Cornwall Austin Seven Club 

Repairing a Lucas PLC Ignition Switch
The terminology for parts is based on Fig 4 in
Repairing a PLC Switch which should be referred to first.

Following on from the excellent first article on the humble PLC (Spade Key) Ignition Switch, which was originally published about 10 years after the introduction of the PLC, remember that now, some 80 years later, there is wear, tear, neglect and deterioration to be considered too.

 

There are a couple of items that normally lead to failure…..and Photo 1 shows a typical example I get to repair:

 

 

 

 

1. The rotor insulator  - Photo 2 - develops a round hole ‘Z#3’ in the Tufnol rotor insulator holder for the small brass contact plate shown at the bottom of Photo 3.

 

2. One, or both the extended lugs are broken off by enthusiastic key/screwdriver turning……Photo 4.

 

 

 

 

 

There is a third one, “It wasn’t me …”, repairs that go wrong.

The first article states that a new rotor insulator can fitted, or if you are adept at fine work, then a steel plate can be pinned to the rotor insulator to reinstate the slot – this seems a good recipe for a short circuit as the contact plate brass tags may/will touch this piece of steel and thus a short circuit will occur somehow.

The rotor insulator is easy to replace with a piece of 2mm thick Tufnol cut and shaped to size, together with a slot to hold the flat tongue – there is also a brass pressing, the contact plate, clipped to the rotor insulator, this does the actual On/Off bit (as photo 3).

The extended lugs are more of a problem, they BOTH have to be there to hold the spindle sleeve as shown in Photo 5, I have resorted to drilling a 0.9mm dia. hole down through the moulded base in approx. the centre of the remains of the extended lug, super-gluing the broken off extended lug back on, and then turning the whole lot over, drilling the hole up into the extended lugs and then inserting a piece of 0.9mm dia. brass wire in the moulded base and up into the extended lug, coated with glue.

This has held good on three moulded bases that I have repaired so far – there’s a limited supply of undamaged PLC’s.  The biggest problem for refurbished PLC’s is the pressed metal spindle sleeve – this has a slot for the key in the top, invariably the key hole is now round from frequent use of screwdrivers etc to turn the ignition on and off (I now resort to silver soldering a cover over this area and then opening up a slot for the key) when the key is turned to the 'On' position, the rotor insulator spring holds the spade key tight up behind the spindle sleeve top.

Although the first article implies the Spade and Yale lock barrels are similar, the rotor insulator in the Yale version is completely different, there are no extended lugs, they in fact are part of the spindle sleeve extended down and through the moulded base.

What is not made clear in the first article is that the slots in the rotor and insulator plate are asymmetric and can only be fitted over the the top switch handle moulding (HS in the diagram) one way, it is this alignment of parts that seems to catch most people out, and in trying to get it together, break one of the extended lugs.

 

Photo 6. The face plate (RP in the diagram) has three lugs.

 

 

 

 

Photo 7 - these too are placed asymmetrically around the rim, they can only be fitted correctly to the moulded base one way, there are three openings in the moulded base rim (photo 8 - arrowed B) for these face plate lugs to be pushed down through, the face plate has to be turned anti-clockwise for about ½” were the lugs should find a ‘safe’ dip (photo 8 - arrowed A) to stop the face plate turning.

 

 

Photo 8 – all held steady by the big strong spring inside the switch (S in the diagram).  Of course when dismantling you have to push the faceplate towards the base and then turn clockwise – DON’T use a screw driver to nip the lugs outwards, they may break off and then you’ll want another faceplate, or even the moulded base side wall may break – I haven’t got any spares.

 

Note - there are some PLC’s of the spade type around that do not have the link strap between the ‘A’ and unmarked centre terminal (The other centre terminal is ‘IG’), You WILL need to connect these two together outwith the switch for normal Austin Seven use.

This article, written by Sandy Croall, originally appeared in CA7C Seven Focus in Sept 2010 pp xxx.

 

 

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