Installing a Reversing Switch - Author Greg Gabb
You know that you are in trouble when your tail light assembly has a reversing light but you cannot find any sign of a means to activate that light.
After careful consultation with the workshop manual and the parts book you again realise that while a part is listed there is nothing in either which actually tells or shows you where it is installed. Lotus strikes again !
The next logical step is to ring Steve, he'll know where it goes.
It is about now that you discover that your particular gear-case is obviously a substituted earlier model which has no provision for a reversing switch.
What to do next, well, having been given some very detailed information and a couple of photographs by Steve as to where and how it should be installed the only thing to do now is get busy.
This small modification would be a piece of cake if only the need for it had been discovered before the entire engine and gearbox had been completely striped, restored and re-machined where necessary and re-built, but that is another story, the job would now have to be accomplished in-situ.
The switch is a normally open plunger type, actuated by the reverse gear selector shaft, which is screwed into the top of the gear-case just forward of the top cover of the gear-case.
The photograph below indicates where the switch should be positioned.
Having set out the position of the hole using a steel rule and eye-balling the centre-line of the selector shaft as near as possible it was a fairly simple task to carefully drill and tap the required port into the shaft gallery. Now we are getting somewhere !
After carefully retrieving the cast iron chips from the gallery using (a) a compressed air jet (this removes the majority of the chips from the gallery and deposits them all over your face) and (b) several cotton sticks dipped in grease, the next step is to see how your handy-work operates.
With reverse gear selected the switch was duly installed using a multimeter to ascertain when contact was made and to avoid inserting it too far and have the shaft strike the body of the switch.
This is simple, what's all the worry about ?
The moment that you move the selector shaft back and try to again engage reverse you find out.
The plunger on the switch has a hemi-spherical end on a parallel shaft and the stroke necessary to actuate the switch is such that a portion of the parallel section of the plunger is exposed to the end of the selector shaft, which is now trying to shear the plunger off.
It's about now that you say to yourself why not just have a switch hidden under the dash.
Further discussion with Steve, he's the only one I know with a gearbox dissembled and the relevant bits exposed as well as a modicum of prior knowledge, reveals that the later selector shafts have a flat machined on the top with a ramp at the end to allow it to slide under the switch plunger before pushing it up.
The question now is how to create a ramp on the end of this selector shaft without taking the whole gubbins apart.
It will no doubt be the case that the selector shaft will be hardened and as such will need to be ground and the whole operation will have to be accomplished down a 3/8" tapped hole without damaging the thread.
Hmmm! just a small ask.
It is about now that I recall having a few 1/8" shaft mounted points that my dental technician brother-in-law had given me back in the dim past when I was making scratch built 64th scale model trains out of brass.
Supposedly these were for use on porcelain and ceramic teeth which would be much harder than what I needed to grind. The problem was really a matter of access in order that a slope could be formed on the selector shaft. In the end the solution was obvious, one of the stones was an inverted cone i.e. approximately 5/16" dia. at the end and 1/8"+ where the point met the shaft.
This allowed the me to lean my Dremel grinder over at approximately 15 deg. to the vertical and grind the top of the selector shaft with the flat end of the stone without damaging the thread due to the taper on the stone.
At the first attempt the stone I had while working like a treat only accomplished about 65% of the job before shattering, it was obviously long passed it's use by date.
A quick call to my brother-in-law, now retired, pointed me in the direction of the dental equipment supply company "Ivo Clare" who were delighted to sell me the required inverted cone mounted points over the telephone with overnight delivery from Melbourne.
As I was unsure what the attrition rate would be I purchased ten stones at some insignificant cost of the order of $3.00 each, well, the new stones were pink not grey (obviously some grade of Adelox) and when I had finished the job I could hardly see where the first stone had worn.
The nett result of all this is that the required ramp was created and when the switch was finally installed and it's vertical position adjusted to just make contact the assembly appears to operate reliably and repeatably but only time will confirm that.
Should anyone like to discuss any of the above in further detail please contact Greg Gabb at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .
Copyright © 2011 Lotus Marques
