Ford Freda Rad Fans and Coolant Switch ISSUE !!

[Rifster]

hello ppl,
I have a 1995 ford freda which is giving me some issues. Recently it started to overheat so I checked the fans on the rads to see if they are working. They were working before but now they seem to have stopped. I located the coolant temp switch above the starter to test the fans and discovered that when I remove the switch plug the fans come on automatically (with ign on) and when I replace the plug they go off. I have driven the motor for around 20 mins to get it up to temp to see if the fans kick in but they dont. When the motor is warmed up there is good heating inside the car so coolant level must be ok. ANY HELP PLEASE !! why would the fans come on when the switch is unplugged ??
Thanks RIF

[g8dhe]

The sensor varies its resistance with temperature, the resistance gets higher as the temperature gets higher. Hence unplugging it causes the ECU to think the temperature is very high, so it turns the fans on. Sounds like the sensor needs replacing.

[mikeonb4c]

My only caution here is that the rad fans don't/shouldn't need to come on much anyway in a healthy coolant system. Are you sure this is the root cause of the problem? One test might be to leave the sensor unplugged so the fans are permanently on and then see if the overheating problem disappears. But whilst that might prove the overheating problem can be held off by fans permanently on, it wouldn't really prove that the fans not coming on is the cause of the problem. As Geoff says, replacing the sensor might be worth doing anyway, then see what happens. Have you got anything suspicious like crunchiness in the radiator top hose, as this might indicate a clogged up main radiator.

[Simon Jones]

With the lower ambient temperature at this time of year, the likelihood of the fans needing to come on will reduce. What symptoms have you got that point to overheating issue? BTW, warm air in the cabin does not necessarily mean you don't have a problem with coolant level or an airlock. What level is it at in the header tank when cold?

[Rifster]

Thanks guys, I will replace the sensor and see what happens. The temp gauge did go over half way but not into the red thats why I thought it was overheating. Before that I did notice that the fans seem to come on and stay on far to long before they went off and stayed off. I will also check for airlock in the system.
regds Rif

[g8dhe]

The sensor for the display is separate from the sensor for the ECU, that display one is mounted on the front of the engine usually accessible from the drivers side.

[Simon Jones]

Bear in mind the rad fans will come on when you switch on the A/C which may add to the perception that the fans are on a lot in hot weather .

[mikeonb4c]

Thanks guys, I will replace the sensor and see what happens. The temp gauge did go over half way but not into the red thats why I thought it was overheating. Before that I did notice that the fans seem to come on and stay on far to long before they went off and stayed off. I will also check for airlock in the system.
regds Rif

To add to Simon's point above, if engine temp gauge is going over half way (and assuming its an unmodified gauge, that is abnormal) and if rad fans are/were coming on when aircon is off then that would suggest to me the temp sensor is/was working properly, but the cooling system is/was not. Suspects might be airlock (an airlock around the sensor could cause confusion to it?) , thermostat (not opening properly), blockage/clogging, or just possibly water pump. A good bleed would see the best first option, then see what happens. The question of what caused an airlock might still need addressing (a leak can pull in air upon system cooling as well as cause coolant loss on heating I believe). Keep us posted, and good luck.

[Rifster]

Many thank again. I did notice a very small leak in the thin pipe that is located on passenger side above th alternator. this pipe is for bleading air from the system I have been told. I have now repaired the leaking pipe. could this have cause an airlock in the system ?
Rif

[mikeonb4c]

Many thank again. I did notice a very small leak in the thin pipe that is located on passenger side above th alternator. this pipe is for bleading air from the system I have been told. I have now repaired the leaking pipe. could this have cause an airlock in the system ?
Rif

This is a classic problem. If the system bleed pipe (the long thin pipe you're referring to I think) is not held back from rubbing on the alternator then it will wear through due engine vibration. There is a clip that should hold it back, but this often comes away from the metalwork and if so then a cable tie or somesuch is needed to hold the pipe clear. it is quite possible/likely that the leak in this pipe could either have caused air to suck back into the system and/or allowed coolant level to drop below the header tank at some point, when air will then get introduced into the system. Better start with re-bleeding the system and seeing if the problem sorts itself out. And obviously, check also that the temp sensor is working properly (though I'm not sure how you check that)

[Northern Bongolow]

a leaking pipe at the position you have mentioned will let air into the system as the system cools down, air entering here will go into the head and get trapped there.

the cooling system when running works with having positive pressure inside it to raise the boiling point of the system. when it cools the pressure eventually should drop to about 1 pound negative pressure due to the spring in the cap, this 1 pound negative pressure plus a leaky system allows a small amount of air to be drawn into the system until the system totally relaxes, this air when drawn in will always try to go to a high point so in your case will travel into the top of the head, if your lucky it will get forced out with the action of the water pump, if it lodges it may continue to grow until it super heats then boil.

[Richard]

The sensor varies its resistance with temperature, the resistance gets higher as the temperature gets higher. Hence unplugging it causes the ECU to think the temperature is very high, so it turns the fans on. Sounds like the sensor needs replacing.

I have the same issue - wondering why the fans come on after the sensor is disconnected. I think this response may not be accurate. The sensor is a thermistor whose resistance DECREASES with increasing temperature. Mine does this as per the book. I am suggesting this conjecture - that the ECU detects the open circuit and switches the fans on as a failsafe.

What I am trying to discover, is why my engine boiled, dumping coolant through the overflow on the header tank on a Swiss mountain pass. No sign of fan operation! We were 2000 metres above sea level and 33 degrees C air temperature.

This was after paying hundreds of pounds to a chap in a Mazda garage to investigate leaking from this overflow and non-operating fans after a run. His solution - too late for me to object was to remove my NEW thermostat, cut it up so he could use the seal and reassembling without the guts. We bled the system thoroughly - I carry a funnel and tube. . . yet it still boiled.

After this, it behaved perfectly (if cool!) all the way home.

[g8dhe]

Yes your quite right, it is a thermistor and does have a negative temp. coefficient hence reduces its resistance with temp. increase!
Not quite sure what I was doing when I made that response, but brain was obviously not engaged!

As to why you boiled at high altitude and high(ish) ambient temperature, could it be that you had a leak in the system and weren't operating the coolant under pressure and hence with a higher boiling point, and so the coolant boiled due to the much reduced pressure typically only 80% of sea level ?