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Fuel Gauge testing

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jclay (RIP 2018)
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Fuel Gauge testing

Postby jclay (RIP 2018) » 28 Aug 2011 17:36

So, here's the deal! The sending unit is actually two switches in one. A contact switch for the low fuel level warning light and a potentiometer for the fuel level gauge.

1976-1979
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1980
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The green/orange wire is for the low fuel light.
Current comes from the light in the panel to the Delay unit (attached to the instrument pod) then through the green/orange wire to the fuel level sender. At the sender the circuit is completed to ground when the float ROD touches the ROD STOP TAB. Testing the circuit. Let the float rod rest on the stop tab for at least 30 seconds, then check the dash for the Low Fuel light. You can also remove and ground the green/orange wire, after 30 seconds the light should come on.

The Green/black is for the fuel gauge.
Power comes from the gauge in the pod directly to the fuel level sending unit. The potentiometer gives full current to ground when the arm is at it's high point As the arm drops, the resistance in the circuit changes and changes the the reading on the fuel gauge.

Testing the gauge. Disconnect the green/black wire and make a good contact to ground. If the gauge reads full, then the gauge is good. If it reads 3/4, install a new gauge.

Once a new gauge is installed and with the sender out of the tank, you can adjust the float rod stop tab to give a better reading. Holding the sender unit in the correct position as it would be mounted in the tank, take a reading off the new gauge with the float rod resting on the stop tab. If the gauge doesn't read empty, then carefully bend the rod stop TAB down in small increments until the gauge reads empty.

Lifting the float to it's highest level should give you a full reading. There is no adjustment at the sender to give a higher reading.

Clay

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DNK
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Postby DNK » 28 Aug 2011 18:09

Clay, any difference in the 7 and the 8 fuel set up as per your info?

Don
"No More Cars For You"
71 TR6- Perpetual remodel
80 TR7 V8 Kick in the pants
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jclay (RIP 2018)
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Postby jclay (RIP 2018) » 28 Aug 2011 18:40

That is the TR8 system diagram, same as the 7. There was a change in 1980, the power comes directly from the delay unit to light the low fuel light.

Clay

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prlee
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Postby prlee » 29 Aug 2011 08:56

Clay, I have the fuel gauge on the bench, which connection is which when looking at the back of the gauge?


To test on the bench do you use a 12v supply to replace the one from the instrument panel, I presume you connect the black and green and black to the negative side of the power supply, presumably if you decrease the voltage the needle wil fall.

Am I correct?


Pete
1981 Carmelian Red 2 litre DHC - out for the summer.
Polybush, KYB shocks, electric fan, electric windows and electronic ignition roller bearing struts and anti-dive kit - otherwise standard
1979 Pageant Blue Spitfire 1500 - In hibernation - Engine overhaul needed.

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jclay (RIP 2018)
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Postby jclay (RIP 2018) » 30 Aug 2011 04:37

Yes, but . . .

Put a potentiometer in the middle of the positive side run. I would turn it down to 1/2 before putting power to the gauge. Now as you adjust the potentiometer, you should be able to see the gauge move. If it never goes to full, then replace the gauge.

Also, don't forget that our cars are old and there can be all kinds of things causing resistance in the systems.

Clay

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prlee
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Postby prlee » 30 Aug 2011 18:11

Thanks Clay, used a variable PSU today, the gauge refused to move, so its on the way back to the supplier.

Pete
1981 Carmelian Red 2 litre DHC - out for the summer.
Polybush, KYB shocks, electric fan, electric windows and electronic ignition roller bearing struts and anti-dive kit - otherwise standard
1979 Pageant Blue Spitfire 1500 - In hibernation - Engine overhaul needed.

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nervousnewowner
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Postby nervousnewowner » 30 Aug 2011 18:54

i've got a new sender and now new wiring right the way through the car to just under the dash and my gauge still reading full and no matter what i fiddle with nothing works............[B)]

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Dave...

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saabfast
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Postby saabfast » 30 Aug 2011 19:40

Mine had the same problem when I bought it, always reading full. I lived with it for a year or so doing other things. When I checked it out I found that the sender was new so assume the PO had the problem too. I changed the gauge for one from a fleabay instrument panel I bought for spares and it has worked fine ever since. Suggest switching the gauge.

Alan
Saab 9000 Stg 1
Saab 9000 2.3 FPT Auto
'81 TR7 DHC
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prlee
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Postby prlee » 30 Aug 2011 21:22

Thanks Alan but I think I have problems with the gauge.

The original goes from full to half only and the needle just does return to empty even when remove from the car, the replacement does not register anything.

Perhaps if I could join them together I could get a fully working gauge [:D]

Actually I have another on the way from S&S.

Pete
1981 Carmelian Red 2 litre DHC - out for the summer.
Polybush, KYB shocks, electric fan, electric windows and electronic ignition roller bearing struts and anti-dive kit - otherwise standard
1979 Pageant Blue Spitfire 1500 - In hibernation - Engine overhaul needed.

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