TR2/3/3A Fuel Guage Sending Unit

Testing the unit outside the tank. Gauge reads full no matter where I put the float arm? I have tried three different sending units that I had as spares...two of them look like they have never been used. Are all the units bad or am I doing something wrong during the testing?

Assuming this is the gauge system that does not use the voltage stabilizer... both the gauge and the sending unit have to be connected to the same ground.

Supply 12V to the gauge's "B" terminal, connect the gauge's "T" terminal to the insulated sending unit terminal. Connect your power supply ground to both the gauge and the sending unit and try your test again.

You can also check just the sending unit resistance. The following values are nominal. Don't be upset if yours don't match. 10 Ohms = empty (float arm down), 90 Ohms = full (float arm up)

The terminals are not marked. Is the B terminal on the gauge the one on the left side with the gauge facing me on the TR3? I believe I have that set up correctly. I had the fuel gauge itself grounded by a wire from the gauge case to the brass dashboard hold down "wing nut" and the fuel sending unit grounded to the tank strap hold down bolt in the trunk. The gauge worked well with that set up for a long time until the sending unit float '"rod" fell off the sending unit. After reconnecting the rod I've had the issue I mentioned in my initial post. Are you saying I need to run a ground wire from a sending unit hold down screw to the ground I have at the dash?

It sounds like you have all the grounds you need without adding more.

You said your car's fuel gauge terminals were not marked. I found the online copy of the Bentley manual and it's not clear in there which terminal is which. The wiring diagram there shows that the fuel gauge terminals labeled B and T.

It is highly unlikely that Smiths/Jaeger would change their tooling. Therefore, look at the MGA Guru website and Barney Gaylord's pictures of the fuel gauge. (link below)
https://mgaguru.com/mgtech/electric/fg_01.htm

On the next page
https://mgaguru.com/mgtech/electric/fg_02.htm
you can see the B & T stampings (B on the right when looking at the back of the gauge).

If you want to confirm this electrically, see page 5 of the MGA site.
https://mgaguru.com/mgtech/electric/fg_05.htm#fg_diag
Disconnect the wires from the gauge. Set your multimeter to measure resistance. Connect one meter probe to the gauge case. Hold the other probe against one gauge terminal, then the remaining gauge terminal. The terminal that measures close to 100 Ohms is for the sending unit's green/black wire. The other terminal should measure something close to 150 Ohms and it will get the green 12v supply wire.

I'm sure Randall will post soon with better suggestions.

The most important earth is the gauge, if you don't have a good earth the needle will go hard to full every time you turn on the key.

Graham

Here is a photo of the sender with the wires marked.

+ 1 on the good ground wire to both the gauge case and sender unit.

David