Coolant temp sensors faulty?

[WadiaAbushanab]

Hello everyone. I have a 1980 Fiat spider 2000. I recently replaced both sensors on the top of the head, and have been bleeding the coolant. The temp gauge shows that the car begins to rise past 190 quite a bit, but once I unplug the sensor closest to the front of the car, the temp drops considerably back to slightly less than 190. Is this sensor faulty? If I leave it unplugged, am I getting the accurate temperature readings? Or does it need to be plugged in to get an accurate reading?

[18Fiatsandcounting]

Try reversing the wires to the two sending units if the wire length allows you to. One sending unit is for the gauge readout, just like always. The second sending unit is an overtemperature switch that causes the gauge to "peg" all the way at the top once a certain temperature is reached. Kinda of a redundant setup to get your attention (an overtemp light would have been a better idea), and it often causes confusion as to which sending unit is which.

To tell which lead is which, try this: With the engine cold and the ignition on, touch one wire to ground (the cylinder head). If the temp gauge pegs all the way to the top, that's the overtemp switch wire. If the temp gauge rises to the top but not absolutely pegged, that's the wire for the gauge readout (the needle movement).

-Bryan

[WadiaAbushanab]

Try reversing the wires to the two sending units if the wire length allows you to. One sending unit is for the gauge readout, just like always. The second sending unit is an overtemperature switch that causes the gauge to "peg" all the way at the top once a certain temperature is reached. Kinda of a redundant setup to get your attention (an overtemp light would have been a better idea), and it often causes confusion as to which sending unit is which.

To tell which lead is which, try this: With the engine cold and the ignition on, touch one wire to ground (the cylinder head). If the temp gauge pegs all the way to the top, that's the overtemp switch wire. If the temp gauge rises to the top but not absolutely pegged, that's the wire for the gauge readout (the needle movement).

-Bryan

The wire lengths do not allow for them to be switched so I don’t think it’s that. Would I be fine just leaving the overheat sensor unplugged if it’s giving a faulty reading? Cause the one that actually gives the correct temperature is the sensor closest to the firewall right?

[18Fiatsandcounting]

Would I be fine just leaving the overheat sensor unplugged if it’s giving a faulty reading?

Yes. The overheat sensor can give many erroneous signals when not working right, so it's fine to delete it from the circuit as long as your main temperature sending unit is working correctly.

Cause the one that actually gives the correct temperature is the sensor closest to the firewall right?

This seems to vary from model year to model year, so I can't give you a definitive answer to which way is correct. I've seen these two sending units wired differently, so I don't know what is "right".

With the engine cold, measure the resistance of the two sending units to ground. One will be infinite resistance (that will be the overtemp switch), and one will be a few hundred ohms (that will be the sending unit for the gauge needle).

-Bryan

[spider2081]

Do you have an Ohm meter
When the engine is cold:
One sensor should measure "open" and should have a gray/black wire on it.
The other sensor should read a value of resistance related to engine temperature and have a green/white wire on it.