Voltmeter/Battery Condition Meter

[Eric Capron]

Has anyone successfully re-calibrated a S II/III voltmeter? Above 13 volts mine reads progessively higher than the true voltage. eg it indicates 16 volts when it has 14.5 volts at its terminals.

Eric

[Heuer]

Eric

IIRC there is a small screw on the rear of the voltmeter case which is sealed with black paint. You have some opportunity to calibrate the gauge with that once you have scraped it clean. It is however a 'null' position adjustment i.e. it will set the park position of the needle which is also usually marked on the gauge face edge by a small white line. You could set it read 14.5 volts exactly with that voltage applied but you may find with the ignition off it does not rest correctly. Let us know how you get on

[Eric Capron]

Thanks David,

I took the voltmeter out to test it with my bench variable power supply. I didn't notice the screw you mentioned but there were two holes with something slotted behind them. Without applying excess force I tried to turn or slide them but neither would move.

I wanted to use the car so I put the gauge back but when I get a moment I'll take it out again and if I don't see the screw I'll take it apart and see if there is anything I can do inside.

This started when I thought the the 4TR voltage regulator had snuffed it and was overcharging the battery so I fitted my spare but it was just the same. Once I started to take voltage reading I realised that it was the voltmeter that was wrong.

Will report back.

Eric

[Heuer]

Eric

If you are not unduly concerned with originality this one is a very close match: http://tinyurl.com/3x8sbqq You can swap the rim with your old one and you would never know the difference. For the money involved it is probably not worth messing about with the original in any case. I used it as you can see in this thread: http://etypeuk.com/forum/viewtopic.php?t=166

[Eric Capron]

David,

I think that a new one will probably be where this ends up but I'm too mean not to try to fix the one I've got first. I have to say the one in the links you sent looks good and you'd have to be very picky indeed to notice the difference.

I have kept my car pretty well to the original but not entirely. It's a driver not a show car.

Eric

[Eric Capron]

As an update to this, I have now taken the gauge apart and having done so can answer my own question. Before I dismantled it I noticed that this particular unit has two small holes in the back of the case and I could see inside that there were two slots that would accept a flat screwdriver blade. I tried turning them but they wouldn't and I didn't apply very much force.

Once inside the voltmeter I could see that thay don't turn but slide from side to side and indeed this is how to calibrate the gauge. One affects the low end of the scale and the other the high end but they do interact so you have iterate until you get a good result. Mine now reads OK throughout the range but I am convinced that there is still a problem.

For those unfamiliar with this type of gauge we used to call them 'hot wire' gauges because inside there is a coil of resistance wire wound around a bi-metal strip. I measured 135 ohms but maybe it should be more - see later. When a voltage is applied the current heats up the wire and causes the bi-metal strip to bend. There is a crude mechanical device that amplifies the movement and makes the pointer move across the scale. Although crude it has the benefit of being cheap to make and it is also not polarity sensitive.

I think that my gauge may have some shorted turns on the resistance wire which is why it went out of calibration in the first place. If the insulation is breaking down then it will only get worse and so I will be taking up David's suggestion and buying a new one.

Eric

[S12+2]

I recently changed ammeter on my S1 4.2 for a voltmeter as suggested in a post on this forum. I bought one from ebay which cost me just ?9 and seems to work perfectly.

Best wishes
Nick