I am getting ready to replace my radiator and fan with a V6 rad and electric fan. The control I ordered for the fan is a Derale adjustable fan controller, but I ended up with the "water jacket" probe not the "radiator fin push in" probe that I thought I ordered. Where is the best place to put this water jacket probe with a 3/8" NPT thread on it? I have a pic of a sensor in the casting on top of my thermostat but I am not sure what the sensor is for. I unplugged it to see if it was my temp guage sender for the dash but it is not. Can anyone tell me what this sensor is and if I can remove it and put my rad fan thermostat in there?

I also have an 89 4runner for parts and there is no sensor in that location on the 89???

This is a link to the fan controller

Here is a pic of the sensor that I think I might be able to remove??

 

I have an 88 with 22 RE and recently installed a Spal fan and the very same Derale adjustable controller. I don't know what that sensor goes to, but I installed the Derale threaded probe in the thermostat housing, just as you show in the pic. On my truck, my thermostat housing was not threaded for that sensor and I don't have one in that location. The only coolant associated sensor I found was tapped into the block at the front of the engine. Anyway, I bought an aftermarket thermostat housing online that was made of cast iron and looked like the one in your truck with a bung for a sensor. Well at least it did in the pictures I saw of it on the website. When I got it, it didn't have a bung and I ended up having to drill and tap a hole for the sensor. I think that location is the best and most accurate location for the sensor as it is right off the motor. So far so good. No leaks and works great.
Edit: I looked in my FSM and it doesn't show what that sensor is in your thermostat housing. I have a Chilton's manual and it shows it to be a water temperature detect switch.

 

Well.. I talked(phone) to 2 toyota dealerships and they couldn't tell me for sure what it was for. I went to see the local 1st/2nd Gen toyota guru around here and he had a couple of engines laying around with one but they were from early 80s cars. He has never seen one of these sensors on a truck before?? I unplugged the sensor and drove home from his place. The truck drove well and my temp guage in the truck worked fine?? I think that I will plug the sensoe back in and drill and tap a new hole beside this sensor or splice a piece of pipe in the middle of my top rad hose and put the electric fan thermostat sensor there.