94 Temp gage only works when truck is off

[spyhawk36]

Hello all,
I bought a 94 Ranger with 154,000 miles last March. The temperature gage has never worked since I have owed it. I just replaced both senors near the thermostat housing with new senors. I thought for sure this would fix my problem. The gage still doesn’t work when I’m driving. But when I turn the truck off the gage will move. Before I changed both senors the temp gage wouldn’t even move after I turned the truck off. Any help would be Appreciated. Thank you.

 

[Curious Hound]

The sensor that is connected to the guage on the dash is the one with only one wire connected to it. The other gives coolant temperature to the ECU. When you replaced the sensors, did you put any kind of sealant on the threads? If so, clean it off. The sensor must ground itself through its threaded connection in order to work properly. I believe that if you take the wire off the sensor and touch the wire to ground briefly, you should see the guage go to full scale. With the wire disconnected, the guage should read at the bottom of it's scale. RonD will probably verify that. That test should tell you if the guage and wiring are ok. You could have a loose connection somewhere.

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[spyhawk36]

The sensor that is connected to the guage on the dash is the one with only one wire connected to it. The other gives coolant temperature to the ECU. When you replaced the sensors, did you put any kind of sealant on the threads? If so, clean it off. The sensor must ground itself through its threaded connection in order to work properly. I believe that if you take the wire off the sensor and touch the wire to ground briefly, you should see the guage go to full scale. With the wire disconnected, the guage should read at the bottom of it's scale. RonD will probably verify that. That test should tell you if the guage and wiring are ok. You could have a loose connection somewhere.

This message composed solely of recycled electrons. Go green!

The one wire sensor came with a factory sealant. The two wire sensor did not. I will clean the sealant off tomorrow and see if this helps. I did use a jumper wire before I bought a new sensor and hooked it to ground. The gage did go to hot. Thank you for helping me. I will see what happens after I clean the sealant off.

 

[RonD]

A temp SENSOR will have a Cone shaped end, this for the computer only, no connection to the gauge, and it runs on 5 volts power from the computer

The temp SENDER will have a cylinder shaped end, this is for the gauge only, it runs on 12volts

So the two are not interchangeable.

When you go to a parts store they will almost always give you a Sensor first, even if you ask for a SENDER.

If you got both at the same time then I doubt they did that.

Yes, temp senders and oil pressure senders were 1 wire until mid to late 1990's then a separate ground wire was added.
Until then the Threads on the Senders are the ground to make it work, so bottom few threads should be left bare, no tape or sealant, its OK to seal the upper threads against leaking.

If grounding the single wire at the temp sender sends temp gauge to HOT, then gauge and wire are fine, sender is the issue