Showing the external pickup on the 64 to 67 RVI tach and the routing of the wire from the coil (SW) towards the ignition switch (A3) for positive earth cars. For negative earth conversions the wire must be routed in the other direction undoing the knurled nut should allow the pickup to be dismantled and the wire reversed otherwise keeping the layout identical.
However some variants have a short length of white in the pickup terminated with two bullets as shown here, labelled as being for a +ve earth MGB. Easy to reverse, but easy to get wrong as well as they don't seem to be marked, although having these connections reversed or the earlier continuous loop going through the pickup the wrong way won't harm the tach. Don't reverse the 12v and earth connections though, which may destroy the electronics. 12v supply spade is at top-right on the case back, the earth would be under one of the thumb-nuts securing the tach in the dash same as the other gauges. The gauge illumination bulb holder is shown bottom-right, and the insulated socket for the ignition warning lamp at top-left.
One of the metal parts of the pickup, and the nylon part that acts as a former for the wire, are detachable and may get lost. The Parts catalogue indicates these are 27H8215 for the 'Core', and 13H784 for the 'Loop-sleeve (nylon)'. Googling the first of those has a couple of suppliers describing it as a 'tachometer retaining strap', which doesn't seem to be the right thing, and they also give applicable dates from 64 to 76 and say it is also for the V8, both of which are clearly incorrect. But A-Head 4 Healeys converts that part number to 1S4150, which appears to be the complete assembly of metal and nylon parts, plus wire, washer and nut, at £22. Rimmer only has the first part, Moss both at £19 but without the washer and nut. (Image: A-Head 4 Healeys)
The 68 to 72 RVI tach, with internal pickup, and male and female bullet connectors for the coil and ignition switch wires and the 12v spade in a cluster at the top. These seem to be smaller than the standard wiring bullets and connectors at 4.5mm as opposed to 5mm (Malc Gilliver). The pick-up 'in' wire from the ignition switch goes on the male bullet on the left, and the 'out' wire to the coil goes on the insulated female on the right. As before the earth wire goes under one of the knurled nuts securing the tach in the dash, the position for the gauge illumination lamp is on the left, and the insulated holder for the ignition warning lamp on the right. Photos: Peter Mayo
Another version this time from a 1968 North American padded dash ...
... this time with earth wires going to a spade spot-welded to the tach case instead of under one of the knurled screws. This type only has the night-time illumination in the gauge. Photos: Bill Mason
Internals of this type as posted on the MG BBS by Steve Church.
Chrome-bumper voltage-triggered RVC tach i.e. 73 and 74, with male bullet for white/black signal wire and 12v spade (green wire) close by. The earth will be a tag under one of the knurled nuts that fix it to the dash. Plain hole for the night-time bulb at the top, insulated hole for the ignition warning light at the bottom.
Rubber bumper voltage-triggered RVC tach with male bullet for trigger wire and 12v spade close by as above, but this now has an earth spade on the back of the case. This type only has the position for the gauge illumination bulb holder, the ignition warning light is now in the binnacle.
RVI/RVC designation and reference number is positioned by the 'SMITHS' (arrow). The original polarity is also indicated (circled), probably on all chrome-bezel tachs, but not the 77 and later plastic tachs. This is an RVI 2401/00B positive earth for 1964 to 1967 as used in all markets. Note that a PO may have changed the internal wiring to convert it to negative earth and not changed the legend on the face.
This is an RVI 2430/00 negative earth for 68 to 72 as used in markets other than North America, but also used in Canada for 1968.
An RVC 2415/00AF (and hence negative earth) as used in markets other than North America, Sweden and Germany from 1973 to the end of chrome bumper production.
An RVC RVC/1810/00/AF used on V8s, still carrying the polarity indication.