which coolant sensor makes the gauge go?

[f85gtron]

i replaced valve cover gaskets on my GT 2.8....which was a couple bolts shy of a top end rebuild.....and now my coolant temp gauge doesn't work.  what do i test and which sensor is which?
Ron

[Fierofool]

The gauge sending unit is the one in the corner of the head just above number 5 exhaust and near the ignition coil.  It has a green and a green with tracer wire.  The sensor will have a round plastic head with a slot in the side.  This is for the temp gauge and light only. 

[f85gtron]

Ok. thanks. the funny thing is, i never unplugged that one.  can i simply ground the leads out to check?
Ron

[Fierofool]

Yes, you can test the circuit to the instrument panel by inserting a paper clip into either.  When you ground it to the block, either the gauge will peg or the light will illuminate, depending upon which you are grounding. 

[f85gtron]

sooooooo that means the two that are left (atop the intake valley manifold) must be for the ecm(double lead) and fan(single lead)? in my head it makes sense...
Ron

[Fierofool]

Right front corner of the lower intake just in front of the thermostat housing neck would be the fan switch and the cold start injector switch.  The fan switch is a single wire.  The Coolant Temperature Sensor (ECM or fuel injection switch) is located on the right end of the engine, just above the water pump.  It's positioned horizontally, point at the passenger fender, has a double lead and the sensor has an oval plastic head.  It screws into a reducer in the lower intake manifold. 

[f85gtron]

i tested the one by the coil using the grounding method. the stupid light came on for one lead and the needle pegged for the other.  i guess it is coincidence that it failed when it dd.  i was afraid i had broken a wire. 
thanks for your help!
Ron

[Roger]

You wouldn't happen to know what size pipe thread is used on the ECM water temp sensor would you?

[Fierofool]

It's GM Part Number 10037037 for cross reference.  It's a standard pipe thread reducer.  I've got a spare temp sensor if you'd like to borrow it.  Find the right inside diameter and the outside diameter will be correct.  If you're replacing one, I'd suggest getting a brass reducer.  Smear the inside and outside threads liberally with anti-sieze.  No need to worry about insulation.  The CTS doesn't ground through its base.  I may have a lower intake with one in it if you need one to compare. 

Another trick to determining the size is to use any of the Fiero temp sensors.  The IAT, Gauge, Fan and Cold Start switch all use the same thread. 

[f85gtron]

i replaced the faulty sensor with one from Napa......it was bad. the replacement to the new one reads around 220.  when ignition on, guage points to zero.  should i take this one back too, or is this normal.  mat and ecm sensors show same when cold via winaldl, and ecm shows 195-202 when warm.
Ron

[Fierofool]

Temp gauges normally point at 100 with ignition on, but engine not running.  Did you use the GM Temperature Sensor Chart to check the sensor for accuracy?  Do a search for Chart in Tech Tips on the forum.

Remember, that the Coolant Temperature Sensor doesn't interact with the temperature gauge.  It gives information to the ECM for purposes of controlling the fuel injectors.

The gauge sending unit is located on the cylinder head, near the ignition coil.  Two green wires going into the harness terminal.  The GM sensor chart can be used to determine accuracy of that sensor, too.  Once you determine the temperature the sensor is telling you, you can compare the gauge for accuracy.  This sensor doesn't interact with anything except the gauge on the dash. 

[f85gtron]

Ok, thanks.  yes, the gauge reads 100 (not 0) when ignition on.  and. reads 220 when warm.  my question is is this normal, or should i strive for more perfect? i.e. 200 indicates when warm?
Ron

[Fierofool]

If you have a 195 thermostat, that's about where it should run.  200 is acceptable.  Let it sit and idle with everything turned off to see at what temp the cooling fan kicks on.  It should come on at about 235.  It will give you a little more insight into the accuracy of your reading. 

[TopNotch]

You can get digital infrared thermometers these days that are very useful for checking engine temperatures. I've seen them at Microcenter and Harbor Freight. Just aim and press the button. Aim one at various parts of your engine, at your coolant hoses, at your radiator (engine warmed up).
I got mine at Microcenter for about 10 bucks.

[Fierofool]

I have the one from Harbor Freight.  They usually sell for around $30 but are regularly on sale for between $20 and $25.  For checking coolant temp, I point it at the brass base of the gauge sending unit.

Point it at each individual exhaust manifold runner and you can determine if each cylinder is burning equally or if you might have a cylinder that's not firing well.  Tape a one inch wide strip of paper across your AC vent as a target when charging or topping up the AC system. 

Check for sticky or non-functioning brake calipers by aiming at the brake rotors.

At night, rather than hiding it away in your tool box, use it to exercise the dogs or little kids.  I just step out on the porch and run the beam around the yard until the dogs are exhausted.  Works even better with cats, but don't shine up on the drapes or another person. 

Warning!  Don't carry the Harbor Freight item around in public or you may get arrested.  It's shaped like a gun.