Speedy reply from Prestolite confirmed the alternator has 12 poles. While fiddling with the iPhone tachometer app I realised I'd miscalculated the ratio of the pulleys. For some reason I'd been working all along with 1.33, but the crankshaft pulley is 5 inches and the alternator pulley 3 inches. So the calculation is 5/3*12/2 = 10 pulses per revolution. I'd used 8 first time around. Once that was in place I got an idling speed of about 800 rpm. That's a bit higher than the speed registered by the old tacho, of about 600, but it should be accurate, if my pulley measurement was correct.
At this point another problem arose. With the constant moving of the control panel, a lead had come adrift – a female spade connector on the red core of a thick 2-core cable running down into the engine compartment, and not part of the wiring loom as such. It was not obvious where it should be connected. The engine started, but now would not stop with the stop switch, so I had to locate the emergency stop lever on the engine itself. The warning buzzer was not coming on.
The cable was labelled in orange marker pen "supply to Q2". Q2 is shown on the Aqualine wiring diagram as a master RCB switch controlling the supply to the horn and headlight, and is located in the electrical cupboard. Its powered end is shown as connected to the "engine ignition switch". It transpired the other (blue) core was attached to the red one at the bottom terminal of Q2, so returning 12 volts back up to the control panel area. It is connected to two white leads which feed (a) the stop switch, and (b) the domestic alternator warning light. That explained why the stop switch did not work.
The engine manual shows which connectors on the ignition switch are active under which conditions, and from that it became clear that the lead should probably be attached to pin 54 (live under Run, Heat and Start). 58 (live under Run)was also a possibility, but seemed less likely, especially as it offers only 4 amps. Making that connection restored all functionality.
I used self-amalgamating tape to tidy up the wires on the tachometer adapter harness, and re-seated the tachometer with silicone grease. The rubber sealing ring doesn't fit very well, and we shall have to see whether the combination stops water ingress. I used the Sikaflex substitute from Kings Lock to seat and seal the panel back in place on the column.
