relay to turn off power

Any time you connect a wire to a power source, you really need to fuse it at the point of connection. The fuse goes before the switch.

Oh- so the thermocontroller has the relay built in; knew I was missing something-    So it just closes an internal relay contact from the battery connection to the output to the fan- I don't know what it looks like, but if it looks like you can open it up, I'd see if you could run a wire in to the coil of the internal relay and use that for switch control. I'd guess it's pretty simple to figure out, but it depends on how easy it is to open up and if you're willing to tinker with it at all. If it's all plastic welded together then just add the extra relay. You could also use the thermo relay output to just switch the external relay coil instead of running it to the 87A contact, because the NO contacts generally handle more current than the NC ones, and it sounds like the fan draws a good amount. Lighter gauge wiring for the thermo relay could then used, but your setup probably already has the heavy wire in place, so I guess that doesn't matter now.

Yeah, the thermo controller has a built in relay.. as for just adding a wire to the controllers relay, well, thats how this whole mess started..  The thermo controller has a seperate wire to by-pass the thermostat, and just close the relay and make the fan run manually.. Unfortunatley, that wire was blowing fuses at an alarming rate.. it seemed to work good for about 2 weeks, but then it would blow a fuse every 3-4 trips.. THEN, it burned up the switch.. my good freind the idiot above, assured me that a diode on the relay would NOT prevent the switch from burning up, so I don't know what it was.. That being the case, I endeavor to supply my own relay, and have it cut off the thermo controller, and turn on the fan manually from a switch, so as not to blow the fuse going to the thermo controller, and hopefully, not burn up the switch. Hence the drawing above... Hopes this makes everything clear, I think this mess is just about cleaned up, and I'll post up the results soon.. just gotta get a new switch, and fire up the soldering iron.Sony Mp3 HU, Dig/Aud 300Wts

(1) 12"sony sub.. Aint much, but it looks good, and thumps like a rabbit on Viagra!

Sounds like either there's an intermittent short on that wire for the thermostat bypass or it's not actually controlling the relay, but instead a direct power output to the fan. If it were just controlling a relay inside, there's no reason it'd be stressing out your switch and burning it up. If you have a meter, what kind of resistance do you measure on that line to ground with everything hooked up? It should be over 100 ohms if it's a relay control.

Glad to help you there kaptan.