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MG TD TF 1500 - Cut out coil on voltage regulator problem

In my infinite wisdom I decided I should just clean lightly the voltage regulator and cut out coil contacts on both of my '53 TD's prior to summer driving. Took just a few minutes. Then I started one of them and it wouldn't generate. Started the second car and it too would not generate. I polarized both units and tested them and got the correct output by shorting out the generator leads and measuring the voltage. Still nothing when I restarted the cars. I figured that maybe I messed something up so I installed a new control box that I had on one of the cars. The dash ignition light went off as it should when started and running at about 1500 RPM and the ammeter showed the generator was working. When I reduced the RPM the ignition light didn't come back on. Big potential problem. I realized the cut out coil had remained closed and quickly removed the battery quick disconnect.
Now, there is nothing too complex about these cars and the control systems. Does anyone have any ideas as to what could be the problem? Obviously I did something when I "cleaned" the contacts but I have no idea what. The circuitry is pretty simple.
The question is why are the cut out coil contacts staying shut and what did I do when I cleaned the contacts?
Is it possible that the coil needs adjusting on the new unit? I'm guessing that the coil probably cuts in at around 14+ volts which would be higher than the battery voltage so it would then drop out when the ignition is off and the voltage drops below the voltage required to hold the contact closed.
So there are two questions. Why did cleaning cause problems and what is causing the cut out coil to stay closed.
Mark Strang

I might have solved part of my problem. As I wrote this query I got to wondering if a brand new control box could be way out of adjustment. It apparently was. It turns out that the cut out relay was closing at about 12 volts and staying shut when the ignition was turned off. Could have burned the car up I think, by staying closed when the ignition was turned off since current would still be flowing to the generator and other wiring. I had to adjust the cut off coil screw by almost a half a turn to get the contacts to stay open until about 13-14 volts at which point the contacts closed which is at roughly 1500 RPM. Now the contacts open when the car drops to idle which is about 8 volts. It is operating as it should. Now on to the second car.
I still don't know what I did to make the control boxes not work properly when I cleaned the contacts. Perhaps the contacts were at the maximum clearances and literally a light cleaning caused the problem.
Any input/comments will still be appreciated.If I'm wrong in my assumptions, I'll take help.
Mark Strang

Mark,

The best thing for you to do is to download and print the attached document, Part 1 and 2 are specific to our units and the step by step guide is very easy to use. You do however require a voltmeter.

John

https://britishcarbreakfastclubcom.files.wordpress.com/2015/11/lucas_generator_and_control_box_tests.pdf

J Scragg

Most common reason for failed regulator or ignition failure after points cleaning is residual grit. Back in the day I used a piece of a matchbook cover to wipe the points clean after filing or sanding. In general, best to not mess with something that is working fine. I generally don't follow my own advice however, and have killed many previously functioning things over the years. George
George Butz III

Well, I put a new control box in each TD and after some minor adjustments both cars are operating properly. I agree with you George. Leave well enough alone.It just seemed like a good thing to do at the time. Very maddening.
Mark Strang

Mark,

Have a look at this document about Repairing the Standard Lucas RB106.

http://nebula.wsimg.com/9b5ab93a42d604fc86461adc4d298878?AccessKeyId=967E22DE049163134A29&disposition=0&alloworigin=1

It refers to installing a 'snubber' diode which substantially reduces burning of the contact points which open/close about 60 times per second during normal operation.

I've tried and it works well. Without the diode, you see continuous sparking (burning) across the contacts. Install the diode and the sparking disappears.

A diode will make the unit polarity sensitive and has to be fitted the correct way for the car's battery earth polarity setup. A bidirectional Transil device (say, a 24volt device), used instead of a diode, will remove the polarity sensitivity issue.

This is the same protection method used in electric fuel pumps.

David Padgett

Be kind to yourself and your car/s build a solid state regulator, a job done once is the reward.

Graeme
G Evans

This thread was discussed between 31/05/2020 and 03/06/2020

MG TD TF 1500 index

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