How accurate is the coolant temp guage?
I have an 86 z28 tpi. and the needle on the guage is well into the red area before the cooling fan comes on.
Is there some kind of thermometer or other tool that I can get to verify the coolant temp? Could the sending unit be bad?


Thanks
John

 

John,

The dash instrument does not have surgical precision, but is fairly close. The variability is introduced by the sensor that detects the coolant temperature and the wiring between the two. Your TPI should have the sensor used by the gauge installed in the left cylinder head. This sensor should have a single dark green wire attached to it and relies on a good engine ground for proper operation.

Since the fans are controlled by the ECM, their operation points may differ from the dash instrument reading because the ECM uses a different sensor to report coolant temperature. That sensor is located in teh coolant crossover in the front of the intake manifold (it has a yellow and a black wire on it).

These sensors can be tested for accuracy by testing the resistance across the terminals (or terminal and ground) and comparing the two.

The sensors should have resistances close to this table:

 

Thanks for the info,Vader.
I replaced the thermostat, and the red zone problem seems to be gone. The temp needle never broke 220. I'll know more by the end of the day.



Thanks again
John