1972 Oil Pressure and Temp Gauges Not Working

[1972--_F100]

New to the group - apologies if I'm in the wrong spot or asking repeat question.

Recent purchase 1972 F100 and novice beginner, late-bloomer.

Last time out, oil pressure gauge and temp gauge needles both stayed pegged low/zero - no movement at all.

Thoughts? Suggestions?

Thank you.

[zgerbic]

Since both are not working suddenly at the same time, it is probably not the sensors for either. A simple test is to pull the wire on the temp sensor, at the front of the intake manifold and ground the wire. The temp gauge should read Hot if the sensor is not working.

If both gauges are not working it will be a power problem. Working forwards from the fuse block, the fuse gets power from the ignition switch. Switch must be working because the charge light is not on. Power then goes to the constant voltage regulator device (AKA: voltage stabilizer, dash voltage regulator, voltage buzzer, etc.) on the back of the dash panel. This device lowers the average voltage to approximately 5.4V to power the gauges. You can check the voltage on the 12V side to see if it is there. You can check the output with the voltmeter but it will not just be a 5.4V reading. It will probably be some voltage reading vs. no voltage reading if it is working ok. It is better to test with a test light. The light should be bright on the 12V input and dim on the regulator output. If it is a solid 12V out, the device is bad and so might the gauges if they had had 12V on them for very long, more likely you would get no voltage out of the regulator. You can check the voltage at one of the gauge connections on the back of the dash to make sure the voltage is getting to the gauges. I believe the 72 uses a flexible printed circuit board (PCB) on the back of the dash. These can have minor breaks or bad connections so check that the voltage are getting to the regulator and to the actual gauge connections.

Constant voltage regulators seldom go bad so I would look closely at the PCB connections for the low voltage to the gauges as the most likely problem. I noticed you did not indicate if the fuel gauge is reading correctly. It also gets power from the constant voltage regulator. If the gas gauge is working properly, then the regulator is working ok. In this case it might be a connector problem from the engine compartment (going to temp and pressure senders) since I think the connection from the fuel tank sender comes in from the back of the truck.

Let us know what you find.

[1972--_F100]

Isolated issue by ground wire.

When ground to block from harness plug exiting firewall - temp gauge and oil pressure gauge both rise to full H reading.

Now searching for replacement harness from plug to
1. Oil Pressure
2. Temp
3. Coil

[bluef250]

Power (about 5 volts) to these gauges comes from the constant voltage regulator with is mounted on the circuit board not the plug. Check continuity on printed circuit board. The CVR gets 12 volts and sends 5 volts through the gauges to the sending units. The sending units limit current making the gauge show current flow.

The plug which is on the printed board carries current to the temp and oil pressure sending units. The pins of the plug are sensitive to proper installation.

[zgerbic]

If I was going to be replacing the harness I would make a 5 wire harness. Temp, pressure, coil, tach and a spare for 12V switched for things like the electronic ignition module, choke or idle solenoid.

If you can recover the connectors to the temp and pressure (or get them from some other vehicle), you can graft them into the harness with shrink wrap insulation over the connections.