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MG TD TF 1500 - Ammeter pinned

I have a 1951 MG TD with a 20-0-20 ammeter. Out of the blue, I noticed the ammeter was pinned at 20 Amps. Upon further inspection, the ammeter pins at around 2000 rpm. If I back off to idle the ammeter readings are proper.
I have checked the voltage at the ammeter and it is indeed increasing to around 17 volts when I rev up the car and the ammeter pins. I have replaced the voltage regulator and the generator and the problem remains. My voltage regulator is digital.
I have read some posts in the BBS Archives which seem to suggest that this is not an uncommon problem and has something to do with the ammeter. I'd appreciate any thoughts on how to fix this problem.
Thanks.
Milt
Milton Babirak

If it is really putting out 20 amps, something will fry. Wiring, dynamo, etc. 17v seems way too high too. Some new regulators need to be calibrated, but I don’t know about electronic ones. Pretty sure the regulator controls dynamo output, if wired correctly. George
George Butz III

electronic regulators can be affected by radio frequency emissions from electronic ignition and ignition leads.
Ray TF 2884
Ray Lee

Milt, an occasion pegging of the ammeter is not uncommon. It's liable to do that when the battery is low, but it then drops down into expected readings. These ammeters consist of a lower range ammeter with a shunt resistor across it. You need to check the current with a known good ammeter. Do you have an ammeter? It doesn't have to be a 20 amp meter. Many multi-meters have ampere ranges. You only need something with a range of about 5-10 amps. Test with the ignition turned off.
Disconnect the brown/white lead from the 'A' terminal of your regulator and connect the meter between that wire and the 'A' terminal. Now turn on the headlights and see if the external meter reads close to the same as your 20-0-20 meter. If so, then your problem is not in your ammeter, but is likely to be in your regulator.

You'll need to watch the polarity of the connections of the external ammeter. If your car has positive ground you'll want to connect the positive meter lead to the 'A' terminal. Best to have a spare multi-meter fuse handy.

Bud
Bud Krueger

If your charging rate should be regulated at 13.6-14v maximum. Your regulator is a problem in and of itself.

The amp meter can register hi amperage relative to the gauge but should deminish as the battery recharges after starting and eventually read near “0” when charged.

Do not confuse amperage with voltage. They are separate entities. You will cook your battery and burn out bulbs and destroy switches if you don’t correct the voltage issue. Ditch the electronic regulator and replace it with an original type breaker system. Even a used one if it hasn’t seen moisture intrusion will not require a simple points cleaning and parameter adjustments. It will be reliable from that point forward
W A Chasser

I tried Bud’s elegantly simple suggestion of putting my 10 A ammeter on post A of my voltage regulator and the brown/white wire. The dash ammeter and my external ammeter read appropriately the same.
W A Casserole recommended getting a new non-digital regulator and trying that. I don’t have a spare regulator and I know Moss sells them. Is there something that can be done with my digital regulator? I read something about cutoffs.
Milt
Milton Babirak

I tried W A Chasser's suggestion above of putting in a breaker type voltage regulator. I bought a new one. As discussed below, I am not sure if the new regulator resolved the problem or not.

I installed the new voltage regulator in the TD and went for a drive. As I shifted through the gears, the ammeter pretty much stayed well below 10 A except it occasionally runs up to about 20 A in first and second gear. The amperage varies depending on the rpm but then eventually settles to around 3-4 A at cruising speed around 3 to 3.5K rpms. At any speed below cruising speed, the ammeter also flicks around a lot. It is not steady.

When I bought this new regulator, I thought the settings would be good out of the box and no adjustment is necessary. However, in light of the flickering, I read the Work Manuel. Section N 12 of the Work Manuel describes a test you can do on the voltage regulator. You disconnect the A and A1 wires from the voltage regulator and connect them together. Then, you connect one lead of a volt meter to the D terminal on the generator and the other lead to ground. You are supposed to start the engine and slowly rev it up until the ammeter flickers and then steadies and then read the volt meter when the ammeter is steady. I tried this test and could not get the ammeter to steady. Only a lot of flickering.

While this test showed something was not right, I don't know where that takes me. I have driven the car with the new voltage regulator as described above and the ammeter stays well below ten amps but it flickers a lot while shifting. At regular cruising rpms, around 3K to 3.5 K rpms, the amperage remains fairly stable at around 3-4 A.

My conclusion from all of this is that something is still wrong but the car can be driven now since the ammeter readings while driving pretty much stay low (around 3-4 A) except for brief moments while shifting in first and second (pretty much less than ten A but occasionally up to 20 A but only momentarily) .

Any thoughts?
Thanks,
Milt
Milton Babirak

It's possible you have a bad connection somewhere. You're also correct to not assume the new regulator is correctly calibrated out of the box. Aftermarket units are notorious for needed calibration before use. Also, the battery itself can have an issue that will cause problems like this although it's usually a steady high reading in my experience. Barring any other bright ideas, you could send your regulator to Jerry Felper and have him make it perfect, then try again.
Steve Simmons

Could be dirty contacts in the regulator. Did you clean them? Some come with corrosion preventative on them. I would think the 3-4 amp charge is about right. Keep in mind there is no damping in these ammeters, and the fuel pump clicking causes a flicker. And check connections as Steve suggests. George
George Butz III

Many thanks to John Scragg and many others for your comments. John sent me a link to an article listing tests that could be run on Lucas generators and voltage regulators. That article was very helpful, even more helpful than the workshop manual. It provided me with an ordered list of tests that I could follow to figure out what was wrong.

I followed the advice of one commentator and bought a new mechanical voltage regulator for my MGTD and installed it. Upon installation, I readily discovered that it wasn’t working properly. Contrary to the workshop manual and to some vendors assertions, the voltage regulator needed work before installation. In my case, the new mechanical voltage regulator needed the points to be thoroughly cleaned and set. Also, the voltage regulator and cut off adjustments were not properly set. Following John’s article, I cleaned up the points, set the gaps and set the voltage for the voltage regulator and the cut off.

By the way, there’s significant commentary in this BBS and in it other sources that the proper voltages to set a new or existing voltage regulator are not those as set forth in the workshop manual. I don’t have the resources or the time to finally establish what those proper voltages should be, and I’ll leave that to others.

I Tested my car with the new mechanical voltage regulator and new adjustments. At this time it seems to be running properly. Thank you all for your helpful comments.

Happy holidays,

Milton
Milton Babirak

This thread was discussed between 03/12/2023 and 22/12/2023

MG TD TF 1500 index

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