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Austin Healey 100 BN2 fuel gauge not working

Martinld123

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Hope someone might help. My fuel gauge has been intermittent for a few years but now has finally stopped working. I have a negative ground car. At the sender to ground I get 1.27 volts. At the gauge I get 1.31 volts to ground so slight loss at sender. On battery side of gauge I get 11.78 volts to ground without engine running. All readings were with engine off. With both wires on gauge I get a resistance 21.4 ohms between both posts. I am hoping for a $57.00 sending unit problem and not a $499 gauge problem, lol. I am leaning toward the $57 solution. Any ideas? Thanks Marty
 

rossco

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My BN-1 fuel guage is not working. How do I test the sending unit to make certain that's not the problem? Sorry to bust in on this thread but some of the responses may be related. Who does the Smith's gauges repair?
 

55modified

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I will join, mine stopped working too. Guy up East rebuilds I forget name, he is easy to find. You can test gauge to see if it spikes. Google that. My problem has to be related to 63 year old cloth wiring. My new tank was installed with a new sending unit.
 

Brinkerhoff

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The fuel tank needs to be grounded which is done through the steel fuel line since the tank sits on rubber pads. Grounding the hot lead to the sender will bypass it and should cause the gauge to read if the gauge wiring and gauge are ok.
 

dklawson

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I have no experience with the specifics of the 100 fuel gauge. However, the gauge system is almost certainly Smiths/British-Jaeger magnetic.

On magnetic gauge systems it is imperative that the gauge case has a good ground and that the tank has a good ground. The sending units typically operated from low resistance = empty to high resistance = full. With the sending unit out of the car you can test the sending unit with a multimeter. Connect one probe to the sender flange or ground terminal, connect the other probe to the insulated sender terminal. Moving the float arm up and down should produce a resistance change... probably in the range of 0 to 90 Ohms. If you see a resistance change that goes to "infinite" somewhere along the travel, the sender resistance windings have an open. Repair is typically by replacement.

If you want to check the gauge, at the sending unit locate the green/black wire. With the ignition in the run position, disconnect the green/black wire from the sending unit. The gauge should go to (above) full. Now touch the bare end of the green/black wire to chassis ground (on the car, not the tank or sender). The gauge should go to (below) empty. If that doesn't happen, check that the gauge case has a good ground and repeat the test. If the gauge still doesn't respond you have a gauge issue to sort out.

All of this assumes you are getting 12V to the "B" terminal on the back of the gauge and that the green/black sending unit wire is securely attached to the gauge's "T" terminal.
 
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Martinld123

Martinld123

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I have no experience with the specifics of the 100 fuel gauge. However, the gauge system is almost certainly Smiths/British-Jaeger magnetic.

On magnetic gauge systems it is imperative that the gauge case has a good ground and that the tank has a good ground. The sending units typically operated from low resistance = empty to high resistance = full. With the sending unit out of the car you can test the sending unit with a multimeter. Connect one probe to the sender flange or ground terminal, connect the other probe to the insulated sender terminal. Moving the float arm up and down should produce a resistance change... probably in the range of 0 to 90 Ohms. If you see a resistance change that goes to "infinite" somewhere along the travel, the sender resistance windings have an open. Repair is typically by replacement.

If you want to check the gauge, at the sending unit locate the green/black wire. With the ignition in the run position, disconnect the green/black wire from the sending unit. The gauge should go to (above) full. Now touch the bare end of the green/black wire to chassis ground (on the car, not the tank or sender). The gauge should go to (below) empty. If that doesn't happen, check that the gauge case has a good ground and repeat the test. If the gauge still doesn't respond you have a gauge issue to sort out.

All of this assumes you are getting 12V to the "B" terminal on the back of the gauge and that the green/black sending unit wire is securely attached to the gauge's "T" terminal.

Doug thank you! I took the wire off the sending unit turned on key and the gauge showed full. Grounded wire to car frame and it went to empty. The new sending unit is on the way. You were a big help. Thanks Marty
 

dklawson

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I hope you did not buy the sender prematurely. If the new sender does not fix the problem, be sure to run a dedicated ground wire between the fuel tank/sending unit and the car's chassis.
 
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Martinld123

Martinld123

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I hope you did not buy the sender prematurely. If the new sender does not fix the problem, be sure to run a dedicated ground wire between the fuel tank/sending unit and the car's chassis.

Doug thanks for follow up. I ran out to garage to ground sending unit but gauge still not working. Wouldn't the fuel line ground the tank? Still looks like sending unit. thanks again, Marty
 

dklawson

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Wouldn't the fuel line ground the tank? Still looks like sending unit.

I believe the original Petroflex (braid covered) fuel hoses were supposed to supply the ground connection to the tank. However, it's not unusual for owners to replace that with simple rubber hose and plain rubber won't conduct. That's why I suggested testing the gauge by grounding the green/black wire to the car's chassis and not to the tank. It's also why I suggested running a dedicated ground wire from the car's chassis to the sending unit and/or tank.

If adding the ground wire did not resolve the problem, I agree... your gauge works and the problem is almost certainly in the sending unit. I'm glad they don't appear to cost too much.

EDIT: Craig, thanks for posting that link. I carried out a similar modification on an early Mini fuel sending unit. Instead of using the coiled single-strand wire shown in the article you linked, I used a length of braided motor brush wire. That material is very similar to the ground wire that typically runs from the breaker plate to distributor housing in early Lucas distributors. It is made for high-flexing. If anyone wants to use fine-stranded motor brush wire for their sender, I suggest you tin all the other parts/surfaces first before soldering to the wire. The fine-stranded stuff will wick solder very quickly and loose its flexibility.
 
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Martinld123

Martinld123

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I hope you did not buy the sender prematurely. If the new sender does not fix the problem, be sure to run a dedicated ground wire between the fuel tank/sending unit and the car's chassis.

Doug just installed new sender and it works great. I think I might have found the problem with plastic float filled with petrol, lol. Sender is older version with just one wiper on one side of coil. New one has wipers on both sides of coil to trap it but same plastic float. I never thought any plastic could survive long swimming in a lake of petrol 24/7. I am guessing 10 years for this float. I should have pulled it out of tank first to inspect but playing with 14 gallons of petrol once is enough for me as you know lots of garages burn down that way. I had lots of fire extinguishers around car and me, lol. Thanks, you made it easy. I always get great help here on British Car Forum. Marty
 

John Kuzman

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I replaced the plastic floats with a brass float. Picked one up at the local Ford dealership. Ford number: C0AZ-9202B
 

dklawson

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I am glad you got it working without mishaps.

The Ford float is a good substitute for the Smiths part. There is one caveat when using the Ford float. The Smiths float arms are coiled on the end to hold the float. Often the end of that loop has a sharp burr on the cut end. That sharp burr chan punch through the thin brass float where it did not punch through the plastic. Use a file to remove the sharp edge on the end of the loop and spread the loop a bit wider so it doesn't crush the Ford float.
 
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