How to verify functioning fuel gauge and level sending unit

[SteveHall]

Gents,

 

My fuel gauge is showing empty, while the tank is full. It was working until quite recently, but became flakey - working one moment, on empty the next.

 

I'd like to verify each component: gauge, fuel level sending unit, and wiring. I know how to check continuity of the wiring, but could use some advice on the other two items.

 

Is there a method (e.g. voltage output or electrical resistance) to verify whether the fuel level sending unit is functioning correctly? What about the gauge itself, how can I test it?

 

Thanks,

Steve Hall

Oakville, Ontario, Canada

[Steve R]

Hi Steve

Try this link http://mgaguru.com/mgtech/electric/fg_10.htm

It takes you through the whole calibration process.

Regards

Steve

[david ferry]

Steves (and anyone else!),

 

The link to the MGA fuel gauge calibration will work in a sidescreen car (method not the link!) as a wheatstone bridge is the method of the gauge providing a reading.

 

I don't think it will help in a TR4 onwards.

 

All later TRs work in a different manner. They use a voltage regulator. Early cars don't.

 

I would be tempted to have a look at the wiring first as you know who to do this. I would also have a look at the voltage regulator. You could even buy a spare one in either original design or modern solid state design. In any event, they are not expensive.

 

Good luck.

 

David

[tthomson]

Hi Steve,

 

In the past I have found that connectors to the fuel sender unit have been either corroded or loose giving very erratic readings. Replacing the lucar connectors on the sender end of the wires sorted the problem. Some have different size spades so different size Lucar connectors may be needed.

 

Checking the gauge is not too difficult and the MGA suggestion should work in a similar way on a TR, you may need to find the appropriate connections on the back of the gauge and I'm not sure the resistance values you need to use, but 68 and 34 Ohms seems reasonable.

 

Checking the sender in situ is not easy. The best option is to remove it and test on the bench using an ohmmeter. As you move the float arm the resistance should change. (The resistance should match the gauge requirements). You will need to replace the gasket when you put it back.

 

Good luck

 

TT

 

 

Gents,

 

My fuel gauge is showing empty, while the tank is full. It was working until quite recently, but became flakey - working one moment, on empty the next.

 

I'd like to verify each component: gauge, fuel level sending unit, and wiring. I know how to check continuity of the wiring, but could use some advice on the other two items.

 

Is there a method (e.g. voltage output or electrical resistance) to verify whether the fuel level sending unit is functioning correctly? What about the gauge itself, how can I test it?

 

Thanks,

Steve Hall

Oakville, Ontario, Canada

[BrianC]

I would also have a look at the voltage regulator. You could even buy a spare one in either original design or modern solid state design. In any event, they are not expensive.

Unlikely to be the voltage regulator unless the temperature gauge is also acting erratically, although it's probably a good idea to carry a spare one anyway. I keep meaning to get one myself, but can't decide whether to go original or modern .

[stuart]

It is possible to test a fuel sender in situ by connecting an ohmeter across the terminals on the top of the sender unit and then open the filler cap and using a torch you will be able to see the float and arm inside the tank, then using a piece of stiff wire with a hook bent on the end carefully hook the float arm up and down slowly and read what the ohmeter is showing.

The usual problem with the senders are either the last winding on the wire wound bobbin has broken away from the inside of the connector tab or the wiped area of the bobbin has corrosion, or the actual "wiper" arm that is connected to the float has some corrosion on it as well. All of the above can be fixed but as the wire windings are very thin (about the thickness of a human hair) then great care needs to be taken to either clean the wiped surface or when re-soldering a break in the windings. I have successfully repaired quite a few but it needs a lot of patience.

Stuart.

[SteveHall]

Thanks everyone. I will verify the wiring first, then do an impedance reading across the sender (ie cause the float to move and watch for change in resistance), then report back.

 

Sorry, I neglected to mention, the car is a TR4a.

 

Steve

[tthomson]

Hi Stuart,

 

Thanks for the info. It is useful to know typical failure points on these units. I had considered the in-situ test, but thought it might be a little too difficult for many. Once again your experience is invaluable.

 

TT

 

It is possible to test a fuel sender in situ by connecting an ohmeter across the terminals on the top of the sender unit and then open the filler cap and using a torch you will be able to see the float and arm inside the tank, then using a piece of stiff wire with a hook bent on the end carefully hook the float arm up and down slowly and read what the ohmeter is showing.

The usual problem with the senders are either the last winding on the wire wound bobbin has broken away from the inside of the connector tab or the wiped area of the bobbin has corrosion, or the actual "wiper" arm that is connected to the float has some corrosion on it as well. All of the above can be fixed but as the wire windings are very thin (about the thickness of a human hair) then great care needs to be taken to either clean the wiped surface or when re-soldering a break in the windings. I have successfully repaired quite a few but it needs a lot of patience.

Stuart.

[SteveHall]

Gents,

 

Continuity across the green and black wires is 65 ohms. Continuity across the fuel level sending unit is infinite, and unvaried when float level is manipulated. Looks like the sending unit is the problem.

 

Thanks,

Steve

[stuart]

Gents,

 

Continuity across the green and black wires is 65 ohms. Continuity across the fuel level sending unit is infinite, and unvaried when float level is manipulated. Looks like the sending unit is the problem.

 

Thanks,

Steve

Steve take it out and open it up. Just needs the folded tags bending gently to get the cover off and I suspect you will see the end of the wire from the bobbin broken off. Carefully unwind from the bobbin just enough wire to reach to the tag and give a couple of turns on the tag. Then with a small soldering iron solder back on and that should solve your problem. Beter to repair than faf with the repros.

Stuart.

[SteveHall]

Thanks for the advice everyone. I removed the sending unit, and opened it up. Inside, I found that the contact arm of the rheostat had somehow become bent slightly away from the windings. I simply bent it back to where it made reliable contact, replaced the cork float ring and tank gasket, and it's as good as new. It worked perfectly throughout a 100 mile drive through the Niagara escarpment in Oakville's 25C spring weather.

 

Thanks,

Steve

[tthomson]

Hi Steve,

 

I'm glad your problem was solved. I envy the drive. It sounds idyllic. I am still awaiting my engine from the refurbishing Co., so I am unable to drive my TR yet. It will be 12 years since I started the refurb, so another month or three will make little difference, but I am hoping this year to finish it.

 

TT

 

Thanks for the advice everyone. I removed the sending unit, and opened it up. Inside, I found that the contact arm of the rheostat had somehow become bent slightly away from the windings. I simply bent it back to where it made reliable contact, replaced the cork float ring and tank gasket, and it's as good as new. It worked perfectly throughout a 100 mile drive through the Niagara escarpment in Oakville's 25C spring weather.

 

Thanks,

Steve