I am having trouble hooking up a fan relay. My question is what drives the relay. Is the relay driven directly from either the yellow or black wire from the sending unit. Or is the relay driven from a computer output.

There are two pins on my wiring diagram that say "pri/lo fan on" and "sec/high fan on". I have tested the pri/lo wire with the engine running and the temp approaching 240deg F. I am not getting any voltage from that wire...

Any insight is greatly appreciated.

Gus

 

Yellow or black wire? Thats the coolant temp sensor (CTS), dont hook up the fan there.

There were 2 setups used, one I am not so familiar with. The earlier single fan setup had a computer controlled fan, set to turn on at about 230F on TPI cars. The reading does come from the CTS, but you have to change the chip to set the temp lower. The later setup has one fan thats run by the ECM, and the other is controlled by a switch in the head. That would probably be the easiest to change to, get a temp switch for a Turbo Buick (turn on temp is about 200) and use that to ground the relay and switch the fan on.