Tuesday, 9 May 2017

Revolutions and resolutions

The time finally came to fit the new tachometer. Over the New Year the supplier had eventually sent me the correct mini-wiring harness to adapt the new instrument to Erin Mae's wiring. The procedure for installing and configuring it seemed straightforward enough from the instruction sheet. Configuring it means telling it how many alternator impulses it will receive for every revolution of the engine, and I'd calculated there would be 8. You use a switch connected to one of the terminals to put the tachometer into "Pulse" mode and then to set the right number. That all seemed to go OK, so I started the engine to test it all. Nada. The needle stayed on zero.

Ouch! Perhaps I had never needed to buy a new tacho in the first place! Out came the voltmeter, and I eventually traced the problem to the simplest of all reasons – the sensor wire was broken just where it connected to the alternator  It was soon clear that a repaired wire would not be quite long enough so I needed a way of lengthening it, plus a new ring connector to attach it to the lug. I was just in time to nip down to the Maplin shop in Stafford and get some bits and pieces.

The wire was in, if not exactly the worst place imaginable, something close to it. The coolant hoses for the engine were right in the way of where I needed to have my wire-stripper/crimping tool. What should have been a simple process of joining two bits of wire became major exercise for both muscles and emotions. Eventually I cut all the plastic covering off a connector and managed to crimp it with my pliers. Then I added some solder for good measure and wrapped it in self-amalgamating tape. Finally I fixed the ring connector to the end of the extension piece and attached it to the lug on the alternator.

Now the engine test gave me a reading on the tacho, but it seemed too high. It showed 1000 rpm at tickover, whereas the old tacho had read about 650. I wondered if my assumption about the number of "poles" in the alternator had been correct. Internet sources had suggested 12, and that's what I'd used in the calculation. I posted a question on the CanalWorld forums and also emailed the manufacturer. 12 was indeed the right answer, but someone on the forum also suggested I could do some testing with a strobe/tachometer app for a smartphone. I found one, installed it on my best beloved's ancient iPhone, crouched down trying to get it to read the engine's speed of rotation, and so found myself eyeballing the engine's crankshaft pulley. I suddenly realised I'd used the wrong pulley data for the pulse number calculation. I measured the pulleys again, and found the number should have been 10, not 8.

So it was back to configuration mode, and this time the tachometer reads 800 rpm on tickover. That still seems a little high to me, but it may be that I need to re-programme my senses. Some time I'll get the strobe tacho app working and see what it says, or get a workshop to check the speed. No doubt that will be the point at which I will be consumed by an obsessive desire to get it pin-point accurate, and will seriously regret having sealed the control panel back in its place, rendering inaccessible the terminal to which I would need to attach my configuration switch. For the moment, however, we have tacho!

There's one final twist in this tale, but that can wait until tomorrow.


  1. I trust you've kept the old tacho as a spare ...

    1. By instinct I am an irrational hoarder. So all my inclinations were to put it somewhere safe, just in case. Then I remembered the state of my garage at home – 10,000 reasons (some of them more than 30 years old) not to hoard. So, at great emotional cost, the old tacho has gone in the bin. I think.

    2. Addendum: Oxley Marine did test the old tacho last year when problems started, and concluded it was an ex-tacho. I think the fractured wire was coincidental.