Triumph Repair: 79 Spit electric fan

Question
Hi Howard, working on my 79 spit and noticed something odd.  When the electric fan kicks on, it some how affects the ignition system to the extent that it greatly reduces the rpms and even stalls the engine.  The car is not original now having twin SUs, a Kent cam and aftermarket distributor.  Any thoughts on that one?  As always, thanks so much for all you do for us Triumph owners.

Answer
Hi Rory,

You should confirm that it is the fan causing the problem by running the engine until the fan starts up and set the RPM at a speed that you can tell that it is changing the running and remove one wire off of the thermo switch. If the engine falters when the fan is running and clears up as soon as the fan it disabled, put a volt meter across the battery and run the same test and watch battery voltage to see if the added load of the fan is causing a excess load on the battery. If not and the battery voltage stays between 13.8 and 14.5 with the fan running then there is a problem in the wiring.

To test the syatem, disconnect the two fan motor wires and put jumper wires on those two fan motor wires and with the engine running again at an RPM that it faltered when the fan thermo switch started the fan. Now you connect the two jumper wires to the battery direct and note if the engine falters any. This bypasses the electrical system when the engine is running and operates the fan directly off of the battery.

A fan motor draws a lot of current and if it is wired into the fuse box on the ignition circuit it can cause a problem for the ignition. Also a fan motor that is has shorted windings can draw too much current thus can cause a problem. This can also happen with poor grounds.

Another test is to leave the cooling fan disconnected and run the engine and turn on the heater fan and the head lights at the same time to see if that causes a problem. You can run this test at highway speed since a radiator cooling fan is not needed above 35 MPH anyway.

If you can not find anything wrong with these tests you will need to start doing "voltage drop" tests on the wiring. if you have not done these before let me know and I will walk you through the procedure. But first run the tests I listed above.

Howard