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MG TD TF 1500 - 51MGTD Ammeter question with 50A alternator

If I changed from my current stock generator to either an improved higher power generator or if I switch to a higher amperage alternator (up to say, 50A) do I need to disconnect my 30A stock ammeter? Would it do it any damage to be wired into a system where 50 amps are being generated? I'm just thinking that if it won't harm the ammeter, there would be no harm to leaving it in place to show current drain when headlights or accessories are on. I'm considering upgrading my generator and installing an electric fan to better deal with Tucson temperatures.
Just curious what the effect of larger amperages would have on a 30A ammeter, if any - and what others have done in this situation.
Geoffrey M Baker

Geoffrey. Good day to you from Norway.
I do'nt( without knowing for sure)think that higher amperage
deliverd from a generator or alternater will
do any damage to your amp meter. The input
from ge./alt. to the battery is not trough
the amp meter. only the output from the battery is shown.ref starter draws 70-80+ amp
when starting, and are not damegeing your
ampmeter. Or am I wrong?
Thoralf Norway TD 4490
Thoralf Sorensen (TD4490)

If you do not add any additional electrical load, the ammeter would function pretty much the same after the change as it does now. The difference, if any would be in the amount of charge going to an uncharged or dead battery. Then the meter could be pegged for some period of time. If there was a short in the system the needle could be bent from the force of the initial surge, and perhaps the subsequent over current, although it would be not much worse than a short with your present generator system.

If you wanted to you could try to measure the resistance of the meter and then parallel the terminals with a length of wire of the same resistance. This would double the meter reading. This resistance would be less than 1 ohm, perhaps less that 0.01 ohms so a special meter would be needed.

There was a recent discussion on this subject. It was within the past 2-3 months.

Jim B.
JA Benjamin

Hi Jim

I am certain I have seen somewhere that resistors are rated both in ohms and wattage. I am currently fitting a WOSP 50A alternator and after reading previous posts was going to risk the 30A meter. I can understand that full ammps would only pass between the alternator and battery if the latter was fully discharged, but if a simple resistor would protect the meter I will fit one. I am not bothered about the accuracy of the meter - a simple charging or discharging indication would be fine. If I fit a, say 0.5 ohm resistor across the meter terminals what wattage should it be. 12volts x 50amps is 600 watts! I do have a spare ammeter - could I use this as a resistor and hide it under the dash board? One thing WOSP recommend is a new wire from the alternator to the ammeter and then to the battery. An auto electrician I have spoken to also suggested checking an the current capacity of wires to the headlamps, horn and aftermarket fan.

Jan T
J Targosz

The resistor in the ammeter consists of a single 10 or 12 gauge wire with a single loop in it. The resistance of this loop of wire would be measured in terms of miliohms. The worst that would happen if the full 50 amps were directed through the ammeter is that one or both ends of the wire loop would become unsoldered from the terminals. To measure the resistance of the existing wire loop, one would need to use a 4 wire miliohm meter to get any useful measurement (case in point, I have a 10 amp ampmeter in my fuel pump test site and figured that if I wanted to turn the meter off, all I needed to do was to put a switch in parallel with the meter - NOT! What I accomplished was to just throw an unknown parallel resistance across the meter causing it to read an unknown amount of current). The idea by someone above to put another ammeter in parallel with the existing ammeter would certainly work, perhaps not doubling the reading of the single meter, it would certainly be close to it. Cheers - Dave
D W DuBois

Hi Geoffrey,
I recently changed my charging system to negative earth on my MGYA and fitted a 50 amp alternator. I had my ammeter uprated by JDO Instruments see link below

There is a brass link inside the ammeter that acts like a fuse JDO increased the size of the link to rate the ammeter for a larger charge. I would also increase the size of the main power feed wiring to and from the alternator to cope with the extra load.

I have an instruction sheet of how to fit an alternator to a MG Y type from Neil cairns. These instructions also apply to the TD. If you would like a copy please drop me an e mail and i will send them to you.

e mail address

Cheers Chris
C A Pick

I had my ammeter apart and between the two terminals there is a 12/2 loop of about # 10 wire. The field from that loop moves an Iron vane wit the needle attached.
That's from memory and I am old enough so mine is faulty. I would guess the loop is made from copper because there is no need for resistance and you would want a little as possible.
Now the loop used around 3" of wire. (Again from memory)
The resistance of #10 copper 0.00118 ohms per foot. 3" is 1/4 foot so the loop resistance could be as low as 0.0003 ohms.

The power dissipated in the resistor is not the supply voltage times the supply current.

It would be better to use "I^2*R" or (50^2)*0.0003 or 0.75 watt.

I would take 3" of insulated #10. put two crimp lugs on the ends. Holes large enough to fit the screw lugs on the meter.

With out the engine running turn on all the lights, blow the horn and any thing else you can find.
That should give you a fair reading on the ammeter.
Record the reading.

Put the little 3" jumper across the terminals.
Re-test. The current should be 1/2 of what it was. If the current is too low go to #12 etc. if its two large, shorten the #10 wire.

Also the loop you form with the wire should be in the horizontal plane so the field from this new, shut, does not add or cancel with the existing loop field. (although that's a possible way to "tune" the calibration.

Jim B.
JA Benjamin

See for Patrick Earles' images. Bud
Bud Krueger

From the picture that looks more like #12 copper wire.

#12 wire had 0.00187 ohms/foot.
3" would dissipate 1.17 watts at 50 amps.

Jim B.
JA Benjamin

Jim B. nailed it!

Keep in mind, the alternator will do a much better job of balancing charge and discharge current than the original generator and regulator box. I doubt you will ever see it move very far from zero. So, if you keep the original ammeter unmodified, that will probably be fine. If you feel more comfortable with an ammeter that handles the full current, then I'd do what Jim suggests.
S Maas

This has been discussed before. For what it’s worth here’s my experience converting various equipment (not MG) that originally were equipped with 20 amp generators.

I always change the wiring to handle the capability of the new alternator. I have never changed the amp meter as it just shows the to/from the battery and not the load items such as lights, heaters, etc., that are in use with the engine running.

Right after engine start the meter will momentarily peg. Even with a badly discharged battery after the initial peg, at idle, the current stays within the range of the meter. I imagine that if you rev the engine it will be tough on the meter movement but even a high amp alternator won’t give its full output at idle.

As already discussed the built in shunt is pretty heavy and can take quite a load.

For anyone who’s interested, aircraft use external shunts. They are 50mv shunts – the meter will be supplied with 50mv at the rated capacity of the shunt. The meters are usually wired as loadmeters rather than charge/discharge and show the actual output of the generator. They’re marked in %. When upgrading from say a 200 amp to a 400 amp generator, the heavy leads and the shunt would be changed but the meter and leads would remain the same. 100% just becomes 400 rather than 200.
JE Carroll

All said on the ammeter. Re Jan T's remark about checking the wires to headlamps etc- the load on the lights is determined by the bulbs, not the alternator, so it won't change. Checking them is a good idea nevertheless, unless you got a new harness anyway

Rgds, Mike
Mike Fritsch

Hi Mike,

Alternators are frequently fitted so headlamps can be updated and possibly an electric fan fitted, hence the reason I raised the issue of wire current rating.


Jan T
J Targosz

This thread was discussed between 28/07/2014 and 01/08/2014

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