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MG MGB GT V8 Factory Originals Technical - Thermostatic Fan Switches

I am starting a conversion of my 1978 MGBGT to Rover V8 power. The engine will be an ex-Rover P5B unit and I will be using a new standard MGBGT V8 radiator. I am not keen on the Kenlowe thermostatic switch with its fragile tube and wish to have my local friendly radiator shop fit an electric thermostsatic switch into my new radiator.

My question is, what should the switch ON and switch OFF temperatures be? The standard thermostat operates at 82 deg C (180 deg F). Would switch ON at say 90 deg C and OFF at say 85 deg C be about right? THe car will be used in the UK/Northern Europe.

Any advice would be very welcome.


Peter Hills

given that a pressurised system won't boil until something like 110 then I'd try ON 96 and off at 90. The OFF temp needs ideally to be higher than thermostat temp which IIRC is 88 (?). If using a lower thermostat then go lower off too.

FWIW: when my V8's rad was recored, I had a 3/8 NPT bung installed for a Painless Wiring 200 deg F on/ 185 deg off switch installed in the top tank. The setup works great with a 165 deg thermostat. In cooler weather the fans rarely if ever come on; the motor runs around 190 degrees. In hot weather, they come on and off regularly. I don't see why the fans should go all the way down to thermostat temp. The BOP/R V8s, given the alu heads, just don't make as much power when they're too cool ... too cool is hardly any better than too hot.

Is the electric thermostsatic switch you are installing adjustable - I assume so.

If so I suggest you start 85/90 as you suggest and adjust over a period of time with experience.

Do have a manual override switch and an indicator light installed, so that you can turn the fans on before reaching heavy traffif etc.

Safety Fast


Nigel Steward


Mine's set at 85c but I fitted an overide switch on the dash, as I was nervous about the temp needle rising too high before it cut in.

I have experienced no problems in 4 years, except a connector coming off,

Michael barnfather

You don't want to get too paranoid about when the fans cut in. Under extreme conditions my temp gauge was in the red but it didn't boil, although I had to use the fast-idle to stop it dying, possibly because it was on the border of vapourisation. The factory stat is 82 degrees and the fan cut-in 90, which about mid-way between N and the red zone, between the two little dots which can be seen on the rim of the dial. The SU Burlen replacement for the factory V8 switch is a couple of degrees lower than that cutting in, and lower still cutting out, which is no bad thing as it keeps the hot idle oil pressure up, albeit the fans run more frequently and for longer. With the sensor in the inlet manifold you don't want to be too low or the fans may not cut off once the stat starts to close. Not a problem on a rad-mounted switch, except for wasting fuel.
Paul Hunt

> Not a problem on a rad-mounted switch, except for
> wasting fuel.

I don't understand, my liege. How does this waste fuel? Do you just mean that by keeping the fans blowing after the stat closes you're making the alt. work harder?

The Range Rover thermostat housing has a temperature switch/sender in it for the Air con', use one of those. I'm not sure what temperature they switch on at but if its too high (I doubt this) use a different sender. I think its the same as the RV8.

Any electrical circuit puts load on the alternator and hence the engine. Depending on specifics you can often hear the engine idle slow slightly when turning on high-current things like headlights, heated rear window, cooling fans etc., although the faster you go the less drain the cooling fans cause ...
Paul Hunt

This thread was discussed between 09/10/2002 and 14/10/2002

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