DailyDriverMods
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- Aug 23, 2020
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- Netherlands
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I wasn't the first to do this, but if you take an old hall-equipped dizzy and cut out three out of four blades, then align the one blade left with the sensor at tdc you have half of what you need for sequential ignition. Not sure if MS3 has a chip on the board but I had to build a hall circuit to accommodate the 2nd trigger.
You'd also need something like the crank sensor on the flywheel to complete the package.
Did you rewire your injectors to accommodate sequential injection?
That looks like a Pertronix knock-off. Pertronix was the original company that offered electronic points-replacement kits for all sorts of mechanical distributors. There's a magnet and hall sensor in the replacement module, and a 4 vane metal rotor in the ring, hidden under the plastic. You'd need to figure out how to mount the module to the distributor housing, and how to modify the ring to give one pulse per revolution. (Note: these use the distributor housing as a ground connection to the block - the 2 wires are +12v and sensor out.)
Depending on local availability, it might be easier to find and modify a LH2.2 distributor.
Are you doing sequential injection and coil on plug (or similar) or are you just doing timing control in MS3 and retaining the conventional coil and distributor?
Something to consider in repurposing an old distributor to provide a cam position signal is wear on the distributor shaft. On my B20E, wear on the distributor shaft contributed to significant timing wander at 3000 crank RPM. Volvo used to sell distributor rebuild kits through GCP which allowed you to re bush the distributor. This worked as long as the distributor shaft itself is not worn, although getting the split pin out of the drive end of the distributor shaft to allow disassembly can be a challenge.
If you are going with coil on plug or a similar arrangement which eliminates the need for the distributor function, consider the Yoshifab adapter to mount an optical cam angle sensor in place of the distributor on OHC redblocks. Gives you both a cam position signal and a medium resolution (12 crank tooth equvalent) tach signal. That was my solution for dealing with a worn distributor shaft although it is significantly more expensive than the $30 Hall sensor. Clearly does not work if you need to retain the spark distribution function.
As bobxyz notes, the Aliexpress part looks like a Pertronix clone. Eliminating the centrifugal and vacuum spark control by locking the breaker plate to do spark timing control via MS3 is trivial. Modifying the plastic collar to give you a single pulse per distributor revolution may be slightly more challenging. If you don't need the distributor function, you are relatively free in figuring out the positioning of the trigger. If you need to retain the spark distribution function of the distributor then life is more complex. I don't know about MS3; but, on MS2 the recommended positioning is to have the cam position signal occur about 60 deg before TDC on #1. Getting that 60 deg timing with an unmodified breaker contact plate and still retaining reasonable alignment of the distributor rotor with the contacts in the distributor cap can be a challenge.
I looked at the MS hardware manual under "Missing tooth crank wheel and single tooth cam wheel" and it sounds like MS is pretty forgiving of the Cam wheel behavior. All MS really needs is a Cam sensor edge before tooth #1 on the crank wheel.
I think that MS is simply setting a flag when it sees the Cam tooth edge. On the next crank Tooth #1, if the Cam flag is set, then the engine is nearing TDC #1 on the compression stroke (and MS clears the flag). If the Cam flag isn't set at Tooth #1 time, the engine is nearing TDC #1 on the exhaust stroke. If it really works this way, you don't need to lock down the distributor advance -- MS won't care if it moves around a little as long as the cam/distributor flag is consistently before Tooth #1 on compression stroke.
I forget the exact numbers, but the original 60-2 crank wheel has a tooth #1 (the one just after the gap) angle of ~80degrees BTDC. With a 1-3-4-2 firing order, I think you want to retain the cylinder #2 flag in the distributor and remove the other 3. Look at the MS guide and crank the engine around to confirm this before cutting up anything.
OK, am I making sense? Maybe someone who's modified a disti for the single cam tooth can say which of the 4 disti teeth/flags they retained?