Temp and Oil pressure gauges not working

[GassMann]

Hey all,

I have fully rebuilt my 1966 Charger. I rebuilt the wiring harnesses as well, nose to tail. Everything is up and functioning except the Temperature and Oil Pressure gauges. Both sensors are NEW.

I think I should be getting about 5v at the sensors since the wiring diag. shows 5v. What I see with my voltmeter is fluctuating 0 to 5v. The power comes from the fuel gauge and that gauge is working.

Cold, the temp sensor reads zero ohms. Warm, it reads 10 ohms which implies it is working. Does anyone know how these gauges work?

 

[RemCharger]

The temp sensor sould read around 340 ohms at room temp. Not sure on the oil pressure range.
You could short them to ground and briefly turn on the key, they should start to go to max, but turn the key off.
Then you know the sensors have issues.

 

[GassMann]

The temp sensor sould read around 340 ohms at room temp. Not sure on the oil pressure range.
You could short them to ground and briefly turn on the key, they should start to go to max, but turn the key off.
Then you know the sensors have issues.

Thanks Rem... I tried the ground and it did not move the needle at all. I will test the ohms again on the temp sensor at room temp.

 

[twecomm]

In the 66-67 Charger gauge cluster, the temperature, oil pressure and fuel gauge uses the 5 volts provided by the mechanical type regulator which is inside the fuel gauge. This mechanical-thermal points type regulator has a coil of special resistance wire wound around a bi metallic strip which has contact "points" that open and close with the heating of the wire. For a brief time on car startup, it is designed to provide a full 5 volts to the previously listed gauges for a brief time to get the gauges to read quickly. After a few seconds the mechanical regulator starts to open and close the points when the regulator mechanism heats up. The regulator cycles from 0 volts to approximately 5 volts which is applied to one side, or terminal of each listed gauge. The other side of the listed gauges goes to it's respective sensor, or sender that provides a variable resistance that completes the gauges path to chassis ground.
If you disconnect the respective sensor lead and momentarily touch the senders lead to ground, the gauge should show maximum deflection.
If at some point the mechanical regulator would fail it could apply 5 volts higher to the gauges which will fry the internals of the all 3 mentioned gauges. Its a good plan to disable the old mechanical regulator and get a RT engineering solid state limiter. Hopefully you haven't burned up the guts of your gauges that aren't working.

RTE limiter - rte

 

[GassMann]

Awe crap... and I just put the gauge cluster back into the dash.

As noted, I see the voltage jump up and down... tough to read though. But I get what your saying Twecomm.

Do you know what to look for in a blown gauge?

 

[GassMann]

In the 66-67 Charger gauge cluster, the temperature, oil pressure and fuel gauge uses the 5 volts provided by the mechanical type regulator which is inside the fuel gauge. This mechanical-thermal points type regulator has a coil of special resistance wire wound around a bi metallic strip which has contact "points" that open and close with the heating of the wire. For a brief time on car startup, it is designed to provide a full 5 volts to the previously listed gauges for a brief time to get the gauges to read quickly. After a few seconds the mechanical regulator starts to open and close the points when the regulator mechanism heats up. The regulator cycles from 0 volts to approximately 5 volts which is applied to one side, or terminal of each listed gauge. The other side of the listed gauges goes to it's respective sensor, or sender that provides a variable resistance that completes the gauges path to chassis ground.
If you disconnect the respective sensor lead and momentarily touch the senders lead to ground, the gauge should show maximum deflection.
If at some point the mechanical regulator would fail it could apply 5 volts higher to the gauges which will fry the internals of the all 3 mentioned gauges. Its a good plan to disable the old mechanical regulator and get a RT engineering solid state limiter. Hopefully you haven't burned up the guts of your gauges that aren't working.

RTE limiter - rte

BTW... EXCELLENT INFO Twe!!! This is the kind of perfect detail I was looking for.

 

[Don Frelier]

I agree with replacing the voltage limiter.
My limiter stuck closed a couple years ago and opened up my temperature gauge.
I replaced the limiter with the RTE upgrade and repaired the temp gauge but it reads low now.
So I'll need to replace that at some point.

 

[twecomm]

The fuel, temp and oil pressure basically all work in a similar manner. Like the mechanical voltage regulator previously described, a small coil of a special resistance wire is wrapped around a flat bi-metal strip. That coil is connected to the rear, screw terminals of the gauge. The 5 volts from the regulator is applied to one side of the three gauges. The other side goes to the respective sensor terminal. The sensors change resistance value with heat or pressure. The body of the sensor is connected to ground via it's screw threads. (Note : don't use teflon tape on the sensor threads). The heat created from the electrical current flow thru the coil causes it heat and bends the bi-metal strip. The bi-metal strip mechanically moves the gauges needle. The regulator, either the original mechanical regulator ( limiter ) internal to the fuel gauge or RTE solid state regulator function in a similar manner. After start up, the constant 5 volts gets the gauges to read rapidly, then the 5 volts will start cycling on & off, but since the bi-metal in the various gauges retains it's heat the reading does not change during the cycle of he 0 volt condition. You can observe the "hysteresis" of the gauges when you shut the car off and see how the gauges slowly drop to minimum with no voltage applied... As far a testing gauges, on the ones mentioned earlier, an ohm meter should show some resistance across it's terminals, but certainly not open..
The alternator gauge works differently.. There is a thin bar (shunt) inside the gauge that spans across the rear screw terminals. Basically, the battery / charging current runs THROUGH this gauge and that current may be many amps depending on battery condition or what the demand on the the battery / alternator is , thus the heavier wire size connected to this gauge. T he current flow thru the bar creates a circular small magnetic field about the bar. One side of the alt gauge is battery +, other side is alternator output +. The ALT gauge needle mechanism has a small magnet attached that is "repeled" from it's normal center rest position to either the charge or discharge direction based on direction of the current flowing thru the bar, or shunt. This alt gauge has no hysteresis, thus it's very rapid acting response. Hope this helps..

 

[Skatpak]

Contact Todd's Restorations for any gauge help.