andyborris
Posted a lot
Freedom is just another word for nothing left to lose.
Posts: 2,218
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Feb 18, 2013 18:07:57 GMT
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Poverty forces me to try converting a BMW 325I tacho to fit into a Smiths case, can't afford a new Smiths tacho that'll work with the coil on plug BMW M52 engine that I've fitted to my Triumph. Here is are pictures of the back of the tacho taken from a BMW, it has been removed from the printed circuit board, I'm hoping I can take a direct feed from the ECU via the RPM Speed Signal wire.   It's not that clear in the pictures, but there are 4 pins in the back of the tacho and what looks like a little electric motor. 1 pin into the centre of the “motor”, one each on the sides of the “motor” and 1 encased in plastic that doesn’t seem connected to the “motor” I'm pretty sure that I need a +feed, an earth and the signal. But how to tell which is which? I'm guessing that the centre pin is the signal and the 2 outer pins are the live and earth for the motor? Will I damage the motor if I connect it up the wrong way? And what's the 4th pin for? Can you see that I know nothing about electronics? |
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Feb 18, 2013 20:11:06 GMT
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Couple of thoughts about using a Smiths unit.
There are two types of Smiths: Earlier RVI ones as used on PI and later RVC as used on 2500S.
RVI ones are connected in series with the coil supply - in fact there is an internal loop of wire that passes through an inductive pickup. I discovered that if I power my coil pack using the same same wire that used to feed the coil +ve, my tacho works just like it used to. Megasquirt switches the coil pack earths.
Therefore, if your individual coil packs are all powered by from the same source with the earths switched by the ECU (quite likely) then you could probably use the RVI taco in the same way.
RVC Tachos are powered by a single wire from the coil negative which picks up the pulses from the points. If you can identify the tacho out wire from your ecu then it might just drive it. However, more likely you'll need to fashion a little amplifier circuit using an old relay coil and a couple of transistors and diodes. I have the circuit diagram somewhere as I made one for my Vitesse as that has an RVC tacho (I hadn't twigged it would be easier to use an RVI one at that stage!).
The BMW one is a stepper motor I think - that needs some bespoke electronics to run it - which might be ok if that electronics is in the engine ECU ("just" need to work out which wires go where) but less ok if it's on some other board that you don't have......
Cheers
Nick
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1967 Triumph Vitesse convertible (old friend)
1996 Audi A6 2.5 TDI Avant (still durability testing)
1972 GT6 Mk3 (Restored after loong rest & getting the hang of being a car again)
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Feb 18, 2013 20:32:43 GMT
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what sort of smiths rev counter is it? there are two types and they changed about 1974 from memory. the RVI (inductive) type are earlier and I couldnt get it to work with electronic ignition, and they are wired in series with the coil. and were replaced by the later RVC (conductive) types, that are wired in parallel with the coil. the smiths meter like the one below are just moving coil meters and need a voltage to operate them. as I needed it to match a speedometer that Id modified, I ended up looking on the net for tachometer drive circuits and ended up making one. so here is me trying the drive circuit  and the compeleted circuit and the original board  |
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andyborris
Posted a lot
Freedom is just another word for nothing left to lose.
Posts: 2,218
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Feb 18, 2013 21:05:12 GMT
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haven't got a coil pack as such, it's a coil on plug system. Although I believe it's switched to earth, I've no proof. The coils are wired as per a BMW and are (as far as I can see!) all controlled individually, i.e. no "lost" spark. Each pair of coils has its own power feed and a combined, via a resistor, earth.
How does this fit in with a RVI tacho? and sorry optima21, you've lost me!
I do have the circuit boards for the BMW tacho, I'm going to and work out which bit on the board is for the tacho and take it from there.
The old coil wiring has gone, with just a switched (fused) feed to the ECU.
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Smiler
Posted a lot
I no longer own anything FWD! Or with less than 6 cylinders, or 2.5ltrs! :)
Posts: 2,492
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Feb 21, 2013 20:43:44 GMT
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www.Auto-tat.co.uk'96 Range Rover P38 DSE (daily driver) '71 Reliant Scimitar SE5 GTE 3.0ltr Jag V6 Conversion '79 Reliant Scimitar SE6A 3.0ltr 24valve Omega Conversion '85 Escort Cabrio 2.0 Zetec - Sold '91 BMW 525i - Sold '82 Cortina 2.9i Ghia Cosworth - Sold '72 VW Campervan - Sold '65 LandRover 88" - Sold
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andyborris
Posted a lot
Freedom is just another word for nothing left to lose.
Posts: 2,218
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Feb 23, 2013 15:04:36 GMT
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That is something like I was hoping to do, fit the BMW innards into a Smiths Tacho.
Haven't really done anything with it yet, been too busy. But I'll try wiring the stepper motor on the BMW tacho direct, without the circuit board and see what happens, if I can get a signal, then I'll have a go at sticking all the BMW gubbins (technical word) into a Smiths.
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assuming you haven't started yet, i've been thinking if a proxy switch would work the original rev counter? i've got one of these to go on my speedo but am going to see if it will also drive the rev counter later on.
2211412987 , should be easy enough to make a bracket onto the cam sprocket wheel or similar.
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