On the www.practicalclassics.co.uk web site Technical Forum, there is a question on what type of resister is required in a 12v system using 6v gauges (fuel gauge in a A7 for example).

http://www.practicalclassics.co.uk/forums/thread.ehtml?i=815023152

An answer directs you to the following web site :-

http://homepages.nildram.co.uk/~vwlowen/java/zener.htm

The full details of 'Fidos' reply are :-
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"You need a zener diode and a resistor. There is a website where you can calculate what you need but you could do with finding out how much current the gauge uses as this will govern the power rating required for the components. This could be done using a multimeter set to the milliamp range and some sort of 6 volt battery (a small one would do). Connect the battery + to the + multimeter wire and the multimeter - wire to one fuel gauge terminal; run a wire from the battery - to the other fuel gauge terminal. Read off the milliamps and enter it into the website calculator.You may need to try swapping the wires on the fuel gauge if its needle tries to swing the wrong way.

Getting the gauge to work on negative earth could be more tricky, depending on its construction.
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Once you have the milliamps reading go to the ....nildram... web site mentioned in 2nd para.

Sandy

Can anyone who knows about these things tell me what will happen if I just put 12v through the standard fuel gauge? I was cheerfully ready to do exactly that on the latest special, until I read this post! Since I was rather looking forward to having a fuel gauge, I'd be sorry to melt it.

Regards, Stuart

The fuel gauge is grounded via the variable resistance in the fuel tank sender unit. As the fuel level goes up and done the resistance alters thus allowing more or less current to flow. The gauge indicates this and is scaled to display gallons. If you put 12 V through it it will not read correctly because there will be too much current flow. I have a resisitor in the Colanders fuel gauge but cannot remember its value? I will try to have a look tomorrow if no one else has answered by then.

Steve.

Very kind of you, Steve, but I did not reveal all! The sender unit in the (non standard)tank of my Special is a 12v unit from a 2cv...I sort of hoped for more or less right at empty and full! As you can gather, I hadn't thought it through. My fuzzy understanding of the fuel gauge was that it works as a "current splitter" but I take the point that it reads as a function of the flow of power through the tank unit. Oh well. I doubt that the gauge will melt, which is all I was worrying about. I shall fill the tank in gallon increments at some stage and watch the results. Start up day approaches!

Regards, Stuart

The Ruby-era fuel gauge has two windings. One from B to ground is 10 ohms and draws about 0.6 amps, to establish a magnetic field in the instrument. The other from B to T is about 18 ohms. This is in series with the tank sender which varies from 0 to 25 ohms, thus the current in this part is between about 0.15 and 0.3 amps. The reason for the two windings is to make readings immune to battery voltage variations. The danger with simply running this lot off 12V is that the first winding will burn out. A series resistor is the crudest way of running such 6V items from 12V (in theory it only works properly if the current is constant, but let's not fuss too much). If we assume an average draw of 0.8 amp, we need a 7.5 ohm resistor between the battery and the B connection. The nearest standard value is 8.2 ohms, which should be a "power" type rated at 5 watts or more. Alternatively try a 6 volt 5 watt bulb, and use it to illuminate something useful! You might like to see my ruby wiring diagram on the Dorset Club website technical pages http://www.da7c.co.uk/

I have read the topic with interest, both because of the number of people who are going, or have gone to 12v, and the suggestions made.

From what I understand, and use as I have been 12 volt in both a Ruby and an RP saloon since before the 2000 Jogle, it is really quite simple.

There are two coils in the fuel gauge used by Austin, this is not he same in all electronic fuel gauges so simply taking instructions from Practical Classics' may not be OK in all cases.

In normal use 6v is fed into the gauge via the 'B' terminal and as long as only 6v appears at this terminal then the gauge will continue to work acurately. So how do we get 6v from a 12v system. There are sveral ways:-

Measure the resistance accross the 'B' terminal and the earth ( body ) of the gauge and add a resistor of this value in the wire before the 'B' terminal. This works by the principle of two equal resistors in series will produce a voltage half of that applied across the outer ends. i.e. 12 volt across two joined resitors produces 6v at the join. ( just what the gauge needs ! )

The use of an electronic soultion such as a Zener Diode which only allows a fixed voltage to flow depending on the value of the diode or the use of an electronic voltage regulator which looks very similar to a transistor or intgrated circuit chip. You need some electronic knowledge to do this.

Another way is to use a readily available voltage control unit ( those designed to charge early mobile phones which used 6v and were intended to plug into your car cigarette lighter socket )

If you have a constant 6volt at the 'B' terminal of the gauge, no matter what your starting voltage is, then the gauge will be just as accurate as it was before, but how accurate was that ????

Good luck and 12 volts is always better than 6volts, all advantages and NO disadvantages.

Thanks to all concerned I now think I know how to proceed. Main concern is not to melt perfectly good instrument and I think I can now go ahead with some hope of success. Thanks again.

Regards, Staurt

Getting 6V from 12 is straight forward, and a 3 terminal voltage regulator to do this is available "off the shelf" at your local electronics store for a few pence (eg: 7806). Bear in mind that the series resistor option (without a Zener) will only work if the "load" is a constant resistance, and this is not the case with most gauge / tank units.

But, the original problem was not this. It was a 12V tank sender to be used with a 6V gauge on 12V. What is the difference in resistance for the 12V tank sender & 6V tank sender? If the same, then voltage reduction (in various forms) is OK, but if not, there will be a deeper problem.

Of course real men would read the tank sender with a microprocessor and translate it to appropriate levels for the gauge (along with linearisation look-up tables too!)

Cheers,
DG

Dennis...

Real men would use a dipstick!

Lets be honest the bit that you need to know about is the "empty" bit. I gradually filled my tank with fuel whilst watching the gauge. It took a bit more than a gallon to indicate one gallon from empty. Therefore I am "safe" to read the gauge at low levels. It probably reads out throughout the rest of the range, but experience has shown this to not be all that relevant, as long as you have a rough indication as to whats in the tank and use the good old common sense then all will be well! .

Steve

p.s I've owned plenty of "moderns" were the gauge was barely readable due to it swinging about all over the place whilst driving. I guess thats were some fancy circuitry could help?

Steve has rather cut to the heart of the matter, but I am grateful to Dennis for his consideration. To tell the truth I might not have used a fuel gauge had the car not had the instrument from its birth. I thought it would be nice if the needle moved about a bit although, as Steve implies, all I really want is a bit of warning when to refuel. I deduce from earlier contributions that, in terms of not melting the windings, a resistor in the line to "B" as suggested will work. Am I right?. Of course, if I site the resistor inappropriately I might melt something else.
In conclusion you will all be pleased to hear that, for my other Special, I have a lovely big dipstick on which I have carved notches.

Regards, Stuart

Of course, Austins being soooooo economical, one only has to wait for the first splutter, then stuff that oiley rag from the tool-roll in the filler spout & drive home! No fuel gauge needed. The other alternative is to fit ALL of those wonderful fuel saving gizmos that the snake-oil merchants pass off to the gullible, and you will travel so economically that the tank will actually fill itself to overflowing!

Cheers,
DG

Yes, Dennis,

My Father actually invented a special container to put below the filler spout in order to catch the petrol coming out! He reasoned that if you fitted eleven of the gizmos that you mention, and that they all saved ten percent, then you could catch and save the resultant product for tomorrow.

It is good to see that after fifty years the theory has surfaced in Oz. Still, they do say the old ones are the best.


Mike

ps - is that 30 grade snake oil?