Overheating - Engine Cooling Fan Not Coming On?

[ATK 137 1 Cars]

Did some more testing today. Bridged the resistor plug. When I did so, both low and high speed fans worked when bridging the blue connector.

 

So can I conclude it's definately the rad fan switch at fault, or could it alternatively be the thermostat?

[ATK 137 1 Cars]

then again, if the rad gets hot, it means thermostat and water pump are both working ok, is that right?

[welshpug 1,627]

If it doesn't get too hot when moving then the mechanical side of the system is ok.

[ATK 137 1 Cars]

Overheating happened when stuck in traffic. On my way home (half hour drive) with little traffic temp gauge was around half. So that indicates all is well with the system (rad, thermostat, waterpump) aside from rad switch I assume?

[unariciflocos 49 1 Cars]

Engine temperature needs to be on the dot at a specific value between 80 and 90 depending on the rating of the thermostat. Anything under or over is not good.

 

The water pump circulates water around the block and heater matrix I believe when the thermostat is closed. When a certain temperature is exceeded the thermostat opens and circulates the coolant through the radiator until the temperature drops to under a certain threshold.

 

A thermostat that is stuck open will not allow an engine to heat up to temp by constantly circulating the coolant through the radiator.

 

It's normal for petrol engines to need extra cooling in traffic where there is not enough air flow through the rad, hence the fans. Petrol engines are less efficient and the heat losses are much greater compared to diesels.

 

I believe there is a bimetalic switch inside the radiator that closes when a certain temperature is reached and activates a fan. This is how I asume it works, don't know for sure, never had problems with mine. The fact that there are 2 fans suggests that two switches activate at two different temperatures.

 

Having a good understanding of how the system works may help you diagnose easier and faster.

[lagonda 41]

Try also cleaning the terminals on the thermoswitch, and gently close up the terminal ends in the switch plug (not all the way!) so that they are a tight fit on the thermoswitch terminal blades. This fixed similar trouble with my fans.

[ATK 137 1 Cars]

All sorted, new resistor and new rad switch and all is well again! Cleaned up the terminals as well.