Fractional Tachometer Signal

[Healey3000]

Hi,

I have a situation where the engine is a four-cylinder but the tach is designed for a six-cylinder. Therefore, the tach is expecting to see six pulses per engine cycle (three per crank revolution) but MS3 naturally can't provide that.

One possible way I could do this is to trigger off a single Tach Out pulse from MS3 and then count crank teeth, generating a pulse every N teeth, thereby creating "extra" pulses (using a separate microcontroller). To do this, it is important to know the timing of the tach pulse from MS3 with respect to crank position. I have searched through the documentation but have not found this information. I assume the mention of "cylinder event" means that the pulse is indeed at a known position.

Alternatively, I suppose I could build a missing tooth detector external to MS3 and generate precisely positioned pulses that way. Does MS3 offer a "sync" output of any kind that would allow me to know engine position at each revolution?

Thanks!

[DaveEFI]

How easy is it to get inside the actual tach? Might just be easier to replace the electronics with a modern chip which allows you to select the number of cylinders. Other way might be a kit from Jaycar designed to correct electronic speedos when you change the axle ratio. It's the same sort of thing.

Unless it really is easy to do it with MS.

[billr]

I have used a CMOS 4046 PLL to increase frequencies.

[phoenix3d]

Hi guys..

have a look at the Latest MS3 firmware (Beta) of course.. it has fractional Tachometer output run it at 130% or so...

Gordon

[kaeman]

why not put a 4 cylinder tach in the vehicle to match the 4 cylinder engine?