Technical Electric Water Temp Gage Help

I'm stumped on my inoperative electric water temp gage. There's no needle movement when the ignition switch is on or after the engine has come up to temp. I've tried two different gages. Here's what I have and what I've checked out.

Stewart Warner 100-280 electric water temp gage; I tried both a 70s vintage gage and a new gage
Stewart Warner 280ED-F temp sender, 100-280 range, it is in contact with coolant and there is no sealant between the sender and intake.
Gage needle does not move with switch on, with or without jumper wire from sending unit body to chassis ground
Resistance, sending unit body to sending unit terminal stud at ambient air temp: 430 ohms; measured same with jumper wire form sending unit body to chassis ground

Resistance, sending unit body to sending unit terminal stud at 170F+ engine temp: about 120 ohms

Resistance, sending unit body to chassis ground: 0.3 ohms

All gages share gage power, ground and light power. All gage lights work and all other gages work (except water temp)

Disconnected temp sender wire from gage and measured resistance from gage wire to ground to be 50 ohms. EDIT: I also disconnected temp sender wire from the sender and I got infinity measuring resistance of the wire itself. It looks like the wire is suspect. I'll run a jumper straight from sender terminal to gage S terminal and see if that makes it work.

Any idea what's going on?

 

Ohm wire end to wire end. Unhook from gauge and unhook from sender and just ohm the wire or run a jumper wire from the gauge to sender. Make 100% the power is to power on the gauge and sender wire to sender pole on gauge, also make sure both studs have the isolation cardboard under the stud base.

 

my OT car did this. I had to ground the sensor housing (not the sensor itself, but the adapter to get it to fit) I am not sure if the ground wasn't getting thru or what, but My dad suggested it and it worked. Might try that if all else fails

 

^^^^^ Had off topic industrial vehicle at work do that as well.

 

Sounds like the sensor to gauge wire is open somewhere. A jumper will tell. Good luck!

 

Either I don't understand, or there is something wrong with your measurements. You said that from the sender terminal to ground you read 120 ohms, but from the sending unit wire at the gauge you got 50 ohms. Then you said the sending unit wire read infinity from end to end. That dosen't make sense. It also points out a problem with continuity readings if you do not completely isolate the circuit. I think when you read the 50 ohms, there was something else connected to the circuit.

A better test is voltage drop. With everything connected and the key on, the voltage you read at the sending unit terminal on the gauge should be the same voltage read at the wire terminal on the sending unit. I suspect the reading at the sending unit will be way less than that measured at the gauge. This points to either unwanted resistance in the wiring or an open circuit.

 

Testing circuit resistance with an ohmmeter will tell you if something is bad. However, it doesn't necesarilly mean the circuit is good. Circuits have to be able to carry current. An ohmeter sends a miniscule amount of current while testing. Testing with a circuit live, voltage drop, will tell you if the circuit can carry the current it is designed to handle.

 

Good point on the ohm meter shortcomings. I just got it straightened out a bit ago. The light gage power wire and sender wire were partially broken from ring terminal crimping and assembly. I was getting erratic readings after taking wires loose to measure.

 

Temp and Fuel gauges are dirt simple [they are basically 2 electromagnets pulling against each other]
They are real easy to bench test .
You need a positive wire and 2 ground wires . But connect the sender ground wire via a potentiometer [in-series]

Approx $2-50 for a potentiometer


If the gauge is working correctly you can dial the gauge up and down using the potentiometer [then measure the resistance across the potentiometer to match the sender values]

I use a 12volt A/C adaptor 2 power up my gauges on the bench.

 

I need to add a potentiometer to my tool box. I bench tested with jumpers straight off the battery but that only pegged the needle to the extremes.

 

They dirt cheap.
I also use them to "tune" my gauges.

If your temp gauge is reading too high or too low and you want the needle to be in the normal range.
After getting the Engine up to running temp [you can also do this with the engine still running ,even on the side of the road after a drive] .

Connect the potentiometer in-series between the sender wire and sender and dial it up and down.
If that doesn't dial it the correct direction, then reconnect the sender and connect potentiometer parallel [from sender to ground] and dial it up and down.
Once you have the gauge where you want the needle , then measure the resistance across the potentiometer.
Then add the same value resistor to the sender wire [either in-series or parallel by grounding it]

This method keeps the needle in the correct position at normal running temperature

 

Good tips, thanks. It's reading about 20 degrees high.