OBD-II toothed wheels for crankshaft position sensing

[Sandy Ganz]

> Why everybodies obsession with external 58 toothed wheels?, you do
> realise that you already have a 120 odd tooth wheel already bolted to
> the back of the engine?. (Yes, I do realise you also need a TDC reference)

A couple of reasons, one is what happens as a tooth gets ground 
down by the starter. This has happed to a couple of cars that 
the starter went south on. Also each system would then have to 
maintain a different setting for each tooth count, i.e., Chevy 
(small and large flywheel), fords, dodges, 4 cyls, etc. Lastly, 
the frequency of interrupts from the flywheel would be 
around 2 times that of the 58 tooth part (I don't think this 
would be a big problem however). 

I don't think that the 58 tooth wheel is anything magical, other 
then I has been used on some production cars (BMW's I think). 
Look at what GM uses on some of it's SFI. The old SSI chip could 
support a vareity of wheel types, even with different spaceing 
between the teeth to provide faster position identification

Also if we are using the 58 tooth wheel (really 60) we have a 
reference location that we can still use even if we lose the TDC 
reference. I think that electromotive uses this as the basis for 
their system. I'm not sure what they use for their 'sequential' 
FI.

Just some ramblings ;-)

Sandy