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MG MGB Technical - Radiator electric fans not working, MGB79

The electric fan motors in front of the car's radiator on my 1979 MGB (USA) are not activated when the engine becomes hot. The temperature gauge on the car is working correctly and the radiator is filled with coolant. Both electric fan motors work correctly when they are disconnected from the car and connected to a 12 volt battery. There are no other electrical problems with the car except the fan motors. The car has 50,000 original miles.
Mike Roe

Mike,
Try pulling the wires off the thermal switch located in the upper radiator tank and shorting them across. If the fans kick on, you've found the problem.

Be aware that replacement switches sometimes have popped out of the sealing grommet. Mine is safety wired in place, just for peace of mind.

Paul K

Mike:
You have a place to start troubleshooting your problem. I have been there myself (1978 B)and have found that besides the Thermal Switch other problems will give you similar symptoms. (ie air/vapor lock)
Try looking up in the Archives on this. There are pages of information that could assist you with a more extensive troubleshotting process. As well, many practical suggestions on "improving" the Thermal Switch set-up on your car.
Good luck.
Richard
R.M. Standish

Mike,

Did you check the in-line fuse; one of the loose ones below the fuse box? Its the one with the green wire.

Doug
D. Cook

Make sure you are filling the radiator by the radiator thermostat housing, not by the reserve coolant tank. I have felt/experienced/believe that filing the radiator via the expansion tank can lead to an air pocket in the top tank of the radiator where the fan switch is located. I believe this has lead in more than one instance of having the fan switch blowing out of the radiator and dumping the coolant all over the engine bay. (It not a MGB question of if but when.)

As suggested above, wire (use armature wire available from Radio Shack) the radiator switch into the radiator as suggested above, then use a thermostat switch from an auto parts shop that can be stuck into the top radiator hose as a replacement.

A safety precaution wire in a switch for manually operating the fans. Switch can be located in the engine bay by the radiator.

There has been a lot of BBS discussion that the replacement radiator fan switch, both Chinese and OEM Rover last only a few years at best before dying and causing problems.

Had this happen on both 79/80 MGBs.


John Long
J Long

Doug and Mike,
The 79 does not have an online fuse originally. It has relay that releases when going too hot. It is situated as Doug told you. Just so you donīt look for the wrong things.
Cheers
Erik
Erik

Thanks for the tips. I tried Paul's suggestion last night. I removed the wires leading to the thermal switch in the upper radiator tank, and shorted across the two wires while the engine was running. The radiator electric fans did not run. There was DC voltage across the two wires. The suggestion was made that I check the in-line fuses below the fuse box. There are two different in line fuses below the fuse box. I checked both as well as all of the fuses in the fuse box last night; they were ok. I noticed in a Moss catelog on the same page as that showing a replacement radiator, the thermal switch, etc. that in 1979 MG-Bs, there is a circuit breaker and a relay that may affect the function of the two electric motors. Is there a way to locate these and check their function? I will check the archives as was suggested. Thanks for the help. Mike Roe.
Mike Roe

Look at my previous posting, for location.
Erik
Erik

Mike, you said, "There was DC voltage across the two wires." If there was 12 volts at the green terminal on the thermostatic switch the fuses and relay must be good. If connecting it to the black green didn't start the fans, you should look for an open between the BG terminal on the thermostatic switch and fans or an open in the ground wires for the fans. You said the fans were disconnected and they ran with a battery charger connected to them. That checked the fans but did not check the associated wiring. Good luck, Clifton
Clifton Gordon

It is strange in that when I cross the green and black/green wire bypassing the thermal switch, I have 12V to both radiator fan motors, and yet the fan motors do not run. With the motors mounted to the car, I can connect the two wires from each motor to an extra 12V jumper battery and they run normal. The green wire from the thermal switch can be traced away from the switch to a 25 amp (12V) circuit breaker. The circuit breaker is connected to the main fuse box in the engine compartment by a white/brown wire. If I connect the white/brown wire from the fuse box directly to the green wire leading to the thermal switch (bypassing the circuit breaker)and keep the green and black/green wires crossed bypassing the thermal switch, the fans run normal. Apparently, the circuit breaker will delivery 12V but not the amps needed to run the fan motors (at least this is my guess). I have ordered a new circuit breaker. Mike Roe.
R.M. Roe

Clausager does that that thermal cutouts were used in place of fuses on late models, and both are capable of showing a 12v output when just connected to a voltmeter but this can drop to zero when connected to a true load, like cooling fans, if they are high-resistance.
Paul Hunt

This thread was discussed between 27/10/2003 and 31/10/2003

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