LUCAS HEADLAMP SWITCH - PR18/4B

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Amanda Hunter
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1 LUCAS HEADLAMP SWITCH - PR18/4B By the mid-fifties Miller had lost considerable ground to Lucas in the competition for the British motorcycle market and Vincents switched to Lucas when the Series D machines were introduced. Initially, one reason may have been the change to coil ignition and the need for the slightly bigger output of the Lucas dynamo and its more reliable regulator. The opportunity to fit a more up to date headlight and more effective rear light was then taken. This only left the Miller wiring loom, ammeter and switch. Although these were excellent items, logistics or commercial considerations no doubt dictated their replacement with Lucas components. This is a pity as far as the switch (and ammeter) is concerned, for the Lucas switch, also fitted to other British machines of the era, is not as reliable or long lived as the Miller equivalent. Although spare parts are not available, new switches can still be found at autojumbles, but note that there are different versions of the switch described here. The switch is located by lugs which engage with recesses moulded in the switch body. The lugs form part of mounting ring SL 17 on the enclosed models, and the headlamp shell on the open models. The switch is held in place by means of an eight-sided spring clip which engages with two recesses in the switch body. The joint is sealed by an O-ring which, if perished or missing, can be replaced using the Loctite O-ring kit. Unlike the Miller switch, the body and face plate is a single moulding. It was made of black bakelite and the outline lettering used for the switch positions and manufacturer's name was aligned on a radius. However, on the drawing, the letters are drawn square to view for convenience. No switch bezel was fitted. The black bakelite switch lever seats on a brass washer at the end of the rotor, fastened by a 2 BA screw, usually chromium-plated brass. The screw has a slightly rounded head and is also relieved to root diameter for the initial 1/16 in. - for reasons which escape us. The length of the screw should not exceed 7/16 in. measured under the head; longer screws may fall to secure the lever to the rotor shaft. Breakage of the lever is rare. The bakelite mouldings on some switches tend to turn brown after long exposure to the weather. The switch seven slotted brass terminal posts are 1/4 in. diameter, tapped 2 BA, and were moulded in the body during manufacture. Each post has the slot facing outwards for easy wiring access and also to ensure that the contact roller runs over a smooth radius. The terminals are numbered as shown in the wiring diagram M0 108 in the Series D Spares List, and also in the redrawn wiring diagram in this book as well as in the 3rd edition of Vincent Motorcycles, fig Together with the colour coded wiring loom, this is a great help when re-wiring. Although seven posts are provided, only five (Nos. 2 to 6) are used on Vincents, and usually only four were fitted with cable screws. The 2 BA brass cable screw is relieved to root diameter at the tip. Apparently, it was official practice to make a ball of the exposed copper strands before putting each cable in place in its slotted terminal; the relieved tip clamped the cable strands without trapping them in the threads or shearing them through. Terminal No. 5 is fitted with a brass stud and washer as shown in the drawing. The 2 BA and 4 BA threads on the post were relieved to root diameter adjacent to the hex. portion, which accepts a 4 BA spanner. A leaf spring (probably beryllium copper) and its stiffener are mounted on the stud

2 between the hexagon and the 2 BA plain washer; after tightening the stud in position it was soldered to the post, presumably for electrical continuity and to lock the stud. The leaf spring forms an electrical bridge between the rotor and terminal 5, and features a small hole at its outer end which engages with a pip in the base of the rotor. The two washers and nut, all 4 BA, are used to clamp a cable fitted with an eyelet, part of the Lucas wiring loom. Also moulded into the body at twelve o'clock is a brass pin 1/8 in. dia. x 19/32 in. long with a small hole, 1/16 in. from the end. This pin - not shown on the drawing - is not connected electrically to any terminal post and has no function when fitted to a Vincent. Presumably it was intended for mechanical retention on other makes of machine. The one piece die-cast zinc alloy rotor body houses a contact plunger subassembly; this allows the contact roller to ride over the terminal posts whilst providing a positive indexing action. The switch fitted to Series D Vincents has three positions: POSITION TERMINAL POSTS BRIDGED LIGHTS IN OPERATION OFF Nos. 5 & 4 NONE L (low) Nos. 5 (via leaf spring), 4 & 3 PARKING: SPEEDO & REAR H (high) Nos. 5 (via leaf spring), 3 & 2 SPEEDO & REAR: HEADLIGHT (via dipswitch) To visualise this sequence, refer to the wiring diagrams mentioned earlier where a bottom view of the switch is shown. Imagine the roller bridging terminals 5 & 4 in the OFF position, (note that current is provided via No. 3), each position moves the roller one step anti-clockwise on the drawing - clockwise as seen by the rider - to correspond with L and H as shown in the table above. It is perhaps interesting to note that when the lights are on the rotor is live, and so is the lever retaining screw! Dismantling and re-assembly Disconnect the battery, detach the wiring from the switch, remove the lever, ease the eight sided spring out of its recesses and withdraw the switch. The contacts can often be cleaned with a purpose-made contact cleaning fluid, and the shaft lubricated without taking anything apart, but as dismantling and re-assembly are straightforward, it is worthwhile paying some attention to the mechanism as a whole. With the lever removed, the rotor is held in place by means of the bridging leaf spring clamped by the stud in terminal No. 5. Removal of the stud may need the solder to be softened, but it can usually be unthreaded, after which the rotor can be pushed through the body; when doing so place a cloth over the assembly or use a finger to ensure that the contact roller plunger and spring do not pingpong around the garage ceiling, wall and floor. After cleaning everything, grease the shaft and roller plunger and apply vaseline to the roller and the leaf spring/rotor contact area. Re-assembly is straightforward. Possible Faults Faults occur more frequently than with the Miller switch and are nearly always due to premature wear or service in poor weather conditions.

3 a) Stiffness - This is most likely due to frequent exposure to rain, grit between the rotor shaft and body, or severe wear of the rotor shaft. Try injecting oil with an aerosol (spray can), and if this does not work, dismantle, ease the shaft with emery cloth, remove all traces of grit and apply grease or Graphogen. b) Poor electrical performance - This is usually due to:- (i) wear of the terminal posts against which the contact roller bears, due to grit; (ii) pitting of the contact roller surface, or roller bearing pins; (iii) poor contact between the spring and plunger and/or rotor housing; (iv) poor contact between leaf spring and rotor. All these can usually be helped by cleaning the parts with a fine emery cloth and checking with an ohmmeter which should read virtually zero. Check that the leaf spring makes firm contact with the rotor; if necessary it can be bent to shape with carefully used pliers. c) Cracked or broken block - This is by no means hopeless provided that no fragments are missing. Degrease and stick the bits together with Araldite (the slow variety preferably). d) Reluctance to move from position, (not to be confused with stiffness). This is usually caused by:- (i) Sloppiness in the plunger housing which, if severe, can cause a complete lock-up or rotor damage, and even lever breakage if forced. The rotor housing can be bored out and sleeved to provide a good plunger guide. The plunger should also be polished to give a good fit. (ii) Wear of the contact roller pins and/or their slots in the plunger. It can be remedied by dressing the old pins flush with the roller and pressing in an oversized roller pin, followed by careful dressing of the plunger slots. (Alternatively, turn a new roller to suit). (iii) A sloppy fit of the rotor in the body aggravates the same fault; sleeving or bushing usually provides a cure.

Maintenance Information

16572679 Edition 2 May 2014 Air Drill QP Series Maintenance Information Save These Instructions Product Safety Information WARNING Failure to observe the following warnings, and to avoid these potentially