Bought a Smiths Petrol Gauge for the Box last summer and when I tested it, it immediately went to Full no matter where the float was positioned on the sender. Even tried reversing the + and - with no difference in the result. Now this gauge is identical to the gauge fitted to the Box except the number at the top of the face is different. As I'm not technically minded can someone explain simply why this happens and is there anything I can do to get it to work correctly ? My test equipment is definitely working correctly by the way. Cheers. Lew

Hello Lew,

As a bit of background, there are two conventions in respect to "Smith's" system Magnetic Fuel Gauges.

The original types , introduced circa 1931 to the motoring world generally, and applied to the dashboards of Austin Sevens from late 1932 operate on a "minimum sender resistance gives Full reading" basis.

Sometime around 1935, Smith's introduced a variation which operated on the same principles, but which had a "maximum resder resistance gives Full reading" basis.

This latter type became the basis of most Fuel Gauges used in British cars from the late 1930's right through till the 1960's when the 'thermoelectric" type of fuel gauge became established as the norm.

What I am not absolutely clear about is just what whas the situation with Austin Sevens. I strongly suspect that on the basis of what happened to gauges fitted to Morris Eights for example, that the Fuel Gauges fitted to Austin Sevens gradually became "Maximum Sender resistance equals Full" types sometime after 1935, but I have no proof.

There is an article presently being prepared on the subject of Fuel Gauges for publication in the UK A7OC Magazine for next year.

I would be interested to know, in respect to the situation you have:

What are the code numbers on the face of the gauge "in your Box' and also on the Fuel Gauge under test?

How is the scale shown (eg E, 1/4, 1/2, 2/4 F, or is in in Gallons or something else)?

Is the Sender terminal on your sender unit on the "long side" or "short end" of the sender unit?

There is a fairly simple way of testing the Fuel Gauge itself.

Connect 6 volts to the "B" teminal and the otherside of the battery to the frame of the unit.

With nothing connected to the "T" terminal you should see either "Full" or "Empty" indication, dependent upon whether the Gauge is a Maximum Resistance equals Full or Empty type.

Then, if you connect the "T" terminal on the Gauge to the case (earth) using a suitable jumper wire or maybe a deftly applied screwdriver, you should get the opposite indication to that obtained in the test above. That is "Empty" if you have a Maximum resistance equals Full type, and "Full" if you have a Minimum Resistance equals Full type.


I hope that the above diatribe is of some help to you,

regards

Barry R.

Not sure what you mean when you say 'reversed + & -'. I would expect the gauge (the bit in the dash) to be marked something like B (battery) & T (tank) if you've got the right unit and not one of the modern thermal types. Before anyone picks me up on terminal markings there have been a number of variations over the years.

It is supposed to work something like this:

The pointer is fastened to a small permanent magnet inside the case. Close to this magnet are two coils. One coil is connected between the battery terminal and the instrument case. when the ignition is switched on, current flows in this coil and the resultant magnetism (like poles repel etc.) pulls the pointer towards empty. The second coil is connected between the battery terminal and the tank unit terminal. Again, when the ignition is switched on, a current dependent on the position of the float flows in this second coil and pulls the pointer towards full.

With current in both coils, a point of balance is reached and the pointer will settle (hopefully) at the correct reading.

It may seem over complicated but the idea is that if the system voltage varies up or down then the current in both coils varies up or down in proportion and so gives a measure of independence from normal voltage fluctuations

If your gauge reads permanently full, ie it whacks straight up to the end stop, it sounds like you've lost the ground connection to the instrument case.

Martin

Just a thought?
Running on 12V I have a resistor on the back of the gauge supplied by a friend in the Cornwall Club. If the gauge is "balanced" as described then would it not work on 12V without the resistor?
Is the resistor there to control or limit the current to stop the gauge being damaged by 12V?

Steve

The set up will "self cancel" the sort of variations one could expect over the operations range of the engine - say something like 5v lower end for a well discharged battery, to maybe 7.5 volts or so at the high end of charging.

But if you were to apply 12 volts to a 6v system gauge you are almost certain to cook the windings in the coils inside the gauge. They are wound of extremenly fine wire and whilst they may not go bang at the outset, you will almost certainly overheat them and end up in their ruin. 12v gauages have a higher internal resistance which limits the current flow.

Magnetic gauges are not "concious" of polarity of connections though. Will work happlily in either positive or negative earth situations.

Very best way of operating a 6v gauge at 12v is to arrange a little reguator curcuit so that the "B" terminal is always supplied with 6v.



Regards

Barry R.

Hello Martin,

I wonder if you might be kind enough to send me an email to the above address so that I could have your email address please. I have an "anorak" matter in respect to Fuel Gauges which I am trying to get the bottom of and which I do not wish to clog this forum up with. You well may be able to answer it for me.

Thanks

Barry R

Hello Everyone,
Thanks to all for your posts and suggestions which I have only just read. Will try out some of those when I get the time.In reply to your question for information Barry, my original Box Gauge is marked X30406, whilst the replacement purchased is X28350. Both have dials painted 0 - 1 - 2 - 3 - 4 - F and also Gallons. To answer your final question, the sender unit has an oval top cover and the terminal is located on the long side of this.
Hope this is of help. Regards Lew

Hi all. Was the car fitted with a new sender at the same time? I know little about the black art subject of elec-trickery, but was told many moons back that there were two different senders fitted to A7 tanks over the years and you have to get the right one to suit your gauge. Or is that a myth? Or have I confused you further? Cheers, Bill

Aside from the guage I had problems with the sender getting stuck.
I dismantled it cleared it and then filled the bottom with heavy oil. The oil does not affect the variable resistor as it is a contact type. Has worked great every since.
Pat now USA with an imported "Opal" 1932