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MG TD TF 1500 - Over-enthusiastic Dynamo

Earlier this month (12/21)I decided to return Lazarus (TD10855) to OEM condition to remind myself what that was like. My pertronix LS Igniter was replaced by a set of regular points. My Dynamator alternator was replaced by a used (but refreshed) dynamo from Abingdon Spares and a comparable Voltage Regulator from A/S was installed.
Set the points to give ~57dwell. Set the regulator at ~16 volts per the WSM (garage was cold). Car started easily and seemed to be anxious to go for a ride. I put a float charger on the battery while waiting.

That opportunity came yesterday when my daughter and I put on our Santa hats and drove around Marietta. A problem popped up. I noticed that the ammeter was readily going full scale (~30 Amps) when the engine went over 2200 rpm. If I kept the rpm between 1500 to 2000 rpm the ammeter showed readings in the 15 to 20 amp range. At idle the ammeter was virtually at zero, no Ignition light. That's not normal.

I'm not sure what to tackle to limit the charging current. I'm certain that the battery had received its full charge. Any ideas of where I should start tweaking? Bud
Bud Krueger

My first test for those symptoms is always for a bad battery, but if you've eliminated that then I would next suspect there is something amiss in the control box. The zero ammeter at idle sounds normal if the battery is charged. My needle almost never moves off zero unless I'm running the headlamps or the battery has drained a bit. I think the cutoff circuit is probably not adjusted correctly, or is sticking. Try operating it by hand while the car is running and see what happens. I'm not an electrics wizard, so someone else may have a better idea.
Steve Simmons

Thanks for the reply, Steve. The battery is only about a year old and has been functioning fine with the Dynamator. I've been meaning to do a detailed study of the charging circuit for a long time. Guess it's time (I'm a retired EE.) Bud
Bud Krueger

Bud, Sounds like the cut-out side of the control box is not working as it should. First clean the points in the control box. (both sides) This may resolve the issue without going further. Had to do mine a couple months ago. I guess after 70 years of operation the points get a little "sticky". Check also with a voltage meter. The cut-out should maintain 13.8 to 14 V output from the generator.

Happy New Year.
CR Tyrell

The best way to clean the points is with a crisp dollar bill folded in half lengthwise. Place between the points, push the points closed and pull through a couple of times.
D. Sander

Learn something new every day. One of the reasons that I went to the Dynamator was that my generator was failing. I ended up replacing it with a 'refreshed' used one from Abingdon Spares. It turns out that the one from A/S is a C39PV2 from 1/58. The C39PV2 is one of the higher output dynamos and doesn't want to be controlled by older Control Boxes, such as RF95 or RF96. It likes to see an RB106/1 or RB106/2. I have been able to acquire a 'refreshed' RB106/2 from A/S.
Great information can be found from Lucas Technical Service, Overseas Technical Correspondence Course, Section 5, Generator Output Control.
When this weather warms up I'm looking forward to applying this new knowledge. Bud
Bud Krueger

This thread was discussed between 26/12/2021 and 07/01/2022

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