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My fuel gauge seems to read accurately when at idling speed but goes more towards empty the higher the rpm goes. The car is a Mk1 S from 1966 with its original loom and original tank sender. The speedometer is from a Mk2 S. I've swapped between a few voltage stabilizers (one of which I know is off a 1965 850) and the symptoms remain the same.

Ideas?

I have heard of this before but cannot remember what the problem turned out to be. I'll look through my files and see if I have any old notes about this.

Thanks. I also seem to remember reading about a similar situation on here recently but can't find it.

I think I may have figured it out. The previous owner fitted a facet fuel pump and I think I remember a loose wire hanging there - I bet it is the earth wire from the tank sender. Will check and report back. If that resolves the issue hopefully this thread will help someone else in future.

I contacted a friend of mine whose fuel gauge had the same symptoms. He determined that with his car it was a fault inside the voltage stabilizer. Somehow the engine vibrations were messing up the operation of the electromechanical Smiths voltage stabilizer. He replaced the Smiths unit with a modern solid-state voltage regulator and his car's problem went away.

I did remember finding one ages back, it was the vibrating reed type stabaliser, I think it show more full as the car was driven. Turned out it was the air passing over it and in doing so cooling it, causing a wrong voltage output.

Thanks for the replies. I attached the earth wire that was hanging loose near the fuel pump to the rear subframe and drove to work - the symptoms remain the same

Although this could have solved the problem, the fact that the gauge went to full when connecting the wires to each other at the tank sender suggests that it was earthing anyway? It doesn't add up.

Don't know when UK cars changed over... but is it a bayonet or bolted in sender??? Early bolt in senders are not compatible with later gauges...

Cheers, Ian

As 1071S said, gauge components from before 1965 are not compatible with those that came after 1965. However, you would know in an instant if the parts were mixed. The gauge would read "backwards" (gauge moving towards full as the tank empties) if you had an early sender and later gauge or vise versa.

Since working with the earth wire at the sender did not address the problem, clean all the terminals on the gauges, sender, and wires. If that does not help, consider trying a solid state stabilizer. Moss (U.S.) sells them in packaging designed to look like the original Smiths unit... although they don't have one with the Mini specific mounting tab. Regardless, you could adapt one of those if you want to keep the original look.

The sender is a bayonet type, original to the 1966 car. The voltage stabilizer is a BR1310/00. I see on the Somerford parts list here http://www.somerfordmini.co.uk/eshop/in ... page&id=83 that the 1310 came in positive earth and negative earth versions. Could it be that I have the wrong type? My car is currently negative earth but was positive earth originally. I can't see any indication anywhere on the outside of the voltage stabilizer to indicate its polarity.

I've just bought a new battery. I'll see if that makes a difference. Earlier today the car was idling very unevenly and after I installed the new battery it is much better. Can someone with more electrical knowledge perhaps tell me why that could be? I'm hoping the old battery was the cause of all of this.

The text on Somerford is misleading.

What the text is trying to convey is that for the ORIGINAL Smiths voltage stabilizer there was only one version and it was NOT polarity sensitive. The modern replacements they sell (C34770A and C34770POS) are solid state (electronic) equivalents that ARE polarity sensitive.

If you have an original Smiths stabilizer marked BR1310/00, it is electromechanical and not polarity sensitive. If you buy a replacement from Somerford, buy the one that is correct for the polarity of your car.

Reference:
http://www.7ent.com/products/voltage-st ... 4770a.html
"... A modern, solid state version of the voltage stabilizer; for negative ground cars only..."

Thanks Doug. Upon re-reading that I see what you're saying. So that can't be the problem then because I've tried two of them and the symptoms are the same. Will take it for a spin with the new battery a bit later and see what happens.

This has got me stumped. I've tried multiple fuel gauges, multiple voltage stabilizers, multiple control boxes, cleaned all terminals and triple checked wiring and still the gauge goes below empty from about half way when the revs are at cruising speed.

I have now installed a different fuel tank sender but not yet tested it - battery flat again.

Any ideas, however long shot they might be, would be welcome

Leaking float on the sender? Long shot for sure.

Just having read back through this thread, I'd actually say you've got a fault with your charging system. If battery volts drop below about 10 volts for the old stabalisers or about 10.6 - 10.7 volts for the newer solid state ones their output will drop to near zero. The car will run with battery volts as low as about 9 volts, below which it will run poorly (if at all).

Thanks Spider. I have recently had charging system problems and although I thought I sorted them out, the battery is still only charging to about 12.25V, enough to start the car a couple of times before it can't anymore. Running with the lights on last night drained the battery to such an extent that the headlamps were very dim at idle speed.

If I installed a dynamo that used to be on a positive earth car (a possibility because I don't know where it came from) in my car which is negative earth (without re polarizing it), could that result in these symptoms? I wouldn't have thought so with the voltage stabilizer being an early/original type that's not sensitive to polarity.

I tried a different fuel tank sender as well and the problem is the same. I think the fact that I've tried multiples of every part apart from the dynamo might mean that's where the problem is.

Yes, I'd say it is the Dynamo, as you rev it, it's output increases, but the wrong way! In doing so, dragging down the battery volts. I'd guess that the Dynamo would be a little on the noisy side and get fairly warm / hot to the touch!

The early stabalisers were not polarity sensitive.

Flash up the Dynamo to reset it's polarity.

I have done my best to stay away from dynamos as I never understood or had success adjusting their control boxes. If you fail to re-polarize a dynamo, is it simply an issue of the voltage output being 'wrong', or do you run the risk of letting the white smoke out of something?

Regardless, I agree that if the charging system voltage is going up and down and drops below a critical value there will be fluctuations in the gauge voltages as well. If you have a digital multimeter you could wire that up temporarily and take a test drive to see what's happening with the system voltage while underway.