After driving on the highway for about 10 minutes, my temp light comes on. Even though the guage shows about 160-180. After a couple seconds the light may go out and flash for a bit. If it stays on, the guage drops to 100 degrees.

The radiator and overflow are full and the car doesn't overheat or gurgle when shut off.

It also seems that if I shut the car off and restart the light goes out and gauge reads correctly.

85 Berlinetta, LG4 carbed, automatic (if that helps)

 

Do those cars have a computer with fault codes to tell you what is wrong?
If you have an anomoly between two temperature indicators, it is best to check the indicator wiring and sensors first.

 

I think that you may have a minor problem with the cluster.......

Just an educated guess really !

I looked into the wiring info & can see that the light is set off by a temp setting inside the cluster. Once the temp reaches a certain point the light is activated by the circuits inside the dash. ( Not set of directly by the gauge ) Evidently, both are supplied by the same wire "to the cluster" but the cluster sends the signal down 2 paths; the gauge (constant output) and the light (or switced output)

Since you said the guage needle drops off to 100 degrees ( bottom of the scale) and the light remains active - my guess is that the 2 indicators might be loosing contact with each other....... & This is probably be happening at the Printed circuit on the back of the cluster.

I know for a fact that all needle guages get thier connection from the printed circuit, so it's possible that this is where the problem comes from.

Did this just start happening ??

 

John,

This started in the last 2 or 3 weeks. Today while fiddling with another carb (another whole story there), I started getting the light. I figure there is no pressure in the system, so I'll take the cap off and stick a thermometer in it. 140-160 degrees, idling in the driveway. I know it is not overheating. Checked the resistance of the CTS and it reads 277, according to Vader that is about 150 degrees. No problem there.

I'm worried that this may affect the ECM and mixture control. Is there an easy fix? Is the printed circuit still available?

 

You should not have to worry about this issue effecting the ECM in any way. The temp switch on the drivers side of the engine block is sending current directly to the dash cluster. I am pretty sure that the ECM gets it's "temp" information from the sensor mounted on the thermostat housing.

As far as a replacement printed circuit - Classic industries was selling them the last time I looked in thier catalog,....... but were asking a small fortune for it. Your better off trying to have your existing one repaired - or buying a used replacement. ( Sorry but I don't have a printed circuit for Ya' !! )