Temp gauge question

[DaveT]

Have a '78 spider. Never had any temp or cooling issues and during normal running the needle was always dead center on the gauge. Recently replaced the Temp Sending Unit & the Temp Switch for the Gauge. Now, during normal operation, needle rests over the '0' of the 190 and if stopped for a bit, needle will move pretty freely towards the red. Today I put an IR thermometer on it while running and the IR is recording at about 40-50 cooler than the gauge is. Did repeated measurements across the whole radiator and it was very consistent. Could the gauge be off that much? If so, is there a fix (other than replacing the gauge?)
Thanks in advance.

[18Fiatsandcounting]

Dave, my first guess is that you've got a large pocket of air in the top of your cylinder head (where the temperature sensor is). Air can get quite a bit hotter than the circulating coolant, so you can get high temperature readings where the air pockets are, but the radiator doesn't "boil over". Also, because of the air pocket, you have less overall coolant volume than is optimal.

Two options to fix this: 1) install a coolant "T" connection at the top of the 5/8" hose that goes from the back of the cylinder head to the firewall heater connections, and do the final coolant topping off through this "T". 2) Raise the front of the car with a jack, which has the effect of making the radiator fill neck the highest point in the system, and then fill with coolant/water. For both of these suggestions, the overall goal is to add the last bit of coolant through the highest point in the system.

Another possibility (less likely) is that your old temperature sensor unit was reading too low, and the new one is reading correctly, and the engine really is running too hot. However, if that were the case, I would expected that you would have blown some steam out of the radiator cap or into the coolant overflow tank by now.

-Bryan

[Turbofiat124]

Just curious but since you replaced those the switch and the sensor to the guage are they wired up correctly? Meaning what happens if you swap wires from one to the other?

I wouldn't even bother with that "switch". I unplugged that wire from mine years ago and replaced it with an insulated connector so it doesn't ground against the cylinder head and peg the guage.

[DaveT]

Bryan, I already have the 'T' fitting installed and have always filled the entire cooling system from it. And prior to switching out the two sensors, it always ran dead center on the temp gauge.

James, I connected them the same way they came off. What's weird is that my infrared thermometer gives a reading that's 40-50 degrees below what the temp gauge reads.

[Turbofiat124]

If there is that much temperature difference , it does sound like there is an air pocket still in the system.

When the heater is on, is there any heat coming out the vents?

Usually if the temperature guage shows hot but no heat comes out the heater, it's low in coolant.

[18Fiatsandcounting]

Does the fan turn on? It should come on just past the 190 oF mark, and certainly well before you are approaching the red zone.

-Bryan

[DaveT]

James,
Yep, I get plenty of heat in the cabin. Never had a problem with that.

Bryan,
Fan does come on - usually halfway between the '0' and the beginning of the red zone.

Also (and i think this is not uncommon) if i turn on my headlights with the car running, the temp gauge moves to the right a hair and stays there. Turn off the lights - and it moves back.

[NSFIAT]

I had a gauge that was reading colder than actual temp. Everything else was normal and topped up as required. I ended up running new wiring (following the stock route) and haven’t had any issues since.

[18Fiatsandcounting]

Fan does come on - usually halfway between the '0' and the beginning of the red zone.

Also (and i think this is not uncommon) if i turn on my headlights with the car running, the temp gauge moves to the right a hair and stays there. Turn off the lights - and it moves back.

If the fan doesn't come on until halfway between 0 and the red zone, that's an issue right there. Either the thermo switch is bad, the radiator is partially plugged where this switch is, or you're not moving coolant through the radiator like it should (many possible reasons for this).

As for the temp gauge moving when you turn on/off the lights, that's pretty common on Fiats. Mine do that, and I've never figured out why. In fact all my gauges except the tach move: gas, oil pressure, temp. Want an extra gallon of gas in your Fiat? Just turn on the lights!

-Bryan

[dinghyguy]

If you put a relay system in for the lights the gauges will not move.
And i too see a significant difference between the heat gun temp reading at the sensor and the sensor itself. I also check that all the temperatures around the sparkplugs are similar on an occasional basis.

cheers
Dinghyguy

[manoa matt]

The temperature sensors and gauge work via resistance. The more resistance in the system, the higher the gauge reads. Resistance in the circuit can also manifest as corrosion on all the wiring terminals. Cleaning up all the electrical contacts on the male/female spade connectors can effectively lower the temp the gauge displays.

One part of the circuit that can be a mysterious culprit is the "Temperature gauge resistor" This item is located up under the dash in the birds nest of wiring. There is a red or blue plastic holder that looks like a single fuse holder. Twist it apart and you will find a resistor that looks like a fuse with a thin copper wire wound around it. Clean the contact points on the ends of the resistor and within the holder, but do not cut or break the thin wire.

[Turbofiat124]

Those grounds for the guages and their lights are Daisy chained. I haven't tried it but I bet if you run separate ground ( just for the guages, not their lights) to that ground pod under the dash , the problem may go away. Mine temp and fuel guages do the same thing when I flip on the park lights.

[spider2081]

The more resistance in the system, the higher the gauge reads.

I don't think this is a correct statement. I believe the lower the resistance the higher the gauge indicates.
The over temperature switch contacts are open until the temperature reaches about 230F then the switch contacts close. This connects the current limiting resistor to ground (minimum resistance) pegging the temperature gauge needle. If the current limiting resistor was not present the temperature gauge would be damaged from excessive current when pegged.
If the wire from the temperature gauge sender is removed from the sender the gauge should indicate zero. With the wire removed that is the most resistance that part of the circuit can have.
The gauge itself does require a good clean ground as has been mentioned. If the meters ground is poor the gauge may display an erroneously high reading. The meters ground is made at the dash grounding spider above the ignition switch. It originates at a terminal riveted to the metal case of the temperature gauge. The gauges dash light shares this ground. If the ground is poor the gauge could display a higher temp when lights are on then when lights are off.

[GeorgeT]

The higher the resistance, the higher the reading: as metal heats up the electrical conductivity decreases (more resistance).

[spider2081]

The higher the resistance, the higher the reading

Without getting into the theory of "temperature Coefficient" Here is a simple test

When the engine is up to operating temperature and the temperature gauge is indicating close to mid range disconnect the green/white wire from the temperature sender in the engines head. The gauge should go full scale left because of an open circuit. This is maximum resistance. If a second set of eyes are available quickly touch the green/white wire to the engine head. The needle should peg full scale to the right. touching the head with the green/white wire is shorting it to ground, this is minimum resistance. Only short the green/white wire quickly as too long a period of time will damage the temperature gauge. The resistor mentioned in a previous reply is a current limiting resistor that allows the temperature gauges needle to deflect full left without damaging the gauge. Touching the green/white wire to ground does not limit the current flow through the temperature gauge and it could be easily damages in a few seconds.