Product Description
Triumph TR6 Top Covers
There are at least 10 TR6 gearbox top cover assemblies, for overdrive/non-overdrive and the electrical interlock requirements of various marketplaces. The TR6 PI, however, uses only 3 basically different types. The applications are non-overdrive, ‘A’ type overdrive and ‘J’ type overdrive.
These types are identified by the quantity and positioning of electrical switches. They all have a reversing light switch, which is either screwed into a boss on the left side of the top cover surface, or in the left side of the case. The ‘A’ type overdrive cover will have a switch screwed into each of the three bosses on the top of the cover: these are for reversing lights, second gear inhibitor and third/fourth gear inhibitor. The ‘J’ type overdrive top cover has only two switches: one to operate the reversing lights, the other screwed into the middle boss on the top surface of the cover to inhibit the third and fourth gear overdrive engagement.
Conversion of non-overdrive top covers to overdrive type is a matter of drilling and tapping to accept a switch(es) in the correct position(s). This can be done by the owner, or by a machine shop thread size is M16.
For gearbox and overdrive electrical details, plus note on switch fitting, see ‘Electrical’ section or ‘Overdrive Conversions’.
Selector (inhibitor) switches are best adjusted with the lid off the gearbox. Ideally use a test meter or an illuminated circuit tester (power source, bulb and wires as shown in various workshop manuals) to test for makes or breaks. The further ‘in’ the switch is screwed the more resistance to ease of gearchange will be felt when the gear-lever is moved from gear to gear. The idea therefore is to screw the switch into the lid the least possible amount to get positive circuit makes and breaks. As the packing washers are thin card, which changes its thickness as it compresses, this can only be achieved by trial and error, and 1 - 3 is the normal range of requirement. Each switch must be adjusted individually.
Don’t take chances with broken insulation on the switches and/or very loose terminals. There’s a lot of ‘stripping out’ involved to get at a rogue switch which will far outweigh replacement cost (ditto: dodgy wiring).
The presence of switches with screw terminals only indicates that an earlier variety has been substituted at some time in the past to keep the TR mobile and functioning safely.
These may require adjustment during the life of the gearbox as the selector forks wear at the top where the switch makes contact. Wear in the gearbox bushes may cause selector fork face wear, which may also cause intermittent overdrive operation. These faulty switch operations will in turn cause the solenoid to chatter and that will certainly cause premature solenoid failure as the points burn out.
Reference was made to the switches providing a safety’ circuit to protect the overdrive. The simple fact is that the unidirectional clutch is exactly that: a unit that works in one direction. It fails very expensively when driven the wrong way, usually destroying the gearbox 3rd motion shaft (or mainshaft) in addition to the whole overdrive. The inhibitor switches are really there to prevent overdrive engagement in reverse.