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MG MGB Technical - How quickly should a voltmeter register?

I'm installing a new voltmeter from the Gaugeguys. When I turn on the ignition, the gauge SLOWLY moves up to 12 volts (i'm talking much slower than the fuel gauge). When I turn off the switch, the gauge goes back to zero in quick "steps", as though it's bound up in some way. I suspect a defective gauge.

Can anyone confirm? Thanks.

Dan

Dan Hiltz

Dan, sounds like a bad gauge. I had a voltmeter on the old Fiero. When you turn the key the volts should jump to 12+ check you wiring again and see if that helps.
James

They might damp it a little to stop it jumping about due to mechanical inputs, however it should register in well under a one second. Usually a meter is set for 0.7 critical damping, which means it just overshoots and then comes back to the final reading and gets there as quickly as possible. Critical is no overshoot but not overdamped. Which your one sounds like. Does it have a spec sheet and if so does it mention response characteristics ?
Stan Best

No spec sheet. I'll call the supplier tomorrow.
Dan Hiltz

Dan, I fitted a NOS Smths voltmeter to my car and it does/did exactly what yours does.

Mine gets quite warm to the touch, something none of my gauges has ever done so I figure there's a resistor inside which is why it creeps up to the desired volatge.

It now registers only 11.5 volts when running but the alternator is putting out 14 volts. (I confirmed that with a voltmeter across the terminals when running.)

So, unless yours is set to read slowly, it too may be defective. I used all new wire when I wired mine according to the instructions. Nothing was mentioned on how fast it gets to the proper reading.
Clive Reddin

If the meter is getting hot it sounds like it has an internal fault leaking to gound. Even with a resistor in the case the meter should still register quickly. A voltmeter on a car should be several K ohms per volt and take negligable current.
Stan Best

Why not ask guageguys?

dave
brdave

Hi all.

Instrument quality meters (eg those used in an analogue multimeter) use a 'moving coil' which is sensitive (takes very little power) and fast-responding. They are generally fairly delicate and not well suited to being shaken about in a car.

Electrical automotive meters are usually rather less sophisticated and use thermal expansion of a wire('hot wire' type) or a heating wire wrapped around a bimetallic strip to move the needle. These are inherently somewhat slow (so don't need damping), and will get a little warm.

My old Smiths voltmeter measures 120 ohms, so will take about 0.1 amps, giving a power dissipation of 1.2W, not a lot but enough to get a little warm.
It takes several seconds to reach full scale from cold.

A meter whose needle moves in steps sounds as if it is 'sticking' which would tend to result in inaccurate readings as well as a slightly slower-than-normal response.

Don
Don

Slow-acting gauges, like the fuel and electric temp gauges used on MGB from about 1965, are thermally operated gauges which have a heating coil wrapped round a bi-metallic strip, so yes they will output some heat, and take several seconds to reach their ultimate indication. I have seen after-market volt-meter gauges to exactly the same design and delay, and they are designed to 'ignore' (up to a certain point) transient changes in fuel level and voltage and give a relatively steady indication of the average value. But the voltmeter on my 1989 Toyota Celica has a much faster rise, and fall, but still doesn't flick about when using indicators, brakes etc. However significantly slower than the fuel gauge does sound too slow, and the stepping does indicate some binding until the tension rises far enough to overcome it. You could take the car round and ask to try another one, but maybe it is just cheap design.
Paul Hunt 2

I spoke to the gaugeguys and they assure me this is normalsince it's bimetallic. I suppose I'll stick with this one.

If it bothers me too much, I'll replace it with a clock!

Thanks to everyone who chimed in.
Dan Hiltz

Dan
Best you put the voltmeter back to the shop, and ask for a voltmeter with an stretchd scale. from 9 to 15 volt.
This meters ar a special disign for auto motor use.
This is the best to look at your board-tension.
wim

The one I bought is marked for 11 to 15, but the scale extends below and above those marks.

Dan Hiltz

This meter sounds horrible. It needs to respond at a reasonable speed, otherwise how can you get info from it while you are driving. There is no reason why a moving coil should not be used in a car, I have a moving coil S meter from a Radio Range ( circa 1940s) that still works fine with 1000s of flight hours on it. The cheap multimeter I carry in the boot is still working fine. The Taut bands that came in in the 60s were almost umbreakable.
Stan Best

This thread was discussed between 30/07/2006 and 31/07/2006

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