My car is a 1969 cutlass, originally a 350 and now a 455/TH400. The car would have originally had idiot lights for oil pressure and coolant temp, but now has aftermarket gauges. The original blue and green sending unit wires are tucked away near the horn relay. I’m currently working on wiring two electric fan relays and was looking for a keyed power source. To my surprise, the green wire seems to have power when the key is on. I thought the sending unit wires were the ground for the lights? Is that incorrect or is something wrong? Or is the green wire not for the coolant switch? If it’s correct to have power, is there any reason I can’t use that for the fan relays?

Thanks!

Dennison

 

There is power on one side of the lamps, the blue/green are the ground triggers. If you put a test light on those wires, not only would the test light show power, but the idiot light would come on.

You are reading the ignition power coming from the gauges fuse, up to the gauge pod, thru the idiot light, and out to the sending unit wire. In other words, the circuit is working correctly.

 

Thanks - that makes sense. The test light is acting as the ground and completing the circuit. I should have understood that, but obviously didn’t. I’ll go back to my original plan of using the accessory terminal on the fuse block for the keyed power source. Thanks again!

 

The fact that you see voltage does NOT mean that there is sufficient current to operate the fans. What is the current draw of your fans? The fan relay should be powered from the horn relay stud via a fuse of appropriate size. Most electric fan systems use a switch to ground to activate the relay manually. If you have a fan controller, it gets wired per the directions. The instrument light wires are not the correct source.

 

My relays I’m using require two power sources. One that actually powers the fans, which I will wire to the horn relay. The other is just to activate the relays and needs to be wired to a keyed ignition source. Very low draw on those.

 

Run a wire to the IGN terminal on the fuse box.