C4 half way through the run to combat air bubbles. The later ones have a radiator overflow and cap above the top of the engine and radiator to help migrate air up and out of the system.