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MG TD TF 1500 - PLC 2 Ignition Switch

This query is directed to those electrical engineers among us, of which I hasten to add, I'm not one. This switch was not fitted to the TC or the TD but MAY have been fitted to the TA and the TB. What I'd like to know is whether I can use a PLC 2 body in lieu of a PLC 6? I do realize they are constructed differently internally. What I do know follows.

The terminal notations on the back of the PLC 6 are A which I believe is fused active, T which is for the tail lights, H which is for the head lights, IG/A3 which is the tall post near the centre of the switch and L which I believe is unused. The centre threaded end of the lock body extension, the lock body, the lock and the key are all at earth potential and remain so when fitted to the car.

On the PLC 2 the notations A, T and H are evident and presumably perform the same functions. L is also evident. F1 I presume stands for Field winding 1 in the genny, F2 stands for Field winding 2. I have no idea what D stands for. One of the two tall posts is marked IG and 3440 and the other tall terminal post has no markings at all.

Why go to the trouble I hear you ask? Well I've now restored quite a few switches for owners of TC's and TD's here in Oz as well as for Land Rover Series 1 owners and I find I have a heap of NOS PLC 2 bodies (that I've stripped from 50 military switches purchased still in their original boxes wrapped in grease proof paper and then used the relevant internals to restore the PLC 6 switches), that I don't know what to do with. I now have a number of original MRN and FA locks and the rest of the goodies, but no PLC 6 bodies. Some of those Bakelite PLC 6's traded in turned out to be too buggered to be repaired.

Obviously the PLC 2 contact plate couldn't be used but I have plenty of PLC 6 internals including the large contact plate. I'm able to record my findings using my ohmmeter to list what is connected to what when the switch is in the various positions. If this is possible I can fill the unused field and other terminals at the rear and rebuild a few more switches for TC and TD owners here. I know Craig Seabrook does great work on these switches in the States but there is a bit of a reluctance from some owners here to send off rare items in case they go missing during shipping. Can anybody out there shed any light? Cheers
Peter TD 5801

P Hehir

Peter,
Just a thought. What about making new PLC 6 bodies using 3D printing technology? I am pretty sure I could do the drawings but I don't have an original switch. My car has the incorrect switch with the window.

Regards
Declan
Declan Burns

Maybe Declan. I have zero skills in that area but. These PLC 2 bodies are pristine and when I nut out the internal connections of the PLC 2 as compared to the PLC 6 I might be closer to a a solution. I'm happy to send you one to play with along with a PLC 6 wrecked body if you'd like to poke about with an ohmmeter. You usually manage to come up with solutions to difficult conundrums. This should be a doddle for an electrical engineer though. You have any thoughts Bud? Cheers
Peter TD 5801
P Hehir

Peter,

The main difference between the PLC2 and PLC6 is the number of positions, the former has 4 (LOW, HIGH, SIDE and HEAD) the latter has 3 (OFF, SIDE and HEAD) Your problem is how to convert from PLC2 to PLC6. At first glance this seems to be good news because you are going from "more" to "less" From memory the number of positions is dictated by the large indented washer, so if you have these available you can experiment. I don't have access to spares so I cannot try my theory out.
Another difference between the two bodies is that some PLC2 switches did not have the 2 circlip slots on the outside of the body, these were fixed to the dash by 2 screws.

John


J Scragg

Peter,

As John said, the PLC2 is a 4 position switch, whereas the PLC5 and PLC6 are 3 position. The thing that limits the rotation is the phenolic/brass contact plate. This is stopped by the tab inside the switch body.

I am not familiar enough with the position on the contact plate to tell you whether they are positionally the same between the PLC2 and PLC5/6, such that if you put a PLC5/6 plate into a PLC2 body, the contacts would make the correct contact. I'm also not sure whether the PLC5/6 handle would fit into the PLC2 contact plate or be in the correct position.

Do you have extra PLC5/6 contact plates and knobs? The PLC2 knob is different from the PLC5/6 one. Of course, the bezel is also different.

If you have the needed PLC5/6 parts, I guess you do a trial assembly and see if everything fits and works, although unless you can solve the problem of the contact plate having different stop positions, all this may be academic.

Lew Palmer

Thanks for chiming in John and Lew. I do feel that it has to be possible. I have tons of PLC 6 innards from all of those military switches that I've taken the lock body covers, bezels and the levers from. After rebuilding so many switches I can confirm that the original bits are extremely hardy with the only fragile components being the lock body cover the lock body extension and once in a while a rusted out mainspring. These, the bezel and the lever are the only items I replace. Of course those with little knowledge have destroyed so many switches by trying to prise the 3 bezel tabs open, which often wrecks the bezel and almost always buggers the very brittle Bakelite body. I shall do what you and John suggest Lew and dry assemble one making extensive notes using the results from the ohmmeter tests. The large rotating brass contact plate is a very hardy item and I have those from the PLC 6 switches with broken bodies. I have also resorted to silver soldering up bezels with windows prior to having them chrome plated. That's a challenging task! The bezels from the military switches also require some work as often the switch positions are indented into the surface and also need to be filled and laboriously sanded smooth. All up a fair bit of work involved. Because I don't have a garage, my time is split between working on the car 8 klms away and working on bits here at home, while avoiding the 'honey do list' as much as I'm able to. I have managed to have made, excellent quality, UV resistant adhesive bezel decals with the perfect original font, covered with an incredibly durable protective film.

If any long term followers of this forum need any innards just let me know. They are free to you guys, except for the levers and bezels. These I shall keep. I can let a few used levers go for not much more than the postage or to swap for something that I may need. I also have tons of On/Off knobs and the PLC 2 contact plate which is an intricate pieces of kit and of no use to me. I also have a limited supply of used Wilmot Breeden Union original keys for the MRN and the FA at a reasonable price. If you happen to hear of a job lot of WBU NOS FA or MRN keys going for sale anywhere I'll definitely be a buyer. I scored over 100 NOS keys a couple of years back and that has enabled me to provide a new key with what looks like a brand new switch. Great that even after 70 years these things can be brought back to life and will still be functioning well into this century after most of those reading this, including me, are long gone... Cheers
Peter TD 5801
P Hehir

This thread was discussed between 13/11/2019 and 14/11/2019

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