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MG TD TF 1500 - Electrical Strangeness

Although I experienced this problem on an MG PA, I believe the same problem could happen on a T-type.
I recently converted to an electronic regulator in my 1934 MG PA. Although the regulator had nothing to do with the problem, per se, I did do a lot of wiring.
The problem was that, although the engine would start fine and all circuits like lights, horn wipers, etc. worked perfectly, the engine would not shut off on the key.
Several days of head scratching and checking did nothing to solve the problem. Today, I removed all non-essential equipment from the wiring (lights, horn, wipers, etc.) leaving only fuel pump and ignition. Yet still the engine would continue to run after the key was switched off.
Considerable tracing with a voltmeter and ohmmeter followed which revealed a steady 12 volts present on the ignition (fuel pump and coil/points) circuit only after the engine was started and with the key in either the on or off position.
The problem was NOT a faulty component (except for the restorer).
CHALLENGE: Can anyone tell me what the problem was and how (easily) it was corrected? As I said, I believe the same problem could well occur on a T-type, but is more hidden on a Triple-M car.
Lew Palmer

Well I will take a guess. Once when I did not have a key I discovered I could put a fuse between the two holders and voila, instant ignition.

But of course this might only be true for cars with a separate fuse block. Not sure if you could make the same 'mistake' with the older 9 post regulator.
Christopher Couper

Chris, you're on the right track, but no.
Lew Palmer

OK, I just checked and I was somewhat wrong. The later PLC6 switch is somewhat different than the earlier PLC2 switch, so the same problem isn't likely on a T-type,

On the pre-war PLC2 switch, there are two posts for the key switch whereas there is only one on a TC/TD PLC6. The jumper to the A terminal on a PLC6 is on the outside back of the switch. However, on a PLC2 the jumper is nicely hidden on the INSIDE.
Therefore, my problem was that I had reversed the leads going to the two key switch posts. This then caused a very hidden short from the accessories position (A circuit which connects the coil and fuel pump) to the generator output.
So as long as the engine was running and the generator putting out current, the coil and fuel pump had power, regardless of the key switch position.

Doesn't really apply to a T-type, but a fun brain teaser nonetheless.
Lew Palmer

Thanks for the heads-up on this Lew. Will be useful when I get around to hooking up the new wiring harness on my PA.

There are just 2 wires going to the 'IG' posts and not designated as to which is which.

Who converted your regulator? Bob Jeffers was going to do mine, but sadly he has left us.

Gord Clark
Rockburn, Qué.
Gordon A Clark

Gord, I used a DVR3 from folks in the UK. It fits neatly under the opening in the cutout so is completely hidden.
Lew Palmer

I knew it was generator power to the ignition, but I could not figure out what path it could take to do this.
D. Sander

Almost certainly you've got something miswired.

The only time I saw something like this was when I installed a relay for the power that is turned on and off by the ignition switch. I turned off the ignition switch and the car kept running--it turned out that the generator was powering the relay through the ignition idiot light. I put a diode in series with that line; that allowed current to pass only in the direction toward the generator, and blocked it from the generator, so all worked well afterward. You don't have a similar relay, do you?
S Maas

This thread was discussed between 01/10/2014 and 04/10/2014

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