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MG MGB Technical - Electric Fan

Anyone know what temp the standard electric fan should kick in?

Or how can you test the sender?

Colin Parkinson

I don't know if what I know will help, but. The sensor should be on the top tank, opposite of the inlet where the coolant will hit it. It should be 165-180.

My experience is with modified tanks is early model cars
Frank Baker

Colin. If you are referring to the thermocouple used on the later model cars to control the electric fan, it can be tested by the same method used to test a thermostat for function. Suspend it so that the bulb portion is below the water level in a pan with the contacts above the water level. Heat the water using a meat thermometer to measure the temperature. Connect up a test light to a battery and the thermocouple. Read the temperature at which the light comes on, indicating that the thermocouple has completed the circuit.

Fan(s) are tested by jumpering the two wires which plug into the thermocouple.

Les Bengtson

Temperature for cut-in of the electric cooling fan should be 90C 194F, which equates to about mid-way between the N and H on the temp gauge.

165F and 180F are the temps of the early and later standard thermostats, not the cooling fan switch.

As Les says linking the two contacts on the switch bypasses it and the fan should come on immediately, regardless of coolant temperature. If it doesn't, there is an electrical fault in fan, fuse (separate in-line), or wiring.
Paul Hunt 2

Colin, the temp switch URP1126 (Lucas SNB712) switches ON at 92C and OFF at 87C (5C Hysteresis). I assume that the later type switch has the same temperature switch points, just alternative connectors.

As we can't get reliable parts any more I *always* test new items such as these *before* fitting them to my car, that goes for thermostats too, using the method advised by Les above.
MG Mike

If the later switch Mike refers to is OE then possibly it will have the same switching points. Certainly the SU Burlen alternative to the original Otter for the V8s doesn't - it switches on slightly earlier which is no bad thing, but its switch off points seems very variable, sometimes as low as the temp gauge needle back on N, which means the fans can be running a very long time. At other times it will switch on and off quite frequently, with very little temp gauge movement.
Paul Hunt 2


On the later model 77-80 I purchased one from Moss. It cuts in at 205F and shuts off at 203F. I tested it also. I did not like it so I changed my set up to a Flex-a-lite with a thermo switch that I can set my own cut in time with operating temperature set at 190-192F.

Don't know if I had a bad unit, but I actually had two of them that had the same 205-207F cut in reading.

Ray 1977mgb

This thread was discussed between 24/05/2007 and 25/05/2007

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