The Lucas Starter Switch – ST10

13 Aug


Diagram 1

Like most things electrical, I guess, we take them for granted until they go wrong. This article by Jerry Austin, which first appeared in Octagon Topics, the journal of the Vintage MG Club of Southern California, in 1984, will help you in the event that you have a problem with the starter switch on your TC. I am grateful to Jerry and to the VMG for allowing me to reproduce it.

“Not much has been said about the lowly but important starter switch used on the TC and some earlier models. The Lucas, model ST10, is designed, of course, to be mounted on the commutator end bracket of the starter. It is operated by a Bowden cable to the trunnion on the switch itself.

If a malfunction of other than the starter motor itself is suspected, connect a voltmeter between the switch terminal and ground. If the reading is zero, there is a flat battery, broken wire or loose connection. Use the voltmeter and an ohmmeter to find the culprit. Check that the pull-cable itself is doing its job and is not broken or disconnected. If the switch moves properly and does not actuate the starter, then, after removing first the ground cable from the battery, then the heavy cable running to the solenoid, undo the four screws that hold it to the motor. One of them is viewable only by moving the rubber bootee aside. Pull the switch cover away from the starter to expose the internal contacts. See the illustrations for details.

The contacts must be clean and smooth where the contact disc meets them. Any raised material caused by arcing must be removed and the surfaces smoothed. It is important too that the two fixed contacts have equal levels so the disc contacts them evenly. Not many places have the once available repair kit any more. It is possible to find them occasionally however, at swap meets, car shows, garage sales and so on. Look for Lucas part # 255912.

Keep your eyes open also for the little bootee, part # 764272.


Diagram 2

Diagram 3

Do not remove the contacts unless they require more attention than can be given them while still attached. If they must come off, take off the starter and remove nut B, that holds on the field coil connection, and the three screws C, and the insulating tubes. To refit the repaired or new contacts, set them into place over the insulating plate, fit the insulators over the securing screws, fit and secure the screws. Lastly, fit the nut onto the field coil connection. Take care not to twist the connection while tightening the nut! If this occurs, the whole starter must come down and the connector will have to be silver soldered back on. If the contacts are new and even if they have been reworked, the faces must be ground so they are dead even and flat. The diameter of the grinding wheel or rotary file used must be 1”, and the depth of the machined surface must end up at .245” +/- 0.005” from the edge of the starter end plate,

To remove the contact disc from the switch for replacement or repair, first remove the jump ring C, from the end of the shaft. The return spring with its end caps will fall off, so don’t lose them. In replacing the disc, note the correct position of the disc relative to the switch fixing plate. To secure the disc, replace the spring and end caps. Affix the jump ring and be sure it locks on the shaft.

The only thing left to do now is to fit the switch cover back on the starter. Note the proper positioning of the battery connecting cable when it is attached. If the cable end cuts through the rubber bootee, watch out!”


Diagram 4

Ed’s note: Bearing in mind that this was written over 25 years ago and even then, spare parts were difficult to come by, it is a real bonus to find original examples nowadays. They come up on eBay from time to time and Paul Beck (vintagecarparts.co.uk) sells “Part number 76428 Starter Switch” described as “as Lucas ST10 style start switch”. Doug Pelton fromtheframeup.com carries a Lucas style switch (and has NOS) as well as individual items, such as the starter switch grommet and the contact set, copper.


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