The previous article sets out details for the fan upgrade for the Series 1 E-Type. Those who undertake this job might run into another problem caused by arcing across the contacts of the fan relay. This article describes the cause of this problem and how to overcome it.

When the fan is running and the fan relay contacts open, a high reverse voltage is induced across the relay contacts and arcing results This process eventually causes a build up of metal on one contact and a crater on the other. The contacts of my relay actually welded themselves together with the result that the fan came on and remained on when I switched the ignition on, regardless of the engine temperature. I was able to break the “welded” connection by tapping the side of the relay, but this was not a permanent solution.

Anyway, this problem can be prevented by connecting a diode across the motor. I used diode type UF5408. This is a 1,000 volt diode, but a 400 volt diode should also do the trick. The end of the diode with the band printed around it is connected to the positive side of the motor, the other end to the negative side of the motor. (The band signifies the end of the diode to which current can flow. A diode prevents reverse current flow.)

When the relay contacts close, thereby applying a voltage across the fan motor, “forward” current cannot flow through the diode. However, when the relay contacts open, the induced reverse voltage can dissipate through the diode and motor rather than trying to jump across the relay contacts.

With relay protection installed, I was able to put my trusty Lucas relay back into service after opening its housing and polishing the contacts. Try doing that with your modern relay in its snap-fit plastic housing!