Triumph Repair: radiator fan, thermo fan, radiator fan

Question
my 80 spit fan will not come on as it should. the thermo switch on the radiator is good. there, i think, should be power to one side of the radiator thermo device with the switch on but there is none. all fuse's that i can find are good . i have it temp wired to come on with the switch

Answer
Hi James,
The thermo fan switch grounds an electromagnet in the fan relay to turn the fan on. It is not directly connected to the fan. The "hot" side of the thermo fan switch may not light a 12 volt test light so it would have to be tested with a voltmeter to see if it is powered.
Turn the key to the "Ign" position and note if the "Ign" and the "oil Pressure warning light" is on. If so the fuse that powers the electromagnet in the fan relay is good. Then check for 12 volts at the "hot" wire (with the wire disconnected) of the thermo switch. If you show 12 volts, then the power is applied to the relay and the electromagnet in the relay is conducting current through to the thermo switch. If you have 12v at that wire with the key on just touch that hot wire to ground, if the fan comes on, then the system is working normally. If not do this.
Check the purple w/green wire at the fan relay for power. (this can be tested with a test light.) If you have power then the fan fuse is good.
If you had 12v at the "hot" wire (key on) of the thermo switch and you had power at the purple w/green wire at the fan relay and you grounded the "hot" wire of the thermo switch and the fan did not come on, then you either have a failed fan relay or a broken wire between the fan and the fan relay.
This is providing no one has altered the wiring of these components.
Let me know.
Howard