The temp gauge on my '76 with 300 barely registers lately. Checked temp with an infrared gun, and it's normal, so it's not a stuck-open thermostat. What's the most likely culprit? And, where is the sending unit? The one for oil pressure's obvious, but can't find this one.

 

If the gauge is moving at all, I suspect the sending unit. I don't know the location of the sending unit on a 300, but once you find it, you can test your gauge: if you ground the sending unit wire, the gauge should go to its maximum position. If it does, there's nothing wrong with the gauge & wiring; if it doesn't, you can try the same test right at the back of the cluster to determine whether it's the gauge or bad wiring in between.

 

Great, thanks for the test info. Did some sleuthing on another forum, and the sending unit is allegedly on the passenger side of the block, up and near the back; not easy to see or access. Guess I know what I'll be doing this weekend

 

My problems with gauges being inaccurate is usually because of the regulator (D1AZ- 10804)

 

Check the wiring connector as well. I believe it's a gray four way connector with three wires in it. The wiring is exposed to the elements and can corrode. maybe pull it apart and inspect it as well. If satisfied, stuff dielectric grease in it to help seal out water and crap.

C-220 Connector details:
31 White-Red - Oil Pressure
39 Red-White - Coolant/Water Temperature
16 Red-Green - Duct Tape Hour, erm Ignition Coil + Terminal

 

Which is located on the Instrument Cluster Printed Circuit. AKA IVR or Instrument Cluster Voltage Regulator.