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Fuel gauge/sender problems


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I have been unable to get my long door TR2 fuel gauge working. 
I am aware that the earth is usually the problem with these things and have put an earth wire in from the sender cover to the tank mounting bolt. The electrician said this earth is OK. Initially it didn't move off empty and the electrician identified the sender as the problem.
I pulled the sender out and dismantled it to find the insulation between the power stud and the top cover was breaking up. I replaced the insulation. With the sender out of the tank I attached the power lead and earthed the cover, with my earthing wire to the mounting bolt, and the sender and gauge worked perfectly.
I refitted the sender to the tank and the gauge reads full all the time. I have tried earthing it through another wire to no avail.
Any ideas please.

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The gauge will read full if the wire to the sender is open circuit, or the sender is not earthed, or if the the sender has an open circuit inside (dirty resistance track, or not enough pressure from wiper to track).  The gauge will read empty if the wire to the sender is shorted to earth, or, as in your case the connection to the sender has shorted to earth.

To test gauge, apply volts to the B terminal, earth the case, & earth the T terminal - it should read empty. Then remove earth (& any other connections ) from the T terminal gauge should read full.

Bob.

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1 hour ago, Lebro said:

The gauge will read full if the wire to the sender is open circuit, or the sender is not earthed, or if the the sender has an open circuit inside (dirty resistance track, or not enough pressure from wiper to track).  The gauge will read empty if the wire to the sender is shorted to earth, or, as in your case the connection to the sender has shorted to earth.

To test gauge, apply volts to the B terminal, earth the case, & earth the T terminal - it should read empty. Then remove earth (& any other connections ) from the T terminal gauge should read full.

Bob.

Thanks Bob. Everything worked perfectly until I put the sender in the tank. Therefore the wire to the sender is OK and it doesn't have an open circuit inside (I also did a visual on the circuit when it was open and it all looked fine). 

As you note it can only be one of the 3 things, wire to the sender broken (it wouldn't have worked outside the tank), an open circuit in the sender (again, it worked outside the tank and the visual inspection didn't see anything) and the earth.

Therefore I assume it is the earth. But I can't understand why the sender isn't earthed as I used the same earth cable to test it out of the tank and it worked OK. Now even when I run a 2nd earth lead to the body it still doesn't work. I have cleaned the connections for the earth till my fingers bleed.

Stuffed if I can figure out why it won't earth.

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Hi John,

                if it all works outside the tank but not inside then look for an intermittent  fault. Check any crimped or soldered joints.

Roger

 

PS - regarding the earth.  Are you earthing to the body or the tank?  Is the tank insulated form the chassis etc etc

Edited by RogerH
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9 minutes ago, RogerH said:

Hi John,

                if it all works outside the tank but not inside then look for an intermittent  fault. Check any crimped or soldered joints.

Roger

 

PS - regarding the earth.  Are you earthing to the body or the tank?  Is the tank insulated form the chassis etc etc

Thanks Roger. Yes the tank is isolated from the body so I have run a solid cable from the sender, under one of the attachment screws, to the tank strap mounting bolt below the sender. The earth worked fine with the sender outside the tank and my auto electrician tested the earth declaring it good.

As the only change is to insert the sender into the tank I am at a loss. 

If I remove the cable to the gauge from the sender the gauge goes to zero. If I remove the earth from the tank nothing happens, the gauge stay on full. It has to be the earth (famous last words) but I don't see why or how.

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35 minutes ago, John McCormack said:

 

If I remove the cable to the gauge from the sender the gauge goes to zero. 

That is wrong - removing cable between gauge & sender should make gauge go to full (assuming you have the correct TR2 style gauge.)

TR2 - TR3A gauges react instantly whereas TR4 on react slowly, they work on different principals, & the later type will read empty if wire removed.

Bob.

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18 minutes ago, Lebro said:

That is wrong - removing cable between gauge & sender should make gauge go to full (assuming you have the correct TR2 style gauge.)

