VCCA Home Forums TECHNICAL - 1945 AND NEWER 1965-1970 Monte Carlo TCS

We are finishing the dash rework on my friend’s 1970 Monto Carlo. The instrumentation includes the warning/idiot lights for oil pressure, generator (really alternator) and temperature.

The problem we are trying to solve is to prove the that temperature light is working properly. Right now the oil and generator lights come on when the ignition switch is turned the to “On” position before starting the car. The temperature light does not light. I cannot “force” the temperature light to come on by grounding the wire that attaches to the temperature sensor on the right (passenger) side head.


I think part of the solution is understanding the TCS that was originally the car. That system used a combination of inputs including engine temperature to control when a vacuum signal would be sent to the distributor. It was a very early form of emission control.

I have a shop manual and wiring diagram. I do not have access to an owner’s manual.

To make it more challenging the have been numerous modifications over the life of the car. The engine is a 350 from a 1975 Chevelle convertible based on the numbers on the pad on the block. The distributor is a more modern HEI unit. The stock manifold and carburetor have been replaced with aftermarket units.

I am not interested in re-installing the TCS. I simply want to prove that the temperature light is working correctly and will come on if the engine overheats.

Any ideas about what to check are appreciated.

Check to see that the bulb for the temp light is good. You could also have an issue with the printed circuit behind the instrument cluster where the bulb socket makes contact. The printed circuits are available for about $60;

Thanks for the basic suggestions. I agree that we need to start simple when troubleshooting electrical issues.

In this case we have installed a new printed circuit. Everything tested fine with the dash assembly on the work bench.

With the dash in the car I can make the temperature light come on by applying 12 volts to the copper strip where the large connector fits.

I cannot get that light to come on by grounding the lead to the temperature sensor. I can measure 12 volts at that wire.

I know I am missing something simple but cannot figure it out.

Made some progress today. I checked my work and did have a ground wire in the wrong position on the back of the instrument panel.

Now I do have a high beam indicator light which I did not have before. The oil and temp lights come on when the key is turned to run. The temperature light comes on when you crank the engine. Based on some additional research that is correct.

I did determine that the dual post temperature sensor in the left head has been removed. The wiring harness has been modified such that there is only one wire to the single terminal sensor on the right head.

My next step is to determine if the TCS time delay relay is still in the circuit. That might explain why the temperature light did not come on immediately when I grounded that wire.

Got it resolved. FINALLY!! It was a generic electrical problem so I will post in the 12 volt forum.