Trying not to use a rocker switch or a adjustable thermo probe. Which sensor wire do I tap to activate the relay? I tapped into the single wire sensor on the back of the engine but it's not working. Looks like it gives off a consIstent 3v.

 

Maybe just a hint of what your working on might help.

What are you trying to do? Have you done a engine swap so there is just no way the mechanical fan will fit?

If this is some attempt to try and save Horse Power the number of electric fans I have thrown away after they failed and caused a head gasket failure .

Best of luck!

 

Have a look at some of the past attempts in the forums, including my build thread.
lots of time and energy has gone into this subject in the past and it is better to leave the mechanical fan alone.
what you are tapping into is probably a temperature sender for a gauge and not an on/off switch. tapping into switches/sensors for the engine control systems and gauges is a bad idea.
tell us what year Kenworth and what engine and we can answer more of your questions.

 

^^^^hahahaha

It's also super helpful for us to know what size birkenstocks you're wearing.