I believe the relay which is sticking on just needs to be replaced. If you are cheap like I am it just needs to have the high amp contacts smoothed/refaced. But check on the cost first as these are very common and you may find a generic one for around $5.

The relay allows a small current (the signal) to activate a electromagnetic switch to pass a much higher current to the fan. The contacts on the high current switch can become pitted and rough with use which creates a slightly longer lasting and more intense electric arc (when activated) than if they are smooth. This is causing the contacts to become "welded" together enough so the spring pressure of the relay cannot separate them when the signal current is shut off. Tapping on the outer shell is enough to break the weak weld and allows the contacts to separate...ending the high current to the fan.

These relays can often be taken apart carfully so you can see the problem. And a "points file" can then be used to file the contacts to a new sooth surface if you wish...but replacing the relay is probably best.

Dana

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Dana Linscott
Vegoil converted truck...vegoil converted 1987 190DT, 300 series next.

http://vegoilconversions.netfirms.com/