14-01-2020, 10:29 AM
Hi Hobbo
I am pretty sure that your car uses the Smiths moving-iron type gauge, as used in the Austin Seven. Within the gauge there are two fixed electromagnet coils wired in series. Battery voltage is applied to one end (B Terminal), and Earth to the other end (E Terminal). The junction between the two coils is brought out to the Tank sender (T terminal). So there are three terminals. Changing from Negative to Positive Earth doesn't affect the operation.
In use, the tank sender acts as a variable resistance to ground in parallel with one of the coils. When it has a low resistance, it increases current in one of the gauge coils, and decreases it in the other. The change in magnetic fields causes a piece of soft iron attached to the needle to swing round and thereby change the reading. The sender resistance range is likely to be from a few ohms at one end of its travel to some tens of ohms at the other end. It is likely that maximum resistance corresponds to a full tank, but I can't be 100% sure of this.
I suggest you check that the gauge Earth terminal is indeed connected to earth, and the Battery terminal is getting battery voltage. Once this is OK, disconnect the wire to the Tank sender at the gauge end, and try the gauge with both an open circuit and earth on its T terminal. This should make the needle swing from one end to the other. Normally gauges are reliable (unless they have been "got at") and it is the Tank sender and its wiring which give trouble.
Cheers, John.
I am pretty sure that your car uses the Smiths moving-iron type gauge, as used in the Austin Seven. Within the gauge there are two fixed electromagnet coils wired in series. Battery voltage is applied to one end (B Terminal), and Earth to the other end (E Terminal). The junction between the two coils is brought out to the Tank sender (T terminal). So there are three terminals. Changing from Negative to Positive Earth doesn't affect the operation.
In use, the tank sender acts as a variable resistance to ground in parallel with one of the coils. When it has a low resistance, it increases current in one of the gauge coils, and decreases it in the other. The change in magnetic fields causes a piece of soft iron attached to the needle to swing round and thereby change the reading. The sender resistance range is likely to be from a few ohms at one end of its travel to some tens of ohms at the other end. It is likely that maximum resistance corresponds to a full tank, but I can't be 100% sure of this.
I suggest you check that the gauge Earth terminal is indeed connected to earth, and the Battery terminal is getting battery voltage. Once this is OK, disconnect the wire to the Tank sender at the gauge end, and try the gauge with both an open circuit and earth on its T terminal. This should make the needle swing from one end to the other. Normally gauges are reliable (unless they have been "got at") and it is the Tank sender and its wiring which give trouble.
Cheers, John.