Temp Gauge Voltage?

[domdart]

I have a 68 Dart. My temp gauge just went from working intermediately, to not working at all.
The fuel gauge is working. I replaced the circuit board and voltage regulator a year ago. I also have a new temp sensor. When I ground the sensor cap the gauge does nothing. When I test the voltage on the sensor cap it fluctuates from 5 volts to 1 volt. Is this fluctuation normal? Is that just the regulator doing its job? I looked under the dash and there are no loose nuts or any signs of trouble. Thanks FABO.

 

[67Dart273]

You need to test this with your mind set "end to end system." In other words keep in mind the entire circuit and what can go wrong

"the path"...............Ignition switch.........harness..........PC board connector..........IVR.......(so far this must be OK as temp and fuel both would quit working)...........out of IVR to board traces..........jumper to both gauge stud/ nut connections..........through the gauge.........out on sender stud / nut connection........board trace.........to sender connector pin at board..........harness connector.........temp sender wire...........through bulkhead connector............to sender wire end terminal connector............to sender.........to engine block ground.

So the IVR evidently works, it could be a bad board trace..............rare but possible
Could be either of the two stud/ nut connections at the gauge
Could be bad gauge
Could be bad harness connector/ board pin for the sender wire
Could be bad connection in bulkhead connector
Could be bad wire end terminal at the sender itself
Could be bad sender--------which I guess you eliminated by grounding the wire

 

[67Dart273]

So far as voltage this can be tough to evaluate, but check BOTH gauges and see if both gauge power terminals act the same for voltage. Ground the gauge sender terminal right at the gauge if you have the cluster out. Check one gauge against the other

 

[BKCowGod]

Rather than just grounding it, I would hook it into a resistance box. Low should be right around 74 ohms, medium should hit at around 23 ohms, and high will be around 10 ohms. Resistance boxes can be had on Amazon for $15 or so. Here's the one I have, one of those tools that sits around forever until it really comes in handy: https://www.amazon.com/dp/B085WS33BH/?tag=fabo03-20

 

[67Dart273]

Rather than just grounding it, I would hook it into a resistance box. Low should be right around 74 ohms, medium should hit at around 23 ohms, and high will be around 10 ohms. Resistance boxes can be had on Amazon for $15 or so. Here's the one I have, one of those tools that sits around forever until it really comes in handy: https://www.amazon.com/dp/B085WS33BH/?tag=fabo03-20

I should have "said" I guess. Grounding it is just for a quick test. A person should not leave the gauge powered up for long with the sender wire grounded. On resistance boxes, you need to be careful. Those gauges draw some power, I do not remember how much. You could burn out resistors in some of the "less expensive" "3rd world" boxes

 

[BKCowGod]

I should have "said" I guess. Grounding it is just for a quick test. A person should not leave the gauge powered up for long with the sender wire grounded. On resistance boxes, you need to be careful. Those gauges draw some power, I do not remember how much. You could burn out resistors in some of the "less expensive" "3rd world" boxes

Oh for sure - eventually mine will die. I just can't justify an expensive box I use once every 5 years.

 

[67Dart273]

You only really need 3 resistors. Last I knew Digikey and Mouser electronics had no minimum order

 

[Bewy]

Before doing anything or buying anything, remove the wire from the sender & measure the resistance between the terminal & the body of the sender. If I am reading post #4 correctly, should read 70ish ohms for a cold engine & 10 ohms for a hot engine.
Repeat test but use the engine block for the ground connection. If you get different readings, you have corrosion between the sender & the engine & is likely the cause of the problem.

 

[AJ/FormS]

As far as the voltage swing goes, yes that is normal; Two of the gauges do not call for anything near 12 volts. There is a little device back there, that shuttles between battery voltage and zero voltage, so that the fuel gauge and the temp gauges get an average voltage of around 6 volts.