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Intermittent Cooling Fans -'78 B


Hello experts. Cooling fans decided not to run for several days on '78 MGB, just as our high desert temperatures are beginning to climb into the 70's. Then, last evening on the drive home, voila! There they were, alive and frisky again? Likeliest culprit? Would it be relay, sympathetic pains for MGR demise, or something else?


Jedi Trainee
Try replacing that thermastatic switch at the top of your radiator. My '78 did the same thing (before I sold it). Beware, I had to get two switches because the first one was faulty right out of the box. Also, get a new rubber grommet to ensure you don't have any leaks. Piece of cake!


Jedi Knight
Country flag
Aloha Midgey,

A quick test of the thermostatic switch is to by pass it by removing the two wires from it and connecting them together. Turn on the ignition and see if the fans run. Another possibility is a faulty or poor ground connection from the fan motors.

Safety Fast,

RF Thom

Jedi Hopeful
The fans run direct through the sensor in the radiator, of which the points are not strong enough to carry the current drag of the fan motors, especially if you have two [2] fans. Utilize a relay and use the sensor in the radiator to trigger the relay. After replacing the sensor twice, I knew something different was needed. I took apart the sensor and discovered the internal contacts burnt to a crisp. I installed the relay and six [6] summers of driving without a problem. Last year I modified the circuit and tied it also into the overdrive circuit on the gear lever switch : 1st, 2nd & neutral - fans on, 3rd, 4th & overdrive on - fans off. Helps in city driving and I don't have to keep an constant eye on the temp gauge.

Regards, Bob


Thanks to all who weighed in on this. It was the radiator switch. Got a Lucas switch from the local MG specialists, which they insisted on pairing with a Jaguar heavy-duty gromet because the gromet which comes with the switch is well known to be inadequate. Anyway, smooth and easy replacement even for a newby car guy. Got it, ran it up to temperature and presto! The sound of those lovely dual fans!

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