Fuel gauge resistance

[Old Dave]

Hi All,

Well I finally bit the bullet and ordered up a replacement petrol tank for my October 1932 RN box saloon, which despite being an RN actually has the rear tank and dashboard of an RP. THe current tank has a leak which was driving me mad.

The new tank is made in Aluminum so when I am long gone it shouldn't rot for any subsequent owner. The tank comes with a "universal" sender unit. This sender delivers 3 Ohms resistance at empty and 150 ohms resistance at full.

Does anyone have any ideal how this relates to the Smiths petrol gauge on the A7? The gauge was working in the old tank after a fashion. Maybe not accurate but providing some "information."

The car is 6 volt still and negative earth.

Any advise would be greatfully received.

Cheers

Dave

[Old Dave]

"Gauge" of course

Dyslexia runs KO   -  sorry

[John Cornforth]

Hi Dave

Up to around 1936 Austin used the Smiths G33 sender and a matching gauge. The sender had a resistance of near zero ohms when full and about 30 ohms when empty. The G33 sender has an oval cover plate and a side terminal, and the gauge terminals are below the centre post.

After that date they used the Smiths G35 sender which used the other sense, i.e. near zero ohms when empty and about 30 ohms when full. The G35 sender has a "rectangle with rounded corners" cover plate and an end terminal, and the gauge terminals are above the centre post. The Morris 8 also used this type of gauge.

I believe the early gauges were marked in gallons and the later ones in quarters, but I can't confirm this.

A 150 ohm sender will give you a reading that varies with fuel level, but it won't be very accurate. There is no easy way round this.

[Colin Morgan]

Would it possible to put a 40 ohm resistance in parallel with the sender? And might this help?

[John Cornforth]

Hi

Adding a parallel resistor would give you the correct reading at full and empty, but the scale between would be very non-linear. The gauge would read half when the sender was at about 30 ohms i.e. 20% of its travel.

[Colin Morgan]

Thanks - so that is the drawback.

There would be the option to 'adjust' the scale on the gauge?

[Tiger]

Would it be possible to fix a matching modern gauge out of plain sight??

[Reckless Rat]

I know some people just like things to work, but to be honest you can live without one. I messed about with my fuel gauge about 35 years ago trying to get it to work and buggered it up, so now it reads 3 gallons, constantly. Most of the time my runabouts are usually in the order of 25-30 miles and a quick check with the dip-stick can confirm whether I've got enough for the journey. However, I always use a belt & braces approach and have a 5 litre (sorry, one gallon) emergency supply behind the passenger seat, just in case. This philosphy served me well on the Santiago run, and I never once had to resort to the "reserve". Just saying.

[bob46320]

You need to get inside the sender unit and make the wire connection to the other end of the wire wound resistance ? Then follow the advice of putting an additional resistor into the circuit - Shouldn't that be in parallel with the sender resistance ?? However, DO NOT FOLLOW my advice until someone else agrees / says that it will not ignite your fuel tank. Just a thought.

This may give you a clue ??
https://www.austin7.org/Technical%20Arti...20Working/

Sorry, scrap this idea,
a) If it is a modern sender unit, you won't be able to get inside it.
b) If you put the resistance in parallel to change from 150 ohm down to 50 ohm, it will not be linear.
On other forums they talk about using an Ardunino (or other) micro computer but the Austin has so much electrical noise, it upsets the microcomputer.
Bob

[Old Dave]

THank you Bob. The drawing of thew sender at that link looks pretty much like the one I have.

I will post some pics as a general reply.

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Hi John, Thank you for that useful info.

Please see the pics attached showing the original sending, which unfortunately has been painted over in the distant past, so there are no identifying maerks of any kind.

The idea of a paralllel resistor ideaappeals but I guess it depends which of the two you mention mine is??

The Smiths gauge is marked in gallons as you can see and has the number ident. x30406 at the top.                

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Thank you everyone. All views are very much appreciated and "Alice" says thank you as well.

Please forgive my mangled use of the reply and inserting images above.

The original setup looks very much like that provided in the link from bob46320.

From John Cornforth's information it would appear that my "old" sender is a Smiths G33 and therefore presents near zero ohms when full and about 30 ohms when empty. The gauge is in gallon markings by the way.

The modern sender is 150 ohms when full and 3 ohms when empty. Is this reversed action handleable in anyway??

If the sender is going to be a problem? My next thought is to try using the original sender (I think they are a common 6 bolt fitting). I would rig a separate earth all the way back from the sender to the gauge in the hope that the irratic behaviour of the gauge might be tamed some what. If all fails the wooden dipstick will have to suffice, but it would be good to get some guidance from the gauge. At least to let me know when to pull in to top-up the tank!!??

If the new sender is going to be no good I will cancel it (£35) and make sure the manufacturer sizes the sender "hole" as per my original tank that he has for a pattern.

Any other thoughts would be welcome.