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+--- Thread: Adjustment of petrol gauge needle (/showthread.php?tid=7990)
Adjustment of petrol gauge needle - Colin Wilks - 01-09-2022
My fuel gauge stopped working properly due to a bump in the road jarring it and putting the fascia and needle out of kilter. I've taken it apart and I think both magnets are working correctly, but the needle needs rotating anticlockwise slightly, since now it registers half full when full and a lot less than empty when running low. I have had a tentative prod with a couple of cocktail sticks but can't move anything except to rotate the small brass ring that sits immediately in front of the needle and its fulcrum. I am at the point where I think further prodding is going to end in tears so have put it all back together.
Does anyone have practical experience of adjusting these needles please?
RE: Adjustment of petrol gauge needle - JoeRowing - 01-09-2022
I had this problem but traced it to high contact resistance
RE: Adjustment of petrol gauge needle - John Cornforth - 01-09-2022
Hi Colin
Your gauge says 8HP so isn't an Austin 7 one. It should work OK though if paired with the correct tank sender (Probably Maximum resistance when Full).
I am fairly surprised that the needle itself could be knocked round on its spindle by a bump in the road, but I guess anything is possible - I haven't seen the bump !
I'd be inclined to test it electrically first, to be 100% sure that it's a gauge mechanical problem and not an electrical fault. If you have the gauge supplied with power in the normal way (6 volts to B, Ground to the case) connect a 27 ohm resistor from the T(Tank) connector on the Gauge to Ground. This should simulate the sender and give a reading of Full. A 15 ohm resistor will give a reading of Half, and a short circuit will give a reading of Empty. This all assumes it's the later "Max Resistance equals Full" type.
If this is all OK, the only other guidance I can give is that there is an internal soft iron piece mounted on the same tiny spindle as the dial needle. This is attracted to one or other of the little solenoid coil pole pieces. As the sender unit changes resistance, the current in one coil goes down and in the other goes up, so the soft iron piece swings smoothly from one to the other. If you check the needle position, when it says half full the soft iron piece should be about midway between the two solenoids. When it says full or empty it should be close to one or other solenoid.
There is a third "solenoid" with no pole piece, this is actually a 50 ohm wirewound resistor in shunt with the "Empty" solenoid. If this became disconnected it would skew off the calibration. For completeness, the two main solenoids are both about 25 ohms, so checking on the bench with an ohmmeter you should get 17 ohms from B to T, 25 ohms from T to Ground and 42 ohms from B to Ground.
With no current in the coils, the pointer sits below empty due to gravity acting on a small counterweight.
As far as I can remember there is no hairspring or similar, it works purely by magnetic attraction.
If the sender seems to be faulty, remember that its earth return is via the sender body to the petrol tank, then from the petrol tank to the chassis. As standard there is no dedicated wire for these links, so they rely on 80 year old bolts etc. Unsurprisingly, many people add an earth wire from sender to chassis.
RE: Adjustment of petrol gauge needle - Colin Wilks - 01-09-2022
Many thanks John for such a comprehensive answer. The gauge plate is a little loose in the body and so had rotated slightly, jamming the needles travel, whether due to one big bump or just steady vibration over time I don't know! If the gauge is incorrect anyway I shall see if I can source one that's right. I'll check the Source Book.