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TR2/3/3A Fuel Gauge

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smdichter

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With the fuel gauge sitting at 1/4, my TR3A coasted to a halt earlier this morning. No gas. First time since I ran my 1961 Comet out of gas in 1964. Sender is about 5 years old, as is the wiring harness. Gauge needle sits full (now) far over to the right, well past the Full mark on the gauge but, before I filled the tank completely, when it had the 3 gallons poured in out of my gas can, the needle bounced. I seem to recall that the gauge is adjustable but cannot quite remember how to do it or even whether it should be attempted. Can it? Should it? Thanks.
 
It can, but it's kind of tricky. Before that, though, I'd make extra certain that you don't have a bad connection somewhere, or a bad sender. The most common problem in my experience is a bad ground at either the gauge or the sender. Try running ground wires to them and see how it looks.

Here's a write up that includes calibration. The resistances aren't quite the same, but the process is.
https://mgaguru.com/mgtech/electric/fg_01.htm
 
Just a thought - you say the sender is about 5 years old. That implies it's a replacement.

Did it always work correctly until now?

Tom
 
There can be several things causing the gage to work well, but not go below 1/4.

1) The float arm can be sticking, or hitting a tank baffle if it is not shaped correctly.
2) A short in the reostat in the tank.
3) Mis-calibration of the gage.

The reostat in the tank should have 0 ohms at full, and about 120 ohms when empty. If you disconnect the wire from the tank to the "T" on the gage, from either end, and then turn on your key, the gage should read past the empty mark. If you then ground the "T" post on the gage, it should read past full. This simple test will allow you to eliminate the gage from the equation.

Most of the issues with the sender unit will require you to remove the sender. The throat of the filler hose is large enough that you can reach a wood stick (or something else non-conductive) into it to move the sender arm to make sure it has full range of motion...and you could even test the resistence while you do it...but you will still have to remove it to fix anything that is wrong.

Edit: please read below for the correct test values...I got them wrong here!
 
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CJD said:
The reostat in the tank should have 0 ohms at full, and about 120 ohms when empty.
Click to expand...
That is backwards. The TR2-3B sender is at maximum resistance when full; and goes to nearly zero when empty. It's the later senders (TR4-TR6) that go to minimum when full. The gauge mechanisms are also completely different, the early gauges are magnetic while the later ones are thermal.

I don't recall the resistance offhand, but 120 doesn't sound right either.

A corollary of this is that with the early setup, a short either in the sender or in the wires will make the gauge read low. It takes a higher resistance to make it read high; like a bad ground at the sender or a worn resistance element inside the sender, or a bad connection somewhere.
 
Right again, Randall. Full is open circuit, grounded is Empty for the check. And about 70 ohms should be the sender resistence at full. I was mixing up my cars!
 
I've tested a bunch of the original senders (Smiths FT3331/02) & if the are functional they measure out at 3 ohms empty & 90-04 ohms full..The aftermarket ones vary somewhat on both ends but are in the ball park.. I use a 1-100 ohm potentiometer rather than the resistor method....infinitely easier. The gauge can be adjusted to the sender but it is VERY tricky..not so much on the electrical end but getting the adjusting nuts loose enough to move the coils can be disastrous. It's been my experience that if the gauge works, the factory calibration is still good. I like the NOS Locators™ aftermarket TR3 sending unit.. The arm is long enough to reach to the bottom of the tank; it has dual contacts for the resistance reading & it reads out at 3 & 90 just like the Smiths.
No, I'm not affiliated with NOS Locators™, just expressing my opinion.
Frank
 
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