TR2/3/3A TR3 Fuel Gauge

[PSUTR3B]

My fuel gauge is pegged at full as soon as I turn the key and I'm sure the tank is not. I believe this is a ground problem. If I disconnect either wire the gauge shows empty. I have a ground wire from the gauge to the steering column support which seems tightly secured at both ends. I recall and the schematic shows no ground wire from the sending unit. The guage had been working fine. What am I missing?
Gary

 

[MGTF1250Dave]

Aloha,

I checked my wiring diagram. There should be a green wiring coming from the fuse block (terminal 4) the the fuel gauge. There is a green and black wire from the guage to the sending unit, the terminal is usually on top of the unit. The sending unit has a ground wire to the frame or body, the ground wire is usually attached to one of the screws holding the unit to the tank. There should not be a wire to ground from the gauge. I believe that is what is giving you the full reading.

Safety Fast,
Dave

 

[dklawson]

I'm curious about this and not sure how to respond since I've never worked on a TR3.

This gauge system should be the same as other Smiths/Jaeger stuff used prior to the addition of the voltage stabilizer. On the cars I've worked with that don't have the stabilizer you do need to insure the gauge case is grounded or you'll have problems. Sometimes the ground connection was simply through the metal dash. Certainly try Dave's suggestion to remove the ground wire at the gauge, but if it doesn't fix the problem... clean the connection points and put it back. I don't believe the gauge should "float" relative to ground.

Another curious thing to me is that you say that disconnecting either wire (from the sending unit?) makes the gauge read empty. My experience is just the opposite. If the gauge case is properly grounded I've seen the fuel gauge read "full" when either wire is pulled off the sender. This again makes me question the ground connection to the gauge case.

I don't know TR3s so perhaps something is different... but the Lucas wiring colors sound the same as does Dave's description of the power connection. Whatever you find, please post your findings.

EDIT:
Sorry... I forgot that I put together a PDF to help a Mini owner friend of mine. The PDF was written to show how the pre-stabilizer fuel gauge responds and works. It may not help, but take a look at this PDF.
https://home.mindspring.com/~purlawson/files/OldFuelGauge.pdf

 

[PlaidMan]

A TR4A I worked on had the same problem, turn on the Ignition and it would travel to past Full. One of the studs on the back of the guage was grounding to its case. There should be a nut on the stud that you can loosen and recenter the stud. Hopefully the plastic insulator is still there between the nut and the case[housing]. Good Luck

 

[dklawson]

Be careful when working on those studs on the back of the gauge (if you're talking about the ones in the slots). Those studs control the position of electro-magnets inside the gauge and therefore... it's calibration. If you decide to loosen them, mark their position with a Sharpie or something first so you get them back in the same position.