Power to electric fan, but it's not switching on...

750hp · #1 (**permalink**) 11-07-2014, 03:14 PM

Hi, I hope someone with greater electrical understanding than me can help out?

I have a dial adjustable thermo switch that feeds to a relay then on to the electric fan.

This seems to be working and I can hear the click at the relay, but the fan doesn't switch on.

There is 11.7V measured at the connector between the relay and the fan. I can rotate the thermo switch to cycle between 0V and 11.7V. The "connector" is just a pair of regular spade terminals for power and earth.

If I run wires straight off a battery to the connector at the fan, the fan starts working...

Seems to me like I've verified power to the connector, and power from the connector, and the spade terminals seem clean and undamaged, so I'd appreciate any thoughts on how I can fix this.

Thanks!

patrickt · #2 (**permalink**) 11-07-2014, 03:31 PM

Craig, you may have 11.7 volts as measured by a VOM on a circuit that is not flowing, but that does not mean there is sufficient capacity to give the load (the motor) the current demand that it needs. That little switch, for instance, could be creating a tremendous amount of resistance and you'd still measure full voltage going through it on a circuit that has no load. Since you know that running wires straight from the battery to the motor works, you know the motor is not the problem. That means there is too much resistance somewhere on the circuit. If you perform a voltage drop test along the circuit, when the motor is trying to run because the circuit is on, then you will find the component that is creating too much resistance. If you Google "automotive voltage drop test" you'll find instructional videos and texts on how to do it. It is not hard at all.

Barnsnake · #3 (**permalink**) 11-07-2014, 08:19 PM

Many systems send continuous power to the fan and the thermo switch controls the ground. Verify the heavy ground wire from the controller. If it appears to be electrically intact (not connected to paint, etc.), the problem is likely the controller or its relay.

750hp · #4 (**permalink**) 11-10-2014, 04:46 AM

Thanks very much for the replies. Patrick, I googled exactly what you recommended and the first video was really useful, thanks! It all made perfect sense and it's a bit embarrassing to realise how little I know about electrics.

Oh, and the video also told me to poke it in from the rear because it was more effective than poking it in from the front, so I think there's a lesson there as well...