Lucas PLC Series ignition/light switches

By Rees Mackay

These switches are fitted to T series MGs, Morrises and many other cars. There are several versions, but they are all essentially the same, and some have more bits than others.

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I have had excellent service from these switches, with never a problem until recently (which you can’t blame on Lucas). The unit had been damaged and the ignition terminal post was loose, this making an unreliable contact internally; and an external link between terminals broken and bent.

Dismantling can be an adventure and an exercise in patience

My first attempt at dismantling and reassembly took the best part of a day, the second half an hour.

1.       Clean the unit, particularly around the groove at the back of the bezel where tabs will be seen holding the chrome bezel onto the bakelite body of the switch. A tooth brush with metho does the job. Stick some masking tape on the body and mark the top tab alignment and light switch position with switch off.

2.       Test the unit, noting all the connections across all the terminals, including the blind ones, in all positions, so that it can be re-assembled correctly. Some may not work because of accumulated dirt.

3.       Remove the solder from the nut in the centre of the unit, this is on the end of the lock cylinder, and holds it together. The nut is probably 6BA (or 5mm fits) and you need a tube spanner or ground down socket to fit between the terminals. To remove the solder hold a soldering iron near vertical in a vise and hold the unit against it, and let gravity do the work. While still hot turn the nut a bit before the solder cools; it will take several goes. It may also help to warm the socket with the iron. There is a tiny washer under the nut.

4.       With the key in place, remove the lock cylinder.

5.       Straighten the two tabs on the back of, and in the centre of the body. These hold the sleeve which carries the lock cylinder and are made of muck metal so you need to be very careful bending them. Check for fractures in the tabs.

6.       You can now dismantle the body of the switch. It is necessary to squeeze the bezel onto the body against the internal spring so that the tabs on the back of the bezel clear their locating recesses, and then turn the bezel until the tabs align with the notches in the body lip. (This is why you needed Step 1) The bezel might come straight off, or it might need some wriggling or even some gentle leverage to get it off the body. At this point it might go sproing and you will be left with the bits all over the table, hence the need for Step 2. Try to hold the bezel against the spring and the body, and gently separate them.

7.       If it hasn’t gone sproing, remove the bits and place on the table in order.

8.       It is probable that all that is required is cleaning off the dried lube, accumulated dust and grunge so that good electrical contact is restored. The switch is well made and is unlikely to have actually worn out. Again the tooth brush with metho does the job.

9.       The bezel and turn knob can also be cleaned. Bakelite responds well to a light rubbing with cutting compound. Some may like to refinish the bezel with new paint and lettering, but this requires more dismantling and much careful detail work, and I don’t think it is worth it, after all, it is an old car.

10.   For the repairs I needed I opted to try the minimum first, peening the rolled end of the post over more to take up the looseness, but this did not last long. For the next attempt I cleaned it all carefully and sweated solder into the joints and hopefully between the post and the body to hold everything firmly, which seemed to work. I left the broken link alone as this problem could be bypassed with a little extra wiring.

11.   To re-assemble, first lubricate the contact rings. There is probably a proper lubricant, but I used petroleum jelly. Hold the body with the open face up.

12.   Place ignition contact ring (Brass contacts down) in off position, then its cover (centre lip up), keeping them aligned; spring (small end on top); and lock cylinder sleeve.

13.   Place lights contact ring (contacts down) in off position (anti-clockwise against the lug on the body).

14.   Carefully place bezel with attached spring in position, guided by the marks from Step 2. The light switch needs to be in the correct position to match the lights contact ring, and the bezel to match the notches in the body. Do not despair if you have lost the masking tape and the relationships, it is possible to work out which goes where as there are different offsets – it just takes time.

15.   Push the bezel and body together against the spring until the tabs on the bezel clear the lip on the body; again, it might need a wriggle or a little leverage, as well as checking the light switch does not foul the lock cylinder sleeve; and then twist until the tabs go into their recesses.

16.   Check that the ignition contact ring and cover are still in alignment and insert the lock cylinder. Push the cylinder home against its spring, then fit washer and nut until it is firm but not tight (the cylinder and nut turn with the key).

17.   Test all the connections in all switch locations to ensure the various bits are in the correct relationship. If not, start again.

18.   Resolder the thread on the lock cylinder to secure the nut. This is essential as otherwise the nut undoes a little every time you switch the ignition off. Use minimal solder to avoid short circuits.

19.   If the tabs on the lock cylinder sleeve are really good they can be bent back into place. If not, leave them as the tabs stop the sleeve turning and the nut holds everything in place anyway.

20.   Sit back and admire your handiwork.

Rees Mackay