Temp gauge not working

[Devin]

My temperature gauge died, it seems. I replaced the sender (the single wire unit) and the gauge won't budge. I'm thinking it is the gauge itself. Is there anything I should check before trying to find a used replacement? I bought a cheap mechanical gauge as a temporary replacement but it seems like it would be better to fix this than to rely on a temporary one (and save myself $22).

 

[hawkeye18]

You are sure the engine is actually warming up and you didn't bust a thermostat, no?

 

[Devin]

Pretty sure. I get heat and the fan cycles. Could those be false positives?

 

[rubydist]

the fan cycles with the a/c or because of temperature? if temp, then you have a working thermostat. if a/c then you may not.

do the other gauges work okay? because a bad ground is the most common gauge problem. the second most common temp gauge problem is corrosion inside the fitting that goes on the stud on the sender - did you clean that up really well before you reattached it?

 

[Devin]

I replaced the sensor and cut the end off of the sensor wire and replaced it with a eyelet and fastened it with a nut (the boot and connector was in real bad shape). The fuel level gauge doesn't work but it never had as long as I had the car.

There is no AC right now. I can warm the car up and see if the upper hose warms up.

 

[Devin]

The upper hose definitely gets hot, so it looks like it is electrical. I found some instructions to test the sender itself, but is there a way I can see if I can get the gauge to sweep by applying an electrical current to it? It looks like it gets the temp reading based on the voltage that goes to ground. I was thinking I may be able to supply it with 12v DC and see if it sweeps at all. Would this work or will I just **** it?

 

[rubydist]

the temp gauge basically measures resistance. Somewhere recently I posted the resistance values required for a "C" and for a "H" reading on the gauge, but I don't recall the numbers off the top of my head. If you search, you should find them.

 

[Devin]

So I think the gauge is fine. First I pulled the gauge and checked the voltage from the three sockets. I was able to get 12v. I then put the gauges back in and pushed the needle up. When the car is on, the needle pushes itself down, so it is definitely powered. I then drove the car for five minutes or so and took a reading using my multimeter with the negative probe on the block and the positive probe on the sensor. With the meter set to 2000 Ohms I got a reading of 130. I have no idea if that was the correct setting.

 

[rubydist]

found it:

according to my factory manual,
a sender resistance of ~74 Ohms should result in a temp gauge reading of "C"
and a sender resistance of ~10 Ohms should result in a temp gauge reading of "H"

it says after connecting the appropriate resistor, wait 60 seconds to read the gauge.