Is my Fuel Gauge Toast?

[DJThom]

When I first wired the car, the fuel gauge jumped to "full" when in fact the tank was completely empty. So, I reversed the leads on the back and it read "empty", so I moved on.

Recently I filled it for the first time. Still read empty. Long story short, I have the gauge out of the car and am using jumpers to power/ground the terminals on the back. It doesn't matter which way I put them, the gauge always reads "empty". I used my volt meter to ensure I have good power and ground.

What now?

[types26/36]

What now?

Suggest you take a Ohmmeter and put it on the tank unit (terminal to unit body) checking the resistance as you move the float arm. If you get no reading then the unit is defective.
If you do not have a Ohmmeter you could connect a 12v bulb through the tank unit (to a battery and check if the bulb goes from bright to dim.

[DJThom]

Yes, that was on my to do list once I established the gauge was working. 12V on the proper terminal should make the needle jump to full, correct? When the tank is full, shouldn't the resistance in the tank unit be very small, such that it approximates a 12V and ground on the gauge terminals?

[types26/36]

Yes that is correct, (sorry I misunderstood your post thinking you were refering to the tank unit as the guage)
By earthing the tank unit wire the gauge should read full or by earthing the corrisponding terminal on the guage the gauge should also read full providing you have 12v on the other side. From memory the live is a red/white wire fed from the live side of the rheostat switch, cant remember the wire to the tank unit.

[DJThom]

I have a small "T" and "B" under each terminal, which I'm assuming is tank and battery. When I took the gauge out today I had them on the right posts, meaning originally I had them on the wrong ones.

I'm not positive, but in retrospect I don't think these gauges can handle 12V and ground with no load due to the coil inside. I may have fried it by grounding it out with no load.

When rooting round the trunk I discovered the wire from the gauge to the tank had come apart, probably while i was rewiring my electric fuel pump.

I'm going to borrow the gauge from the MG and try it with the correct hookup to prove the theory.

This is a classic case of "ready, shoot, aim". Action without thinking...

[RotoFlexible]

Had a similar problem when I got my gauge back from rebuild. Upon consulting Nisonger, I discovered that I needed to ground the fuel gauge case. Upon doing so, the gauge worked. Try grounding the case before you assume the worst. I had it hooked up every which way before I got it working and it didn't fry.

[DJThom]

Andrew, I owe you one. I grounded the case and sure enough, needle jumped up to full. Attached the wire going to the tank and it dropped down to 3/4 which is about right.

So seems like it's all working so I didn't have to continue with Brian's suggestions on troubleshooting the sending unit. One less thing to worry about. That leaves just 10,000 more....

[DrEntropy]

DJThom! I'm somewhat surprised. The grounding pigtail should have been danglin' right beside the other two wires! As an FYI the gauge can take a 12V power-up, but as with the MG there is a voltage stabilizer in the instrument feed circuit... ~rumored~ to be attached to the rear of the speedo. Some are, some aren't.

[types26/36]

Doctor,? (Dr) there was never a stabilizer fitted on S1/S2 cars, it only came with S3/4 cars .......are you telling us that MG's are a more advanced car then an Elan .....I think not although I agree a ground wire should be obvious as it would be needed for the light to work

[DJThom]

Yes, there "should" have been a ground. However, I made a new dash with extra gauges instead of a radio cutout, which meant adding/modifying my harness (don't worry, I got the proper colour wire from www.britishwiring.com and modified my wire diagram).

The old type indirect lights aren't available anymore where you power the bulb and ground the case, so my ground ended up in the plastic bulb recepticle.

Easy on the MG knocking! Acquired 14 years ago at the tender age of 19 my B is still my first love

[Emma-Knight]

...I had this fuel gauge issue too, last year the same time - the cause was a bad electrical connection behind the wiper box at the front/rear loom joint. These gauge needs a resistance of 90 ohm to show normal full, so 45 for half and full flow for empty. Just the other way round as expected. A full resistance / isolator will lead to an unpleasant bang of the needle to full. So all connetions have to be sound. My S2 has no additional voltage stabilizer apart the box in the engine bay. There shouldn't be a second one for the S2.

Cauition! The later cars have other arrangements and resistances and parts mixed up will lead to false readings, like full for empty etc.

some helpfull guys in this forum helped me to sort that out.