1989 GMC s15 - radiator fan constantly running
 

When I start the truck ...it immediately starts to run and than runs until I turn off...I believe it should come on only when temp. is high enough or ac is on..

Before this happened I was trying to fix ac but now ac connected is not connected any more but still fan starts immediately I turn on the truck..

temperature sensor or anything else?

Where is temperature sensor located that turns on that radiator fan?

Re: 1989 GMC s15 - radiator fan constantly running
 

I'm not sure of where they are with an S10/S15, but I'll move this to that area and maybe someone over there can help u out.

Re: 1989 GMC s15 - radiator fan constantly running
 

If it is a stock 1989 S15 fan set up, then it will run whenever the engine is running because it is a mechanical fan attached to the waterpump.

If you have some aftermarket/custom electric fan set up, then it could be the temperature switch that controls the fans is not functioning properly.

Now where that switch is on your truck? I don't know, as it is not a stock set up. Start following wires from your electric fan, and you will find it somewhere.