1955 6v temperature gauge troubleshooting

[dcplums]

Gauge reads 5.8V on input side. Reads 1.8V on sender side with sender disconnected Gauge reads Hot when grounded. Should both terminals on gauge read at or around 6V with key on? Gauge bad?

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[59tbirddavid]

Remember that car is positive ground. Gauge reading hot when sensor side terminal is open, and that terminal is grounded, this is a correct statement. Also in that state, sensor disconnected, you will read 6 volts to ground on both terminals, this is a correct statement. Use an ohm meter on a low ohm setting, connect one lead to where wire connects to sending unit and the other lead to sensor body. With sensor in hand move float through its travel and watch meter movement. Meter reading should go from a higher reading to a lower reading or vice a versa. Sensor in tank is usually at fault along with bad grounding being the causing issue. Temperature gauge is heat driven by a wire wrap around a bi-mental strip that gives the temp reading, they do fail if voltage regulator fails closed. Putting a constant 6 volts to meter. You never did describe the meters problem.

 

[Fhower]

Gauge reads 5.8V on input side. Reads 1.8V on sender side with sender disconnected Gauge reads Hot when grounded. Should both terminals on gauge read at or around 6V with key on? Gauge bad?

Hi, The temp gauge on my.55 was Reading cold all the time. I used a 100ohm variable resistor to validate the gauge. 0 ohms should read cold. As you increase the ohms the gauge should move to hot. BTW you bypass the sender until for this test. Also check the ohms on the sender unit. I found 1 out of 3 actually worked.

Best of luck,!

 

[dcplums]

Remember that car is positive ground. Gauge reading hot when sensor side terminal is open, and that terminal is grounded, this is a correct statement. Also in that state, sensor disconnected, you will read 6 volts to ground on both terminals, this is a correct statement. Use an ohm meter on a low ohm setting, connect one lead to where wire connects to sending unit and the other lead to sensor body. With sensor in hand move float through its travel and watch meter movement. Meter reading should go from a higher reading to a lower reading or vice a versa. Sensor in tank is usually at fault along with bad grounding being the causing issue. Temperature gauge is heat driven by a wire wrap around a bi-mental strip that gives the temp reading, they do fail if voltage regulator fails closed. Putting a constant 6 volts to meter. You never did describe the meters problem.

Thanks a lot for your quick response. I am not sure what you meant by "not describing meters problem. I just wanted to know why I was only reading 1.8v on the sender terminal. Do you think I could repair the gauge (if it is at fault) if I removed it? Thanks again.

 

[doug7740]

Temperature and Fuel Gauge Calibration

A new temperature or fuel gauge must be calibrated prior to installation. If you fail to do this your gauge will read inaccurately. When you receive a new gauge, there is nothing that tells you that the gauge hasn’t been calibrated.

To calibrate the gauge, you will need a new 1.5-volt C type battery for a 6-volt system, or two new 1.5-volt C type batteries, taped on top of each other, for a 12-volt system. Next tape one wire to the positive side of the battery and another wire to the negative side of the battery. Make sure you avoid having the two wires touch each other as they can rapidly heat up.

Before you can begin to calibrate the gauge, the mounting plate must be removed from the back. This will allow you to gain access to the two holes where the toothed adjustment cams are now visible in the holes.



This is the mounting plate removed exposing the toothed cams.



Place one of the wires from the “C” type battery (or batteries) on one of the gauge studs and the other wire to the other stud. It doesn’t matter which stud the wires are connected to, you are now feeding 1.5 volts of power to the gauge on a 6-volt system, or using two batteries, 3 volts of power to the gauge on a 12-volt system. If the gauge is calibrated, you should read ½ full (for the fuel gauge) or ½ on the temperature gauge.

If the gauge is not calibrated, you must insert a small screwdriver into the back hole of the gauge and moved the tooth cam until you reach the ½ way mark. Move them very little until you get to half scale. Only one hole adjusts the calibration, the other one holds the tension on the spring. So if you move one and nothing happens, move it back to the starting point and again very little at a time move the other one until it reads half scale. Just remember to move them very slowly; a small move will make a large adjustment. You now have a gauge that you can install with confidence and should not have to be removed or replaced again for many years to come.

doug7740
1955 Thunderbird Blue

 

[59tbirddavid]

I’m sorry but I don’t want to get into this in too much detail. The fuel gauge is in series with the tank sensor and this 2 part series circuit is across the battery. If wiring is correct and grounding is good the reading of 1.8volts dc is the voltage drop across your meter. Your test meter is now in series with the fuel gauge. The remaining 4 volts is dropped across the fuel gauge. 5.8 minus 1.8 leaves 4 volts dropped across fuel gauge.

 

[dcplums]

I’m sorry but I don’t want to get into this in too much detail. The fuel gauge is in series with the tank sensor and this 2 part series circuit is across the battery. If wiring is correct and grounding is good the reading of 1.8volts dc is the voltage drop across your meter. Your test meter is now in series with the fuel gauge. The remaining 4 volts is dropped across the fuel gauge. 5.8 minus 1.8 leaves 4 volts dropped across fuel gauge.

The fuel gauge is not the problem. It is the temp gauge

 

[59tbirddavid]

Again I am sorry, I stated that the temp gauge changes resistance with temp change. The correct statement is that the sensor changes resistance due to temperature change, or changes fuel gauge reading because of float changing location in tank in turn changing resistance reading on wire wound resistor, these changes result in the change of current in circuit.

 

[doug7740]

I’m sorry but I don’t want to get into this in too much detail. The fuel gauge is in series with the tank sensor and this 2 part series circuit is across the battery. If wiring is correct and grounding is good the reading of 1.8volts dc is the voltage drop across your meter. Your test meter is now in series with the fuel gauge. The remaining 4 volts is dropped across the fuel gauge. 5.8 minus 1.8 leaves 4 volts dropped across fuel gauge.

Both the fuel gauge and the temperature gauge are in series with their respective sending units, but the voltage to the gauges is supplied through a parallel circuit coming from the ignition switch.



Since dcplums problem is with the temperature gauge and not the fuel gauge, I would assume that his fuel gauge is working correctly. The temperature sending unit is a Negative Temperature Coefficient Thermistor or sensor. This means that the resistance decreases as temperature increases. So, if you remove the R/W wire from the sending unit and ground it, the gauge should move and read past H. When touching the wire to ground, touch it quickly and remove it. Leaving the R/W wire grounded for more than a few seconds could damage the gauge.

If you determine that the sending unit is defective, remember a 6-volt temperature gauge must have a 6-volt sending unit.

doug7740
1955 Thunderbird Blue