Amps vs. Volts?

Having had problems with battery discharge I decided it would be advantageous to fit some sort of gauge to test the performance of the electrical system.  In any case, period accessories look great!  The only question was which instrument to choose?

In a fight, who would win?  Amps or Volts?
Ammeters are widely fitted to classic MGs and Minis, with a variety of indicator ranges (for example, +/- 45 amps).  But what does the ammeter show?  These measure the electrical current flowing through the circuit.  Typically they are wired in series with the load.  This means that the current flows through the ammeter itself, requiring beefy, heavy duty cables connecting to the device to withstand the potentially high currents. "ABW" has been retrofitted with an alternator.  The electrical current is therefore even higher than might be expected from the original dynamo system.

Whilst useful for measuring the drain under load (for example, from head lights), the car already has a red ignition warning light, which gives a good indication when charging is not occurring.

Voltmeters are connected across a current load and measure electrical "pressure".  This makes fitting a simpler proposition, because they use standard car wiring.

The voltmeter principally provides information about the status of the battery. With normal alternator / voltage-regulator functioning, the voltmeter will read around 14v, falling to 12-13v as the load on the alternator increases (for example, when lights, wipers, and heater blower are used together).  If the alternator fails, then the voltage reading will start to drop as the battery takes on the work and begins to discharge.  Ratings below 12v, particularly on starting, can also indicator a problem with one of the cells on the battery.  A high reading indicates that the alternator is failing to regulate output and possibly cooking the battery and wiring loom.

Given the relative difficulty in fitting an ammeter compared to a voltmeter, and the information that can be gleaned from each device, the latter seems the obvious first choice.  A scan on the internet confirms this opinion.

Smiths voltmeters
MGs were originally fitted with Smiths gauges and I'm keen to keep that period look and feel - so Smiths it is!
The Smiths voltmeter gauges work by internal bi-metal resistance, similar to fuel and temperature gauges.  Like the fuel gauge, the voltmeter does not respond instantly, but the needle climbs slowly when power is supplied.  The gauge draws some power, so is best connected to the "ignition on" circuit with the other gauges.

Being standard on many British cars, especially popular BMC models, a wide variety of suitable 2" gauge designs were sold over the years, some still available new today.  These are variously marked "volts", "battery", "battery condition", or a combination there of!

Various Smiths voltmeter designs

Sourcing a Smiths voltmeter
Several firms supply new or refurbished units.  Perhaps the most extensive range is available from the Gauge Shop (http://www.thegaugeshop.com), who specialise in period Smiths and Jaeger gauges.  Speedy Cables (http://www.speedycables.com) are manufacturers and suppliers of automotive instruments, including Smiths gauges, which are skilfully crafted to the original specifications using the original tools and dies. They also do a cracking business recalibrating speedos.

Being a cheapskate I plumped for ebay!  The result was a rather battered gauge, probably from a mid-70s MGB.  Still it only cost me £7!  Unusually it came complete with a bulb and fixing bracket.

I hunted down some very useful instructions entitled "Smiths - The Care of Instruments" on PDF from Mk1 Performance Conversions (http://mk1-performance-conversions.co.uk), a website dedicated to early Minis. The guide, from 1966, is helpful for a range of instruments.  The voltmeter page is reproduced below, showing a wiring diagram.

From "Smiths - The Care of Instruments", published 1966

The ebay voltmeter has been dismantled, glass polished, and the grot cleaned out.  The face is not perfect, but has come up presentable.  The v-profile bezel was difficult to remove. It is retained with three small tabs.  Ideally the bezel should twist to register with corresponding gaps in the lip of the casing, but some hard caulk material effectively glued the bezel to the body.  Carefully prising two of the tabs up I was able to pull the bezel off.  It has cleaned up reasonably well, with black paint to the inner side coming off with paint brush restorer fluid to reveal clean chrome.

I've reassembled the unit, with a fabricated piece of rubber to seal the glass and bezel.  If the face was perfect, I would invest in a new bezel and new rubber o-ring (all of which are available from several suppliers, for example AES:  http://www.autoelectricsupplies.co.uk).  As it is, I'll probably make do and replace with a better specimen later. Now all I need is a a dashboard panel - more to follow...

Refurbished  Smiths voltmeter