Coolant Temp Stuff

Ok, I’m resigned to being the guy asking “eye rolling” questions. I have a new temp gauge, new sender. Key off, nothing, key on, after a few seconds, full hot. Checked wire to ground for shorts. What am i missing

 

I'd be tempted to pull the wire off the sending unit and check for power.
It should be low voltage, 6-7 volts if I recall and pulsating with key on.
If not, wiring/dash gauge problem. The voltage regulator for both dash gauges is built into the temp gauge ?

 

You can test wire at sending unit with a regular 12 volt test light; it's just that the lamp will be dim when pulsating.

 

Jim is correct.

The TSM has a complete section on testing the gauges. The sender is a thermistor, varying in resistance with temperature. The resistances by temperature are given in the section.

The gauges work on 12V power pulsed to time-average about 5V. If the pulser does not work, you could get full 12V and see full deflection even if the sender has the right resistance. Full 12V will destroy the gauge after a while. The pulser will also not pulse if not grounded, and its default without gorund is on.

You should be able to see the pulsing with a test light, analog meter (like a Simpson) or a scope. A good digital meter like a Fluke should sample and convert fast enough so you can see the voltage pulsing - a cheap meter will freak out. The layout of what terminals have what voltages is in the TSM.

I wrote an"article" about building a gauge tester. Not exactly what you want, but might be helpful. http://earlycj5.com/xf_cj5/index.php?threads/143458/

 

Well, per the ladder diagram, in section 3-61, my new sending unit is bad. Seem reasonable?

 

Anything is always possible.
If you disconnected the wire from sender, the proper pulsating voltage was present, and the needle dropped back to "C" I would say you are correct.

 

If you have access to resistors or say a 100 ohm potentiometer or rheostat, you can test the gauge like my article shows. If the sender is bad, the gauge should still respond to changing resistance, whether it's a resistor or the sender.

 

As a reminder, don't use teflon tape or sealant on the threads.
For fear of bad ground.