Repairing a PLC Switch

(With acknowledgements to the wartime issue of "Automobile Electricity" 1940).
Thanks to Kevin Wissett for sourcing this important article. (Dorset A7C)

The breakdown of a PLC panel switch may be due to several possible causes, and when no replacement is immediately available, means must be sought of making a serviceable repair.

It is often possible to utilise parts of a replacement of another type when they can be interchanged, but a close comparison and inspection to ascertain the full extent of faults is first necessary.

External design varies according to type. One of the commonest types comprises a cylindrical moulded body with metal front and rim, the switch being held in position by a spring clip with eyeleted ends secured by a screw and nut.

This encircles the back of the switch body after the latter has been fitted in the panel from the front, and two flat sides of the clip engage grooves in the switch base so that the clip exerts pressure to hold the switch firmly against the panel (Fig. 1 below).

In another design, the moulded handle can be removed from the front when a slotted ring nut has been unscrewed. Beneath the handle a larger ring nut secures the switch in position. Projections on the handle engage slots formed in the metal rotor sleeve. In each of these designs the key-operated ignition switch is concentric with the lighting control handle. Where a regulator box is fitted, the normal F1 and F2 field terminals are not used and may be blank studs, but the D terminal is sometimes used as a junction point to the warning light.  Fig. 2 outlines this type of mounting.



Testing for Defects

This article is being reproduced in full as it appeared in 1940. HOWEVER, UNDER NO CIRCUMSTANCES USE MAINS TO TEST IN THE MANNER DESCRIBED BELOW - IT IS POTENTIALLY FATAL. Instead, use a CAR battery and a headlight bulb. This will test the switch adequately for our purposes. Sufficient current will flow such that a poor contact will show up. In the years since the switches were manufactured, the insulation is likely to have reduced such that TESTING WITH MAINS WILL RESULT IN LETHAL VOLTAGES BEING PRESENT ON THE PARTS YOU ARE HOLDING. YOU HAVE BEEN WARNED.

With the switch removed from the panel and wires uncoupled, the nature of the defect can be ascertained by simple tests.  Both ignition key and lighting control action should be snappy and decisive.  Excessive stiffness or slackness indicates lack of lubrication on stud surfaces or weak springs respectively. A mains test lamp of about 60 Watts should be used to check insulation between charging and lighting sections, the prods being applied to terminals D and A, between which there should be no contact in any switch position.

In the case of a “three-rate” switch, terminals D, F1 and F2 should be all mutually insulated in "Low" position, D and F1 should be bridged by the switch rotor contacts - indicated by test lamp lighting - in "High" and "Side" positions, and D and F2 should be similarly bridged in "Head" position.

Lighting combinations are checked in the same way, the prods being applied to A and T to light the lamp in "Side" position, and to A, T and H in the "Head" position. With the ignition switch on, the centre terminal A and IG should be bridged.

Fig. 3 illustrates these tests.  Remember - use a car battery and headlight bulb - NOT MAINS.


Dismantling the moulded base from the rim plate assembly for internal inspection in case of faulty action or bridging, is effected by lifting the locking tabs from their slots and rotating the rim plate until the three locating lugs are opposite the open slots, when spring pressure will force rim and base apart.  Care is necessary to avoid loss of spring, ignition key spindle, tube, spring or rotor.

Fig. 4 shows the components in “exploded" order. The moulded base should first be examined, the following points receiving special attention.


The three charging terminals form concave studs inside the base, these having two intermediate studs with no terminals.  It should be noted that D is bridged to its right-hand neighbour - viewed from inside, as also is F1.  It will also be seen that terminal A is internally bridged to central A terminal ignition switch stud. These bridging connections need checking as a break or bad connection in either of the charging bridges causes low output (Summer or Low rate only) in "Side" position when High rate (Winter) should obtain. A broken A terminal bridge will affect ignition feed and any circuit including it, as when ammeter feed is connected to central A and auxiliary or cut-out lead is wired to lighting A. When wiring up, it is best to connect all A wires to lighting A, leaving central A unconnected, when the bridge carries ignition current only.




Dummy Contacts
Between the two groups of studs are "dummies" - two concave indents in the moulding at one side. These receive rotor contact pads in off position, and must be clear of metallic tracking or dust likely to cause leakage. The stop ridge, controlling the limit of rotor travel should be inspected, and the two extended lugs which carry the ignition sleeve checked for fracture - a likely occurrence when the switch has been strained or forced. 

The rotor and insulator plate
The lighting section comprises a brass ring R secured to the insulated rotor frame RF surmounted by a C spring with end pads CS standing clear and embracing a fixed rivet stud or pad P, the three forming a group spaced to correspond with stud positions in the base.

