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MG TD TF 1500 - Generator to alternator

Hello everyone

During the last year I've expierienced lots of troble with my generator. First the one fitted to the car broke down, and I bought a new one, as the old one was beyond repair. However, the new one did not charge from time to time, and I ended up getting a new unit from my supplier. This second one stopped charging all of a sudden about two weeks ago, and I got fed up with Chinese stuff, which also showed play between the axle and the pulley.
So this week I've installed an alternator, British made by Dynalite (type RAC006T, lifetime warrenty)which fits perfectly, but here comes my problem.
The new unit generates some 40 amps compared to the generators 20 amps. So to be sure the wire from the D terminal to the battery can carry the current, I've fitted a new (extra) heavy wire from D to the battery directly. But my ammeter in the dash now shows only the current passing through the original wire and not what passes through the new wire.
Does anyone know if the ammeter (scale +/- 30) will handle the current from the alternator?
Or must it be shunted in order not to burn up?
If shunt is needed, how is this done best?
As always thanks in advance

rgds
Torben
Torben Olesen

The internal current path, in the ammeter, is a half loop of (about #12) wire.

The ammeter position is designed to measure ONLY the difference between
the current used and the current generated. you would need to have either a totally dead battery or a good short for the meter to get beyond 30 amps.

Jim B.
JA Benjamin

Current is DRAWN, not "PUSHED". Therefore, if nothing demands a current approaching 40 amps, all will be fine just as is.
Lew Palmer

Torben, I did the switch from generator to alternator a few months ago and it has been a very favorable switch. Like you, I have a 30-0-30 ammeter. I did not change the wiring of the ammeter. I only changed the wire from the alternator output to the 40 amp fuse. It is only for a brief moment when I may see the ammeter pegged in the 30 amp charging position. It drops right down to about 15 amps as the battery charge used in starting is replaced. I have never sensed a low battery condition since I did the switch. IMHO the Dynamator regulator is better than the OEM regulator with the generator.
See http://www.ttalk.info/Dynamator-for-Lazarus.html and
http://www.ttalk.info/Alternators.html for some of my experiences.

Bud
Bud Krueger

Torben,

I did this change about 4 years ago, I just swopped out the generator for the alternator and installed a dummy control box, no wiring changes. I did change over to negative earth at the same time. With normal starts the ammeter will hit the stop for 10-15 seconds, then slowly drop back to a few amps. I you have a real problem starting then yes the ammeter will peg out for a lot longer and may require a light tap to free the needle. Its the best thing I ever did, I can now run all night with my halogen headlights on, it used to be -10amps with main beam on the generator.
One thing to watch is the warning light, the exciting current runs through it (on the neg model, relay? on pos model) and if the bulb blows or is lose in the holder you will get no charge.

Bernie
B W Wood

Now here is an interesting thought. A dynamo doesn't put out much charge at tick over but an alternator does. This means an alternator is sapping power from the engine at low revs. I wonder if this will affect the tick over speed - especially just after start up with a cold engine.

Jan T
J Targosz

I haven't found any difference, The alternator isn't generating anything significant at idle according to the voltmeter.
Dave H
Dave Hill

Let's assume the you start the MG. That you needed to crank it a bit.

The Alternator would like to charge it ASAP. It would need to put out about 14V to get to its limit of 40 amps. Thats 560 Watts. 1 HP is 750 watts (metric). So the Alternator would need 3/4 HP at 100% efficiency. Nothing is that efficient, say 80%, that's 0.933 HP gross. call it 1 HP.

But that lasts only for a short time. If you used the enrichment knob, perhaps by the time you can return it to normal the Alternator output would drop off to a small fraction of a HP.

If the engine torque remains constant with RPM then you can relate RPM's to HP.
HP = Torque x RPM ÷ 5252 (says HP increases with engine RPM.)

Let's say you idle at 800 RPM. The XPAG is supposed to deliver around 57 HP at 5500 RPM. So you could expect around 8 HP at 800 RPM and if you bumped your idle speed up tp 870 RPM that would deliver 9 HP.

Yes over the RPM range from 5500 to 800 the torque is not flat so the HP at 800 RPM is a poor approximation. However between 800 and 870 it's not too bad.


Just some napkin calculations.

Jim B.
JA Benjamin

This thread was discussed between 21/07/2019 and 23/07/2019

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