TR2 - TR3A gauges react instantly whereas TR4 on react slowly, they work on different principals, & the later type will read empty if wire removed.

Bob.

You are right. It does got to full, or rather it stays on full.

That must be a clue. As I noted in my first post, the gauge is on full all the time the ignition is on.

But why would it work OK outside the tank?

 

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John,

I’ve worked on many electrical and mechanical problems in my life and on several occasions I’ve found things work “On the bench” but not when fitted to what they are supposed to be fitted to.

Usually the answer has been because when bolting the item in place there has been some minor distortion due to the mounting screws causing something to be where it should not be.

Try loosening the sender mounting screws with the ignition on and the sender connected and see at what point (if at all) it all starts to work correctly.

WARNING… It would probably be best to not remove the sender completely with the power on as the slight chance of a spark from somewhere might just cause the petrol in the tank to explode.

Charlie.

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19 hours ago, Charlie D said:

John,

I’ve worked on many electrical and mechanical problems in my life and on several occasions I’ve found things work “On the bench” but not when fitted to what they are supposed to be fitted to.

Usually the answer has been because when bolting the item in place there has been some minor distortion due to the mounting screws causing something to be where it should not be.

Try loosening the sender mounting screws with the ignition on and the sender connected and see at what point (if at all) it all starts to work correctly.

WARNING… It would probably be best to not remove the sender completely with the power on as the slight chance of a spark from somewhere might just cause the petrol in the tank to explode.

Charlie.

Thanks Charlie. Unfortunately that didn't work.

But I removed the sender again and pulled it apart. I cleaned up all the surfaces and removed the bit of insulation I put on the stud under the cover. In doing this I noticed that the gauge stayed on full until the float moved up a bit at which point the gauge went to empty. It then rose normally as you lifted the float.

The tank had 15 litres in it so I thought the float should be high enough to get the gauge to work. I put another 10 litres in and now the gauge reads, but only about 1/8 of a tank with 20-25 litres in it.

It will now read full when it is full but it will read empty with about 15-20 litres still in the tank. Reading empty with 5 litres left would be good.

I tried to bend the arm down but I didn't want to strain it for fear of it snapping, it is 64 years old.

However, the gauge does show a fuel level which is better than yesterday. In the absence of any breakthrough ideas I will leave it and see what happens as the car gets used.

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Ok mystery solved then. The float is dropping too far, & the wiper is moving beyond the end of the resistance wire.

There are matal tabs on the Base of the sender case which should limit the movement of the float rod. The lower limit tab needs bending to stop the wiper moving so far. After that the easy  way to improve matters is to bend the float rod (wire) to push the float lower in the tank without upsetting the adjustment you just made. That will move the range of the fuel level downward.  If the gauge does not then read full with a full tank then the other tab could be adjusted, or the gauge it's self can be adjusted (but this is not simple)

Bob

Edited by Lebro
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5 minutes ago, Lebro said:

Ok mystery solved then. The float is dropping too far, & the wiper is moving beyond the end of the resistance wire.

There are matal tabs on the Base of the sender case which should limit the movement of the float rod. The lower limit tab needs bending to stop the wiper moving so far. After that the easy estate way to improve matters is to bend the float rod (wire) to push the float lower in the tank without upsetting the adjustment you just made. That will move the range of the fuel level downward.  If the gauge does not then read full with a full tank then the other tab could be adjusted, or the gauge it's self can be adjusted (but this is not simple)

Bob

Thanks Bob. I'll bend the shaft tomorrow. The tabs look to be right but if I can move the lower one a bit I will.

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10 minutes ago, Graham Harris said:

You need to make sure the gauge is earthed, you are working on the tank and sender earth but don't say anything about the gauge earth which is viral.

 

Graham

Thanks Graham. The gauge appears well earthed via the mounting bracket. I find a poor instrument earth is revealed by tapping the gauge.

 

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