I replaced my cooling fan switch but my fan still isn't turning on when the car get hot 240+. As a test I connected a ground to the switch's wire and the fan turns on, so I believe the wire and fan are OK. Anyone have any ideas how to figure this out? Could the new switch be bad too? Could the block not be grounded or something like that?

 

What kind of car?

What's the ACTUAL temp of the motor? Not what the sporty-wavy-pointer-trim-package thing in the dash says, butt, the ACTUAL temp of the casting near the switch, as measured with your IR temp gun?

 

92 RS with a 305. I'll check the temp with IR gun tomorrow. Thanks.

 

Good chance temp indicator is more of the problem than the fan switch. Install a mechanical temp gauge.

 

IR gun read 250 around the fan switch. Dash temp was well beyond 260. I suppose I could test the ground by bypassing the switch and try to ground the wire to the engine instead of the battery like last time. If that isn't it I guess I'll have to drain her again and pull the switch out and test it.

 

I have had this issue also... Grounded wire... Fan comes on... Took out the switch, heated with a hot air gun....could hear it close. Ir temp was reasonable... Tested good electrically.... So switch, relay, and fan all good. Tried 3 different switches, all tested good but no fan operation.

I bought a tap set for pipe plugs to clean out thread oxidation...it was super tight...trying to improve grounding.

.since the switch and head are different material... Galvanic corrosion can occur.

I put it all back together but it's too cool here so the engine never heats up....time will tell

 

Did you use teflon tape or another thread sealer on the switch when you installed it?

 

Chasing the threads will probably take care of it. Being cast iron, the threads in the head often get VERY rusty and cruddy. The brass threads on the switch are too soft to cut through that and reach a point of making good contact with actual metal.

You HAVE TO use some kind of sealer or other on the threads. I prefer Teflon tape but good thread sealer (NOT silicone) also works. To some extent, this can also interfere with making a good connection. It's possible to solder a wire to the sensor body in some way that doesn't interfere with the hex or the threads, and then ground that to some nearby spot that offers better (cleaner) connectivity.