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TR2/3/3A Tr3 fuel gauge sending unit

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sp53

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Tr3 fuel gauge sending unit is challenging my electrical understanding. I have two extra ones and I am trying to figure out which one might work best. One of them comes completely apart. However, on that one there appears to be a short because the hot lead shows ground with a continuity test back to the case. Again, the bottom assembly unbolts from the flange and I can see the little variable resistance coil dealie with the two arms coming out from the float linkage and just rubbing on the coil. I presume the problem is in the coil dealie shorting to the case. This is where I start showing my lack of understanding because if the arms are touching the coil is not the unit supposed to show some resistance or short? In addition to that, it looks like the whole unit gets submerged in fuel and why would that not cause a short. Even more problematic, how come the vehicle does not blow up if there is a short in the unit if it sparks?
Sp53
 
The tank sender IS the grounding for the circuit. Hot goes to the gauge...through the gauge to the sender, and then the sender varies the resistance to ground.

Use an Ohm meter to check the sender as you move the arm up and down.

I would have to say that gas is not a very good conductor, or they wouldn't be putting electric fuel pumps in cars either.
 
Ok I kinda get what you are saying. The gauge is actually the ohm meter? If I take an ohm meter and put it on the hot post of the sending unit, It still should be insulated from ground right? I mean it should not show continuity?

Sp53
 
The sender is really like a variable wire-wrapped resistor. As the arm moves up and down, the resistance (measured with an ohmmeter) varies. So measure across the terminal and the body of the sending unit with an ohm meter and you will see the value change. Select the "R" scale on the meter that shows a value change when you move the sender arm.

I am not an engineer but I did stay at a Holiday Inn once.
 
The sender for the TR3 should have two terminals on it. One is insulated, and the other isn't. The one that is has the wire that comes from the gauge. The other just goes to ground. Just take an ohm reading across the two terminals and move the float. You should see the reading go from (IIRC) 0 ohms when empty to about 80 ohms full.
 
All the information you've been given above should be helpful. The MGA Guru site is the best resource I've found on the web for the pre-voltage stabilizer gauges. Browse his site for information on checking and calibrating those early fuel gauges. It will be time well spent regardless of your comfort working on electrical things.
 
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