spasmodic petrol guage needle

[richgilb]

I was cruising down the M18 M62 today in my car, doing about 90 for most of the journey (that's because I am selfish and irresponsible and have one of those 1990s designed twincam engines). I had about a fiver left in the tank and then I noticed the petrol guage needle started going ape in the clock. And then it settled down for a few minutes, and then did it again. And then again. Is this a well-known issue? Can anyone shed any light on it?

Cheers,
Rich

[oldelanman]

Hi Rich,
My guess would be the voltage stabiliser is faulty so the supply to the gauge is fluctuating. If your car still has the original item it is mounted on the back of the speedo so a bit tricky to get at. Some folks have replaced theirs with a modern solid state device which can be mounted anywhere. Do a search and you will find several threads on the subject.

Best regards,

[neilsjuke]

Check and clean the fuses then the earth to the sender unit in tank ,also the earth to the instrument that the voltage stabilizer is fitted to and give it a try.
then I would earth the feed from the sender unit to earth will go full scale and retry as it could be the sender being faulty at low level
Neil

[69S4]

I noticed the petrol guage needle started going ape in the clock. And then it settled down for a few minutes, and then did it again. And then again.
Cheers,
Rich

My petrol gauge is so heavily damped that "going ape" would be beyond its capabilities. Voltage fluctuations, wires falling off, sender jumping up and down in the tank would all move the needle but over a 15 -20 sec time scale. If the needle is moving rapidly I'd suspect something loose in the gauge itself.

[billwill]

I think there are two kinds of fuel gauge.

The heavily damped one is I think a bimetalic strip moving the pointer heate by a wire wound around the strip & carrying the current to the tank sender. More current bends the strip more.

The second, possibly earlier one is a magnetic deflection coil which has two electromagnets. One has a constant current, the other the current from the sender. More current from the sender pulls it to that side. This kind of gauge operates quickly and so can go-wild. I seem to remember it is actually wired as a bridge inside, but that isn't important for this explanation.

I suspect that the most likely cause of that behavior is the sender in the tank. It is a small rheostat, i.e a resistance wire wound around a flat former; the float moves a slider along the resistance wire.

If there is a break or a rusty spot on the resistance wire you get the wild behaviour effect.

The two kinds of gauge require opposite types of tank sender, one is high resistance when the tank is full and low when empty, the other is low when full and high resistance when empty.

[paddy]

You can tell which type of fuel gauge you have from the Smiths part number. If it starts FG then it's the early wild type; if it starts BF then it is the "Bimetallic Fuel" type. The FG type was only on early Elans as far as I know.

Paddy