Today i discovered something interesting with my temp guage (22re, sr5 guages). When i first start up the truck it will be cold for a few moments then jump up to normal operating temperature, even though I am posative that the engine is cold. Just to see if it was the sensor i unplugged the connector (the sensor on top of the thermostat is for the guage correct?) and the needle didn't budge. Shouldn't it drop to nothing? I'm assuming i have a problem with the wiring at this point, any ideas?

This is a common problem we all face... There have been many theories over the years as to the cause for the temp gauge failure, but nobody has come up with a fix to date.

You gotta get an aftermarket temp gauge.

That is not the temp gauge sending unit. The sending unit is between # 2 and # 3 cyl down on the intake, beside the injector plugs for those cyl.

So is the one on top for the ecu? and is there any way to check to see what the ecu is seeing for temp, i think mine may be getting incorrect data, causeing it to "hang up" on the high idle/rich idle circuit.

The one on the top of the manifold, near the valve cover is for the gauge.

The one at the front of the manifold (green) is for the ECU.
It influences how rich/lean your truck runs.. I don't actually know if toyota uses an open/closed loop based on temp, however.. Basically the colder your truck is the more fuel it'll be given by tht ECU.

Note - LC makes a little "variable resistor" box for $50 that ties into the water temp circuit... It's marked rick/lean and all it does is impact what the ECU is reading for temperature.

If you want to know what temperature the ECU is reading, you need to check the resistance at that sensor with the connector off of it.

Broadly, it's a linear relationship between temperature and resistance. Plot these values on a graph and then you can figure out what temperature your ECU is reading based on the resistance value of that sensor..

32 F 5750 ohm
66 F 2830 ohm
212F 200 ohm


If you want to test the sensor itself, pull it out.
Test it at room temperature, test it in ice water, and test it in boiling water. I think a failure is unusual.

There is another temp sensor for cold start - brown - it's the same type of sensor, but uses different values. Someone else will have to fill you in on how that works.


Other notes: At operating temperature, the coolant temp sensor will almost always read slightly colder than your gauge sensor. I think this is due to position - it's farther out from the head and in a good path for coolant.

dcg9381 - thanks for the detailed description, that's very helpful. BTW, what is the sensor in the thermostat housing for?

Thanks again

A single sender goes to the TSW pin on the ECU, that is the water thermo switch ued on 4WD trucks to bump up the fuel pressure, via a VSV, while starting to prevent a possible vapor lock type condition.