OK, I'm going to install an aftermarket water temperature gauge soon. I can't really find a definative place to put the sensor. Some people say tap the waterpump housing, but don't really explain. Also, can someone explain the stock setup.

There is a sensor on the rear iron, and looks like 2 on the back of the waterpump housing. Where do these go?

I believe that the sensor on the back iron is to the cabin gauge and the others go to the ECU and the fan switch.

the sender for the guage is in the rear of the engine, by the oil filter.



the sender for the ecu and the fan switch (if its an s5)



i would think that the ecu gets the "better" temp, i have mine in the upper radiator hose, with a 38mm hks adaptor thingee

So If I replaced the one on the back iron it would work but the cabin gauge would be dead? Would that trigger any other problems?

nope wire goes right to the guage, with no stops in between.



i have seen a Teed setup where you have a little pipe in the iron, and then the stock and aftermarket senders in the pipe

Isn't that bad though? Like slower or wronger readings since coolant isn't really flowing, just sitting?

If you tap waterpump housing you will get the hottest reading better than the rear iron .

yeah, but tap it where?

anywhere below the thermostat...

PICTURES!! Actually I think I figured it out today by examining an engine that was together.