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MG TD TF 1500 - Ammeter

The question I have, could it be possible for the ammeter to cause the lights and the ignition to cut off. If I tap above the ammeter with the car running and the lights on the car and lights cut off. If I continue tapping the lights will come back on and I can start the car again. I have checked all the connections to be sure they are tight. Could the ammeter being going open?
I was hoping to have all this ironed out before today, but for now I am off to the hospital for a full knee replacement. Will check the thread next week for hopefully a solution.

Thanks for everyones help.
Jeff McWhirter


Simple answer - dont tap on the ammeter! Sorry, couldnt resist it! Hope you findan answer though as I am searching for one on a TD fuel gauge too!


Paul Barrow

Jeff - Yes, it is possible that there is a bad internal solder joint in the ammeter. The ammeter cosists of a sort piece of heavy gauge wire, twisted to form a loop inside the ammeter case. There is a vane of magnatized metal that is on th esame pivot as the needle. As current flow through the single turn coil, it causes the vane (thus the needle) to deflect in one dierction or the other. Since both ends of the coil are soldered to the terminals, it is possible to develop a fractured joint at thesolder connections. I had just this happen in the ammeter of a TD that I had some years ago. Resoldering the joints in the ammeter cured the problem. Cheers - Dave
David DuBois

Dave and all,

There is acually no such thing as an 'ammeter'. It is a voltmeter no matter how one slices or dices it.

The "piece of heavy gauge wire" is actaully a shunt, which is designed to absorb the current (energy) consumed as it passes through the shunt. The voltage is measured across this shunt and should under normal circumstances, be directly proportional to the current.

Only difference is that this voltage is displayed on a gauge scale that reads in amperes.

And that's why an 'ammeter' must always be in series with the load it is supposed to measure.

Unfortunately, today's cars no longer measure current (amps), but volts. While that will tell you a lot, it also misses a lot; and its much cheaper (and I use the word in the "less expensive" sense) than an ammeter. Sometimes shunts can get big, heavy and get hot. Considered impractical for today's cars which need voltage, mainly to measure the power supply of the
Gordon A. Clark

Gordon Your observation that there is no such thing as an Ammeter, that all are voltmeters. Is certainly not incorrect it's all a matter of how you look at it.

But I will nitpick at your explanation that the shunt "absorbs" the energy that passes through it. It does not, it develops a voltage drop which is measured by the meter movement (voltmeter calibrated in Amperes).
V=I*I*R (Volts equal I squared R)
So the 'R' must be a low value so as not to subtract the voltage drop 'V' from the energy passing through the ammeter.

Also there are meters that respond to the magnetic lines of force around any wire carrying current. These I think can be called Ammeters.

Also there are devices called Hall Cells that respond to magnetic lines of force. If these are placed near a wire carrying current they will respond with a voltage that is proportional to the current in the wire. The advantage of these devices is that they do not have a voltage drop as the series resistor (Ammeter described above) method does.
R. K. (Bob) Jeffers

To me, an ammeter is an instrument which has an impedance, or a resistance, close to 0, while a voltmeter has an impedance which is as high as possible. So, the instrument on the dashboard is definitely an ammeter. If you don't believe me, apply 12 V to it and see the smoke coming out!

Of course, in reality, instruments measure power, so an ammeter has a slight voltage drop at its leads while a voltmeter lets a small current pass through. But it's better not to confuse the issue and to "believe" than an ammeter has no voltage across and that a voltmeter has no current flowing inside. Electronically you can get pretty close to that ideal case.


P.S. Bob, I got the regulator, thanks, will let you know.
Denis L Baggi


You're right. I just tried to simplify it. And I deliberatly kept away from Hall Cells.

Gord C.

Gordon A. Clark

If I remember my first year of college electrical stuff, E=I*R, or V=I*R being that electromotive force or 'volts' is equal to the current I times the Resistance R.

Apparent power Watts is P=E*I, or P=I*I*R, not V=I*I*R


Dave Braun

To me, my ammeter shows me my generator is charging, but not if my battery is bad!
A voltmeter shows me my battery is bad (for whatever reason?) New cars with voltmeters still have an 'idiot' alternator light?

I like ammeters!
gblawson - TD#27667

Dave B. You are absolutely correct. It is really power that is I*I*R. Unfortunately I always seem to think in terms of the voltage drop and refer to it as I squared R loss, which as you point out is not really true.

Thanks for the correction, we would not want incorrect info on this BBS would we?
R. K. (Bob) Jeffers

Another view -- virtually all analog meters are current meters. Their operation depends upon the magnetic field created by current passing through a coil. An ammeter of the type in a TD contains an internal shunt that 'shunts' the excess current around the meter's winding. The actual current through the meter's winding is only on the order of thousandths of an ampere (or less).
Bud Krueger

"Another view -- virtually all analog meters are current meters."

This is correct, but beside the point. An ammeter, no matter the technology used - maybe I could build one with water or with whiskey - is a device that indicates current and which has ZERO resistance, or at least negligible.

Stray from that and you'll get into any sort of troubles! I warned you!

Denis L Baggi

This thread was discussed between 27/09/2007 and 05/10/2007

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