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MG TD TF 1500 - PLC6 Alternate Assembly Method

PLC 6: ALTERNATE REASSEMBLY METHOD

Note: Prior to assembly ensure you have noted & marked the OFF position on the body of the switch. This coincides with the single grub screw near the top of the switch. If using a decal it is necessary to complete this step before fitting the decal to ensure it is located correctly. Temporarily refitting just the face plate will ensure the decal is not incorrectly placed. The decal supplied by “From The Frame Up” is a good reproduction of the original lettering

The phenolic insulator disc (10) may or may not be fitted. Later models saw this disc (10a) relocated to the small end of the large spring. Do not omit this insulator or smoke will be released! To assist in reassembly of an earlier switch such as the PLC2 it is recommended that the phenolic disc with the 4 ears be replaced with the later disc & relocated.

During disassembly check & ensure that the two tabs on the Mazak lock body cover aren’t broken. Be careful here, as these two tabs are unbelievably fragile! If the tabs are at all bent over at the back of the switch, it may be possible to very gently straighten them enough to remove the cover. Great care must be used here! With one tab broken it can still be used but if both tabs are broken then the switch is toast!

(A) To reassemble the switch first start with the key switch contact bridge, also known as the ignition contact ring (1). This is a small circular brass bridge with its arc shaped projections facing the bottom of the switch. Two insulated washers (1a) & (1b) then sit on top of it. Carefully place this assembly into the recess in the body, followed by the metal cover (2) with its projections also facing the bottom of the switch, then fit the small spring (3) large end first.

(B) There is a small brass button (4a) protruding from the lower part of the cylinder lock barrel (5) into a corresponding hole in the bottom half of the lock barrel (4). On some PLC6 switches there is also a protruding spring (4b). Insert the key & depress the brass button (4a) to release the cylinder lock barrel (5) from the bottom half (4). Be careful not to lose the spring (4b) as this is easily done. Apply a little Vaseline internally to (5), (5a) & (4) as an aid to assembly & then reassemble it.

(C) Place the brass contact plate (8) into the switch body making sure the contact plate cutout (9) engages with the molded stop in the body. This is what provides the stop for the 3 position lighting switch and prevents it from going too far clockwise or counterclockwise. Follow this with the phenolic insulator disc (10), if fitted. This is relocated to the other end of the spring in the later PLC6. The large spring should now be first compressed & held with two thin wire twist ties opposite each other, (I used those from a loaf of bread), & then placed in position, with the tails protruding from the 2 slits in the side of the body.

(D) Assemble the bezel face plate (12), the knob (13) and then the U shaped knob retainer (14). The cupped shaped keeper (15) slips over & secures the retainer (14). Note that the retainer (14) must be fitted first. The keeper (15) will have its convex side facing the back of the switch. If a decal is used it must also be applied before (12), (13), (14) & (15) are assembled. Place the cylinder assembly (4), (5) & (5a) through the knob then position the hollow lock barrel (5b), also known as the Mazak lock body cover, over the cylinder & use the key to lock it in place. Finally fit the circular insulator (10a) & the assembled face plate is now complete.

(E) It’s critical to ensure you have the 4 projections on the back of the knob (13) lined up with the corresponding 4 cutouts in the brass contact plate (9) (and phenolic plate (10) if fitted). It will be noted these projections & cutouts are different sizes. It is also essential that the deep cut out in the hollow lock barrel (5b) fits over the central molded stop in the body. Now gently press the two halves together. Rotate the knob (13) a little to get it to drop into the contact plate (9). There is only one way the 3 tabs on the face plate bezel (12) will line up with the 3 notches on the switch body. Check to ensure the lettering on the bezel face plate (12) is properly aligned with the body of the switch. As noted above the OFF position should be marked on the switch body prior to disassembly.

(F) With the two halves of the whole assembly together so the 3 tabs have engaged with their notches, ensure the face plate (12) is fully home. While maintaining slight even pressure give the face a ½” counterclockwise twist so that the three bezel tabs now rest in their slight detents. Release the pressure. Gently untwist & pull the two ties to remove them thus releasing the tension on the large spring. Check the switch operation mechanically then secure the nut (17) with two 6BA brass hex nuts, the second acting as a locking nut. The use of solder just makes subsequent disassembly more difficult than it needs to be.

Alternate method by Peter Hehir based on text originally supplied by Lew Palmer. There are 3 numbered pics to follow which identify the components.









P Hehir

Second pic

P Hehir

3rd Pic

P Hehir

Great information. Thanks, Mike.
Mike Hart

PLC?
efh Haskell

I don't know what it stands for Ed but this ID is molded into the back of the LUCAS T type ignition switch. (Here in Sydney everyone uses it as an abbreviation for the Presbyterian Ladies College!) :) This is the combination ignition & light switch in use with M.G.'s since the 30's. The PLC2 was fitted to the TA & evolved into the PLC6 used on the TC, TD & YT. The MMM, J & P types used a very similar switch made by ROTEX. The method outlined above can also be used when restoring a PLC2. Cheers
Peter TD 5801
P Hehir

Always know PLC as an abbreviation for Programmable Logic .Controller, do not think this is applicable to the MG era.
G Evans

Re the name "PLC", if my memory serves this refers to "Primary, Lamps & Charge".

"Primary" referring to the ignition circuit (in our case, the key switch).

The "Lamps" reference is obvious.

As to "Charging", on earlier versions of this switch you had the ability to select either a 'summer' (i.e. 'half') or 'winter' ('full') rate of battery charging from the dynamo (generator)... this was, of course, on switches used before there were automatic regulators that did this job for you. With the "lamps" turned on you obviously used 'full' charge.

Here's a photo of one off an early Austin as an example:

Kevin McLemore

Just discovered an error in the instructions above. The "6 BA nuts" should read "4 BA nuts". The use of 6 BA nuts came from the Bob Butson article that appeared in the December 2011 Issue of TTT2. I only discovered the mistake today when the 6 BA's arrived & I found that they are too small! Apologies for perpetuating this error & I hope nobody else was inconvenienced. I've now pulled 4 of these things apart & it's worth noting that the retainer (14) & keeper (15) arrangement was reversed on two of the switches. There seems to be some logic in this reversal as the keeper acts as a buffer ensuring that the knob never makes contact with the bezel thus protecting the silk screened artwork (or the decal). Once assembled however this is not an issue as the large spring ensures this separation. The design intent was I'm sure to have the keeper, hence the name, sit over & secure the retainer. Seems to work either way as all four switches had functioned prior to dis-assembly & all showed signs of extensive use. I also made my own insulators (10a) from a plastic ice cream container. Cheers
Peter TD 5801.
P Hehir

Just received the following info from Lew Palmer. "It may be worth noting that the 4BA nuts that we spoke about have a smaller diameter (across the flats) than the standard 4BA nut. This is because the standard 4BA is large enough to potentially short to the ignition terminal. That may be why the confusion existed referring to that nut as 6BA. I haven’t checked, but the nut may be sized for a 6BA socket, but the thread is definitely 4BA. And of course, the nut is brass." Thanks Lew. Cheers
Peter TD 5801

P Hehir

Lew has just confirmed the above i.e. that the thread is indeed 4BA while the nut is 6BA! Don't you just love English eccentricity...
P Hehir

This thread was discussed between 02/01/2015 and 19/01/2015

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