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MG MGB Technical - cooling fans

I am a new B owner and the cooling fans never seen to turn on. I havwe tried touching the two wires together that are on the switch on the radiator and tied grounding each wire and nothing. Is the somethng else I can check that will make these fans turn on when the car heats up.
GK George

GK - The fact that you grounded each wire without producing a shower of sparks or released all the smoke from one of the wires would indicate to me that you don't have any power going to the switch. Go to Advance Auto Wire at: and click on Stock Schematics, then MGB/MGC and scroll down to the print for your car and use that to start troubleshooting. Cheers - Dave
David DuBois

NEVER test for voltage by shorting a wire to ground! Use a test-lamp or a voltmeter. Joining the two wires at the thermal switch is OK as that is exactly what the switch does when it closes, and is a normal diagnostic step for diagnosing a faulty switch. But as Dave says in your case you don't seem to have any power feeding the switch.

What year? If 77 or later with the twin factory fans then there is an inline fuse, probably by the main fusebox, with white/brown one side and green the other. Both should be showing 12v when the ignition is on, power comes from the ignition relay on the white/brown, goes through the fuse and then to the thermal switch on the radiator on the green.

If an earlier year with after-market fans then it depends on the make.
Paul Hunt

Many of the 77 on MGBs have a circuit breaker instead of the inline fuse. See the attached image. It is the little box under the fuse box.

GD Perrone

Trying again to upload the image

GD Perrone

True enough, apparently the fuse holders used to melt!

Unbelievably (to me at any rate) the V8 with the same twin fans had them powered off the main green fuse, along with everything else fused ignition including the heated rear screen! No ignition relay, and no separate fuse. Not only did they drag the voltage to everything else down but they suffered from low voltage themselves. I took a separate fused supply direct off the spare output spade on the alternator, and that plus putting local earths to the bonnet slam panel for each motor as well as the original earth (shared with the headlights!) upped the voltage at the motors by about 4v. That made more difference to the cooling than did an uprated radiator.
Paul Hunt

The car is a 1980. I will keep looking but when I took the wires off the sending unit on the radiator I found that someone had cleaned off the insulation and twisted them together. What some people do to these cars is unbelievable.
GK George

Well after playing around with a test light I found that the fuse was bad. It looked good but was bad. Everything seems to work fine with a new fuse.
GK George

GK "Everything seems to work fine..."Did you wire in the switch? Or are you running with the fans on all the time? If the switch is in the circut and functioning as it should you might want to install a circut breaker to replace the fuse as this might be the reason the PO was twisting wires.Let us know. RIC

If a PO took the wires of the radiator switch and twisted them together because he was having trouble with the fuse that would make him a DPO!
Paul Hunt

This thread was discussed between 16/10/2009 and 19/11/2009

MG MGB Technical index

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