Smiths Fuel Gauge - Ruby Mk2 1937

[Pault]

Can anyone advise  of both the gauge Empty & Full ohm readings.
Any advice greatly appreciated.

[Ivor Hawkins]

That one appears to have foxed the forum electrickery experts!

[John Cornforth]

Hi

The sender units for the later Smiths moving iron gauges are about 25 to 30 ohms for Full and about 2 or 3 ohms for Empty.  Earlier ones were the same resistance but with Full and Empty values reversed.  The two types are not readily interchangeable.

Moving iron gauges continued to be used up until the late 1950's, but often with senders having somewhat higher resistance.  The MGA used about 70 to 80 ohms.

Later still (e.g. Austin Mini) thermal type gauges with a separate voltage stabiliser were brought in.  These are slow responding and don't "dance" as the fuel moves about whilst driving.

[Pault]

Hi John,

Thank you so much for the information. 
Would i be  right in assuming the resistance reversal (earlier models Empty :25-30 ohms; Full: 2-3ohms) was due to a polarity change - Negative ground?

[John Cornforth]

Hi

No, it's nothing to do with polarity change. 

Both early and late gauge systems will happily work with either positive or negative earth.

[Pault]

Hi John 
Would the gauge model / serial reference determine early / late?

[John Cornforth]

Hi Paul

There is undoubtedly some connection between the gauge numbering and whether it uses the early or late system, but I don't have any reliable data.

You can bench test a gauge in the following way:

Connect a 6V battery to the B (Battery) terminal, and the other terminal of the battery to the metal casing of the gauge (normally this is earthed via the dashboard). 

With nothing (open circuit) connected to the T (Tank) terminal, the early gauge should read Empty.  Now connect a short circuit wire from T to the casing, the early gauge should then read Full. 

If you get Full with an open circuit and Empty with a short circuit, you have a late gauge.

These gauges draw less than half an amp, so the testing battery doesn't need to be anywhere near heavy duty and thin connecting wires are OK.

[Pault]

Hi John,

Thank you so much for your help. 
I'll pursue the testing route & see how the gauge responds.