Fitting a Voltmeter

I am really surprised that the MGB wasn't fitted with a voltmeter as standard. They are a useful piece of equipment as a recent alternator failure showed me. The alternator failed but the charging light didn't come on at all as it was an internal wiring failure. Consequently I didn't realise it had failed until the battery had run down perilously low. Luckily I made it home but had I received earlier warning I could have driven to conserve battery power and it might not have been such a close run thing! A new alternator solved the problem but some more information on the state of the charging system would be very useful.

I tried to get a Smiths voltmeter but these are silly prices, so in the end I opted for a cheaper Chinese meter from Ebay, which even comes with its own bezel. LED illumination is pre-installed as well.

Chinese Voltmeter and accompanying bezel

I decided to fit it under the dash on the left. It would look better in a dash-top pod and I may eventually relocate it there but this Chinese meter has really bright illumination and I think would dazzle so close to the windscreen*. Furthermore its not possible to use a dash-top pod without making holes in the top of the dash (doooh!) which will be obvious if the instrument is ever removed. At least fitting it underneath hides the fixings. If I can ever get a nice Smiths meter and a vintage double pod from Harry Moss then I may reconsider as I'd like a vacuum gauge too.


* I have to say that I don't know why the original instruments are so dim- this new one is illuminated by a single LED of the type I have installed in the other clocks on the dash, but is far brighter!

Testing the position for the mounting bezel

I soldered some extension wires onto the lighting leads

Extending LED lighting leads

Instrument lights are all fed by red and white wires, I detached the tacho light and made a soldered internal T join to connect the red lead up to the new voltmeter. I subsequently bound the join in tape and used a cable tie to hold the wires together so that the join won't be stressed if the wire gets pulled.

Tacho light bulb dangling- T join to connect voltmeter illumination.

I installed the bezel by drilling two holes and screwing it on with self tappers.

Bezel installed

I connected leads to the meter terminals using crimped eyelets and routed the light to earth via the meter's own negative terminal.

Wiring added- note that the LED earths via the meter's own earth terminal.

I drilled a hole in the metal of the chassis below the dash to connect the earth lead

Meter earthed to chassis under the dash- black wire with crimped eyelet.

I could then install the meter and tidy up the wiring.

Meter in position

The final lead has to be connected to a live source. There are several views on this; some suggest using a permanently live un-fused (brown) wire, others a permanently live but fused wire (purple) and others a  switched live only energised when the the ignition is on. The problem with any permanent live is that the meter would constantly show battery voltage even when the car wasn't running and although the current drain would be small it would still be a drain. I opted for a switched live feed and checking my Lotus, which has a voltmeter fitted as standard in the factory, I saw that the meter is connected to the voltage stabiliser. I think its actually connected to the power input terminal of the stabiliser which is used simply because its an ignition controlled live feed. The stabiliser is too tricky to access on the MGB so I opted simply to route the red wire out of the cab via the wiring grommet on the RHS, up through the right wing (beside the brake master cylinder) and out into the engine compartment. I could then connect this to the  second terminal on the fusebox (fused side) using a piggy-back connector. This inserts the connection into the green wire feed to windscreen wipers.- Result a functioning voltmeter!