Electric fan wiring

You should use a HD relay to power the fan(s). The coolant-sensor power-source would only be to trigger said relay.

Most fans consume 20-40 Amps at steady state. This does not play nice with a 15 amp donor circuit, even if nothing else on the circuit is being used. This is why one cannot use a donor circuit like that for the load-side fan wiring.

The relay trigger will draw less than one amp however. This is why it's ok on the control side.

I would NOT try to run the fan again until its got fused power from the battery. There's a real fire hazard otherwise.

Good luck.

Your controller should have a relay built into it (the instructions probably noted terminals like 30, 85, 86, and 87). Just be sure that only 85 or 86 are connected to the trigger (your overflow bottle circuit or other donor of your choosing). Do NOT connect 30 or 87 to this circuit.

Be sure to fuse the wire going to the overflow circuit (a 2 amp fuse should be fine). Be sure to fuse the thick wire going from the controller to the battery. The controller instructions tell you what size fuse to use (or it might include a fuse).

A relay is a high amperage switch. A circuit breaker is a resettable fuse. They do not perform a similar function.

You might be able to eventually get the car to heat up enough while idling to set the controller temp setting. You might have to safetly and carefully block the radiator with a piece of cardboard.

Be sure to have a gap between the temp at which the t-stat is all the way open and when the fan powers *off*.

Good luck.