MGB Electric Fans

Once the ignition is switched off, the engine and water temperature in a car rise. Heat, once concentrated around the combustion chambers and exhaust manifold, now begins to spread generally throughout the engine block. Coolant is no longer pumped through the engine and can easily reach boil-over point. The temperature rise can be substantial, overheating engine oil, bearings, gaskets, and unnecessarily stressing nuts and bolts.

A number of manufacturers have dealt with the problem by installing electric fans which operate independently of the ignition circuit. After the ignition is turned off, if the engine’s temperature rises to a critical point, the cooling fans turn on. The fans remain on until the engine has cooled. Even if the water pump is stationary,water will still circulate. (In prewar days, MGs often ran without a thermostat, fan or water pump. Hot water in the engine rises, and cool water in the radiator sinks. Put the two together and you get circulation through the system. The process is known as thermosyphon – Ed.)

An MBB’s electric cooling fans are connected to the white/brown wired “switched” circuit and not the brown wired “always hot” circuit. As a result, the fans can only run when the ignition is switched on.

To make the fans operate independently of the ignition switch, do the following:*

  1. Check that the ignition is switched off.
  2. Remove the connector at the thermal fan switch (do not pull the whole switch out of the radiator). Use a piece of wire to jump across the two female terminals in the connector.
  3. Turn the ignition on, and the fans should start. If they don’t, check the fan line fuse (green wire on one side of the fuse, white/brown on the other).
  4. Remove from terminal #5 of the fuse box (hot side) the connector with the white/brown wires on it. The fans should stop. Identify which lead runs the fans by separating the wires. The smaller diameter white/brown should be it. Reconnect the other wire, adding a female slide connector if necessary. The fans should not start at this point. Add a female slide connector to the end of the fan circuit wire.
  5. Make a small 2″ long “Y” pigtail of 14 gauge wire with two male and one female connectors. Remove a brown wire from the #7 fuse terminal and plug the female lead from the pigtail into the fuse block. Reconnect the brown wire to one of the male connectors of the pigtail. Connect the fan circuit lead to the other. (The pigtail allows the wiring to be easily put back to original. Another approach would be to cut  the connectors off the brown and fan circuit wires, then combine them in one female connector – Ed.)
  6. Remove the jumper from the thermo switch plug and plug it back in. The fans now run when needed, regardless of whether the ignition is on or off.

*Your car’s wiring may not match Mr. Lewis’ description. Read the additional notes and look at your car before starting. 

Don’t worry about draining the battery when the engine is not running. The fans can only run until the engine cools (usually a few minutes or less), and will not run again. They won’t run at all if the engine doesn’t overheat. This is how the fans should have been wired in the first place.

A word of warning: if like me, you are apt to park the car and immediately open the hood to look at or adjust something, be aware that the fans can now come on at ANY time, even a few minutes after the engine is shut off. Stay clear of the fans whenever the engine is even slightly warm.

SOME ADDITIONAL NOTES:

While looking at a 1979 MBG to confirm Mr Lewis’ tip, we noticed something storage. The wire on our car didn’t quite match his description. A look at a wiring diagram labeled “1978 and later” showed a discrepancy. Mr Lewis’ 1977 car matches the late diagram, but our late car (with 70 original, documented miles) has slightly different wiring at the fuse box. In step 4, he refers to three wires, two of which are combined in one plug. Our car has only two wires, with individual plugs. This does not match the diagram. If your car is like ours, you have two identical white/brown wires, one of which is the fan circuit. Without tearing things apart, our best guises is that the two wires in the double connector (as shown by the diagram) must be connected elsewhere in the wiring harness.

Another point which could be confusing: The in-line fuse referred to is, on our car, a sealed metal box which was made in Chicago, Illinois. We have never yet found a proper replacement for it. If you have one of these and it blows, do not bypass it. You have no other protection if something goes wrong with the fans. Install a normal in-line fuse holder and 37 amp fuse to replace the metal box. Moss doesn’t currently have a source for these, but we are working on it.

by David Lewis

David will receive a Moss gift certificate for his contribution. 

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