Testing radiator fan switch

[bran100]

Debugging fan issues. When the coolant reaches around 190 should the sensor at the bottom of the radiator ground out? In others words, zero out on the ohm meter when connecting the two leads from the switch?

Thanks,
Brandon

[joelbert2k]

In a word, yes

[bradartigue]

Here is a long-winded description of the coolant temperature fan, switch, gauge, and sensors, from an update to my book:

The Veglia gauge and its two sensors are notorious for reading incorrectly if air is in the cylinder head. Fill the system and bleed out the air and then see how that worked. If not, remove the plug from the rearmost temperature sensor. This sensor is designed to peg the needle on your gauge to 260 degrees – the red zone – if temperatures get too high. You can say it influences the other gauge; and sometimes this influence is a bit overzealous, causing the gauge not to peg but to read quite a bit higher than reality dictates.

With the rearmost sensor disconnected you can determine if the forward sensor works. If the fan is kicking on and off regularly and the car is not actually overheating but the gauge is still reading oddly then consider replacing the sensors. Replace them in pairs, they last 20 or more years under hard use!

Here is another tip on the gauge - if your gauge cluster is not grounded correctly then the gauge will read high if the overheat switch is connected. This is one of those odd behaviors of bad grounds - or "Dwight's Rule" as it is commonly referred to. If you have dim dash lamps or gauges that bounce when you turn on the headlamps or the blinkers, you may have a bad cluster ground vs. a bad sensor.

If the fan is not kicking on and off regularly and the system has been filled to eradicate air then inspect the fan wiring (using a voltmeter helps). Continuity between the two wires on the fan switch should occur when the temperature of the coolant reaches around the middle of the gauge sweep (the “9” is the best reference). Jumper the fan’s negative wire to determine if it works. Replace the sensor if you suspect it to be the issue. Keep in mind when testing that the fan switch does not ground to the body; it connects the two wires coming into it together when heated.

It is prudent to consider that the three coolant sensors (four on fuel injected cars; see the FI section) should be replaced together and if they look old they probably are. When replacing the two on top of the head also try to replace the boots that protect the connection (boots such as these were also used on other European cars if you cannot find the FIAT part). If you replace the FI sensor inspect and replace the wiring to it as well.

[courtenay]

On my 80 Fi, the sensor that pegs the gauge is the one closest to the front of the engine - i.e. the radiator. I have disconnected it and the gauge works just fine, although the fan doesn't come on until the engine temp (according to the gauge) reaches about 210.

[bradartigue]

On my 80 Fi, the sensor that pegs the gauge is the one closest to the front of the engine - i.e. the radiator. I have disconnected it and the gauge works just fine, although the fan doesn't come on until the engine temp (according to the gauge) reaches about 210.

I need to correct that since it seems that, depending on who assembled the car, one wire is longer than the other. The green one is the temp sweep and the gray one is the overheat. That is, when they are green and gray. I need it to read "figure out what is what through grounding..."

[manoa matt]

Another thing to note if it seams like you are running hot but the fan comes on and the stat works:

The temp gauge sending unit in the cylinder head works via resistance. As the temp rises the sending unit increses in electrical resistance. The increased electrical resistance is measured by the dash gauge and corresponds to the movement of the needle. The wiring for the circuit is as follows: Temp gauge sending unit is wired in with the temp gauge sensor that pegs the needle. Both sensors are then wired to a "temperature gauge resistor" which is located up under the dash in what looks like a fuse holder. The wiring then goes to the dash gauge.

The senders and gauge rely on resistance for the reading at the gauge, therefore any additional resistance in the wiring circuit will result in a higher than actual temp reading at the dash gauge. Corrosion at the terminals or hard cracked wiring near the cylinder head sensors will add resistance. To correct the situation and make the gauge read a more accurate reading you need to clean all the electrical contacts in the circuit and replace any wiring that is corroded or exhibits hard cracked insulation. You also need to gently clean the contact surfaces in the temperature gauge resistor. When you open the holder you will see a ceramic core with a tiny copper wire wound around it with caps on each end. You only need to clean the caps at the ends and the corresponding contact points inside the resistor holder. Do not break the copper wire.

Once complete you can verify the results with a non contact laser thermometer. You want to point the laser at the metal cylinder head between the sensors. Its the metal temp you are looking for not the water temp.