Directly opposite are two other fixed studs C1, C2, acting as assembly rivets but insulated by collars from the brass ring and C spring. They are spaced two base studs apart and are bridged together by a strip at the front of the rotor frame.
Check contacts by applying prods to the two charging studs C1 and C2, when the test lamp should light, and check insulation with prods on C1 and P (no light). A cutaway sector at the edge of the rotor frame allows rotation over the stop ridge extending to four successive stud positions. Four wide "key ways" in the rotor frame engage keys on the moulded handle sleeve HS - allowing end freedom for spring pressure on rotor. Clean thoroughly and check spring tension and wear of pads. Insulator plate IP keeps spring S clear of stud rivets and bridge strip. The spring is located on the handle sleeve by a collar and the assembly is held in the rim plate RP by a circlip engaging a slot.

The switch illustrated has a flat ignition key. If a Yale lock barrel and key is fitted this must be dismantled before the switch is opened by taking off the spindle nuts and washers located on spindle end between ignition terminals, when the barrel can be withdrawn complete, leaving ignition rotor in place. In the design sketched the rotor insulator carries a contact plate located by lugs engaging slots, and having two punched-out dimples which register with the two ignition studs. The rotor is held in contact by a compression spring bearing against a shoulder on the key spindle.

The flat tongue at the end of the key spindle engages a slot in the rotor insulator, and the whole is assembled in the order indicated inside the spindle sleeve. The outer end of the latter is closed except for a slot through which the key is inserted into the slot at the end of the spindle. The key can only enter when both slots coincide. Damage to the sleeve slot is caused by attempts to force the switch without the proper key.

Faulty Ignition
Faulty ignition switch action is sometimes caused by shearing of the tongue slot in the rotor so that the spindle does not rotate it sufficiently. A new or replacement rotor must be fitted unless the operator is quick and handy at small work, in which case a steel plate, slotted to fit the tongue, and secured by 1/16 in. pins, can be located between rotor and spring. With the necessary small tools and material, the job can be done in half an hour.

When all parts are cleaned and wearing surfaces lightly coated with Vaseline, the switch can be assembled. The spring pads CS should be urged out to ensure sound contact, and the stud faces polished clean. A film of lubricant should be applied to the handle sleeve keys and a spot of oil between collar and rim plate will ensure free action. The ignition key spindle should be lubricated before assembly in the sleeve and the tongue must be a smooth easy fit in the rotor slot. If any of the internal interstud bridges are doubtful these should be re-soldered. If there is any difficulty in making a neat job on the inside of the moulding a better way is to solder short leads to each of the hollow rivet heads securing the blank stud and to bring these leads across to the terminals to which each stud is bridged. The same can be done in connecting externally the two A terminals.

Inspection of the dismantled switch will make the action quite clear. In Low or Summer position, the three lighting pads, P and CS, occupy the studs L and the two adjacent dummies, while charging pads C1 and C2 occupy the left-hand dummy and the blank stud bridged to F1.

In High position, C1 and C2 connect F1 and D, while CS and P lie on T, L and the lower right hand dummy. Side position brings CS and P on to A, T and L, while C1 and C2 join the two blank studs - which are virtual extensions of F1 and D. Head position causes H, A and T to be bridged by CS and P, while C1 and C2 lie on studs D and F2. This will explain the interlocking combinations described earlier in this article, and emphasises the importance of good insulation between the charging pads C1 and C2, and the lighting pads CS and P.

Terminal L (Low) is very seldom used, being originally incorporated for three lamp lighting sets where head/-side lamps on the wings carried bulbs with dim and bright filaments, the former acting as side lamps. Terminal L is alive in side position only and cannot be used for normal side lamp feed as it is dead in Head position. This terminal is used on motor cycle sets where it feeds the pilot bulb in the headlamp. Another application is on cars having a separate headlamp switch, wired from terminal L, while a pass light is wired from terminal H. In this case, position of normal "Side" becomes Sides, Rear and Headlamp, while position "Head" becomes Sides, Rear and Pass light.

Assembling the Switch
Assembly of the switch is expedited by first fixing the ignition parts in place, holding these in by means of a long screwdriver, while the main rotor, insulator spring and handle sleeve are got into position and the bayonet mounting of the rim plate secured. The rotor and insulator plate should be fitted with the cutaway edge engaging the stop ridge at limit of the left--hand rotation while the handle is set to "Low". The keys and keyways will then engage without trouble, and the rim can be rotated to bring the extended lugs opposite entry slots in the base edge.

When a faulty switch occurs on a vehicle urgently wanted, a good temporary substitute comprises a standard PLC fitted with strap fixing for mounting on steering column - like a radio control dial - short cables being taken from terminals and jointed to wires removed from the vehicle switch. The latter can then be repaired at leisure, or replaced, while the vehicle can be operated normally with the service switch.

This article was originally reproduced in Seven Focus Dec 2009 pp16-19. 



[1] Note: The Editor of Seven Focus is an Electrical Engineer and so is well qualified to offer the warning not to use mains electricity for testing.


See also:

 Repairing a Lucas PLC Ignition Switch for more information on this subject.