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MG TD TF 1500 - Ammeter over range

While we're on the subject of ammeters, I have an early 20-0-20A meter that tends to jam against the left-hand pin at about the time the cutout operates and just the brake lights are on. Otherwise the meter operates normally.
Adjusting the cutout voltage doesn't seem to work, nor does tightening the needle pivot screw slightly to hopefully increase damping.
Presumably the 30A meter was introduced to avoid this kind of problem, but does anyone know of a cure for the 20A meter?
Tim
Tim Wilkinson

You could put a shunt across the 2 poles of the ammeter. That would bypass part of the current. Obviously the reading of the meter is then less than the real current, but at a fixed ratio, e.g. 0.5.
Shunt can be a piece of wire or metal. If shunt and ammeter have the same resistance, you get your factor .5, which should address your problem.

rgds Mike
Mike Fritsch

Mike, the TD ammeter is not a classic meter reading the drop across a shunt. It's a magnetic device with a loop of heavy wire. Image below. Bud


Bud Krueger

Does not make a difference to the electric current ;-). Bypassing with a shunt will reduce the current through the internal wire and reduce the magnetic field that moves the needle.
Rgds Mike
Mike Fritsch

Tim

With the engine accelerating from idle to 2000rpm I had a similar problem on my TF with the needle hanging on the RH (charging) stop, it would release after a tap on the bezel.

After building a solid state regulator and fitting it the problem has disappeared. The original Lucas technology is fairly primitive in the manner it functions and the make and break of the relay contacts slow in their response time.

A regulator swap might be an answer. Calibrating the ammeter would be a difficult chore without the correct equipment.

Graeme
G Evans

Mike

Thanks for the suggestion. I agree in theory, but I think Bud is correct in practice - I'll be trying to match the resistance of a short piece of thick copper wire (some fraction of an ohm) and I don't have a high wattage one of those in my box of spare resistors. I guess I could use a similar piece of wire in parallel, but I think I'd rather find the true source of the problem.

I hoped there was some way of mechanically damping the movement. Without that it seems I'll need to have a closer look at the regulator.

Tim
Tim Wilkinson

What series of T is this. Early TD's had 20-20 while later ones were 30-30.
DARNOC31

1950 TD. It's the original ammeter.
Tim
Tim Wilkinson

Mike, you have me puzzled. The image that I posted is for the in nards of a late 30-0-30 ammeter. There is no needle pivot screw. It may well be that the early 20 Amp meters are a different configuration. I don't have a 20A meter to look at. Sorry. Bud
Bud Krueger

I meant the previous post for Tim. But, Mike would probably be interested. Bud
Bud Krueger

Bud - I don't have an image but my 20A meter has a tiny brass screw on the back at the level of the pivot. I believe some Smiths ammeters used this for damping, but sadly tightening mine a fraction just locks the needle.

I don't particularly want to open the ammeter unnecessarily to tinker with it. Maybe someone with experience of this unit can elaborate.

Tim
Tim Wilkinson

Donīt know what the screw is for, but might it just be for adjusting the zero point ?
The fact it locks the needle speaks against that, though

While trying to get the regulator to work as desired
Iīd still try the "short piece of wire" approach in parallel - start with a ft long cable across the meter poles and shorten until you see some effect on the meter. I donīt think anyone actually tries to read the correct amount of amps flowing - I am happy to see the needle moving in one or the other direction as an indicator when power usage is above supply.

Electrically dampening is difficult - capacitors of the size required here are typically uni- directional and the meter works both ways.

Rgds, Mike


Mike Fritsch

Would a digital ammeter clamped aroung the wire connected to the A terminal of the voltage regulator give an accurate reading of what the dashboard ammeter should be reading? I don't even know if those clamp around digital meters work with DC electricity.

Thanks.

Jud
J K Chapin

Mike, what do you think of the possibility of a clamping diode across the meter? Problem is that I have no idea of the magnitude of the transient.
Tim, do you still have a 9 post regulator? Bud
Bud Krueger

Bud - I have the original 9-post regulator (possibly showing its age in this respect). I haven't measured the excess current but it's causing enough swing to jam the needle, requiring a sharp rap on the glass to release the movement.
Tim
Tim Wilkinson

Bud, Voltage across the meter terminals will not be big, it is a current driven device. Probably well below any clamping diode limit. And the peaks that clamping diodes are made for are typically much steeper than what we see here - if the needle can follow, it's slow for a diode...

Jud, A digital ammeter with a current clamp should read correctly if made for DC. If the A wire is the one that leads to the ammeter, then you have the right measurement. Digital meters are bad for detecting fast changes, though, you get readings that don't tell you much

Simplest alternative is to get a cheap analog car ammeter (those aftermarket ones that make the car go faster ;-) with a higher range, and wire it in series with the original, then you can compare the readings and see the movements.

Mike
Mike Fritsch

Mike, thanks. I read the booklet on my cheap digital ammeter and it is only for AC so a trip to Auto-Part-R-Us will happen.

Jud
J K Chapin

Tim, the early TD wiring diagram (WSM N.21) shows that it's fairly easy to connect an ammeter in series with yours without having to get behind the instrument panel. The wire from the ammeter connects to terminal A on your 9-post regulator. All you have to do is to remove the wire (brown w/white) from terminal A, connect one side of your external ammeter to it and then connect the other side of the ammeter to terminal A. Be very careful to keep these connections insulated from ground. The external meter should read the same as your panel meter. Bud
Bud Krueger

Tim,

I had the same problem a few years ago. I fixed it by moving the copper wire loop downwards, this gives less sensitivity to the meter. You can calibrate by using the lights as known loads. You just need to total the power of the lamps(Wattage)) of each of the following:
1/ side lights only
2/ side lights and brake lights
3/ side lights and headlights

(switch off the rheostat and for 2/ you will need to disconnect the coil.)
Once you have these figures divide them by 12 to get the current.

This will give you three point to read from on your meter, move the wire loop until you approch the values.

John





J Scragg

Many thanks to all for your suggestions. Plenty to go on with there.
Regards, Tim
Tim Wilkinson

This thread was discussed between 25/08/2013 and 28/08/2013

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