1970 351C std model gauges.
Sometime over the past two months my temp gauge went from working perfectly to working but reading about 1/4 lower for any given temp than before.
With ignition On, grounding the wire from the sending unit cause gauge to go to full hot.
I replaced the sending unit as that’s easy and cheap. No change.
Ideas?
BTW - infrared thermometer reads 190-195 at thermostat housing when engine has been idling for 20 min.
Do any of your other gauges show similar behavior? If so, it could be the constant voltage regulator, corrosion on contacts, corrosion on CVR input power line (it uses a resistor wire as well), or a poor ground.
Just the temp gauge. All other gauges working as expected.
Substitute a 10 ohm resistor for the sender and it should read right at the top of the scale.
I’ll try that this afternoon.
I didn’t have any 10 Ohm but had some 15 and some 47 Ohm (both +/- 5%)
This is the reading with a single 15 Ohm connected between the wire which is normally connected to the temp sensor and ground:
This is the reading with a pair of the 47 Ohm in parallel so ~23.5 Ohms:
Looks pretty close Todd. It would have been a bit higher with 10 ohms and 20 seems right about the middle from my measurements.
Gauges read correctly with proper termination resistance. That said, your problem likely resides in the temperature sending unit itself.
Did you recently install it, perhaps wrapping the threads in Teflon tape (a no-no)?
I did change it out with one I got from WCCC (the one I took the measurements on). I’ll drive it around for a good warm up as soon as this rain passes and report back. I also noticed the wire was a bit loose so I crimped the connector slightly.
A loose connector can cause your problems…you’re probably now good to go!
Perhaps this is a somewhat idiotic idea, but have you checked your thermostat or changed it? A new one isn’t that expensive. What about your radiator? Has it been flushed out lately? If something was stuck in your thermostat preventing it from closing, that could cause an actual lower temperature. So could one that is simply stuck open all the time because its tired and needs to be replaced.
Actually good questions. I’ve not changed the thermostat since I’ve owned the car (about 5 years). No idea when the radiator was last flushed. I’ll dig deeper if it continues to act up.
The thing that has me scratching my head is how it just changed between times I had the car out. For as long as I’ve owned it, the temp when warm was always just shy of half way unless it was really hot outside and I had the AC on, then just over half way. Suddenly it was just going up 1/3 of the way.
I’ll see how it works next time I get it out.
At least it didn’t get stuck closed!!
That would be my concern. They do weird things as they fail. Besides, you can choose thermostats that open at different temperatures.
Is there a way to bench test these with the unit completely removed from the dash pod? I have a spare I’d like to compare measurements to.
You need to have the instrument voltage regulator to test the circuit.
You can measure resistance from the post to the chassis of the sending unit while using a heat gun; at least you can see it go from about 70 ohms to about 13 ohms when it gets hot.
Realized I wasn’t clear. I meant can you test the temp gauge itself with it removed from the instrument cluster.
Yes, you put the Instrument voltage regulator on the positive side to supply power and the resistors you used before in between the ground and the gauge.
I would just test it at the harness where it is routed to the cluster from the sensor. Removing the cluster is a PITA.
You can test the thermostat with one of those thermometers you use to tell what the temperature of a turkey or ham is.
Just use a stove to get the water up to boiling. Drop the thermostat in and watch it as the water heats up. Use the thermometer to see if it matches the water temperature. For example, a 180 degree thermostat Doyle fully open when the water is 180 degrees.
Finally got out for a drive. With the new sending unit and connector crimped a bit tighter, the reading came up closer to middle.
Temp readings with the infrared thermometer throughout the engine compartment seem normal so I’m going to leave it for now.
Thanks for all the advice.