Petrol gauge problem

The gauge shows full tank since filling up but I know it ought to be down to 1/2 to 3/4.
Today, in a roundabout (right hand traffic), the needle went down to 1/2 while turning, and then back to full.
What is going on here?

You have a 1950s Smith’s undamped fuel gauge.
I’ve got three very different cars fitted with those - you get used to them. They start to swing more frequently - it’s when they stop swinging you need to start worrying. I tend to judge refuelling stops using the odometer.

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The instrument board on this 140 DHC is down, as the clock has been away for repair. And I took the opportunity to set the needle for the temp rading, firm against the resting pin, by pushing it onto the central axle.
Might look at the fuel gauge which was fine until this summer (see first comment). Could be the float on the sender in the tank…
Or just leave and live with it. The 0-1/2 section works, and is more important.

The instrument board is back in place, with working clock and temp needle.
Still puzzled about the petrol gauge. Cant be a part filled float on the rheostat arm, as it goes all the way to 1/1 when full.
But why does it remain there until the tank is under 1/2?


Mine does the same thing, I put it down to a peculiarity of the gauges. I just got back from a drive and while out I put 10 imp. gallons into an almost empty tank, the gauge reads full and it will sit there probably until I am below half a tank, then it will drop like a stone. This is why I re-set my odometer every time I get fuel and have come to rely on the indicator light. When it comes on and stays on I have calculated over the years that this means I have 2 gallons left in the tank. The last time this system caught me out was about 8 years ago, but I always carry a small can of gasoline at this stage so it simply meant pulling over and throwing it in. Crude perhaps, but effective.


Your gauge is not matched to the sender. It is possible to recalibrate them, but it’s a fiddly job and best done with both gauge and sender on the bench, and an element of luck. Some tips here:

The float support wire could be bent wrong and could be hanging up inside the tank.
Or the needle may be hanging up inside the gauge.
If you want to adjust the needle movement, you need both the gauge and sending unit on your workbench with a 12V DC source and some test leads. Clamp the sender in your vise so you can move the float and it will stay in any position.
Wire up the gauge and sender so when you move the float you get the needle moving.
The needle movement is controlled by two small elecro-magnets, which are attached by studs and nuts.

Loosen these nuts just a little bit, then you can tweek-move the magnets around so they are almost but not touching the needle spindle.
Move the float and try how the needle moves.
When you have the needle motion set right, tighten the two nuts.
Then check it again.

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