There are a few other issues.   There is a mode that increases the idle
to a pre-determined
amount to anticipate the extra load of the AC, and I bet the PS pump
load is also involved.
Also my guess is this circuit is turned off once the idle switch is
tripped, but not at full throttle.
Once all these factors are taken into consideration, you end up with the
brainy looking
think that VW made.  Hopefully  some has the time to engineer this thing.
                  THe VW digifant  Jetta 88-92 has this built into the
ECU, and these  are
plentifull, and cheap.   I'm tempted to try this injection on a water boxer.
                                                                  ERic

Pensioner wrote:

>Perhaps not so mysterious a design.  Your tachometer uses a pulse per unit
>time to voltage conversion no digital stuff here nor D-A conversion.  So if
>the transfer function of pulses per unit time  to voltage is known and the
>desired range of ppt is known then the range of voltage is known.  A
>comparator circuit can give you a measure of whether the voltage is within
>the specified range.  If below then you have a signal that indicates engine
>speed too low.  If above then engine speed too high.  Detailed design left
>to David B or those with that sort of "bent".
>
>Pensioner
>
>Tau Beta Pi (bent), Eta Kappa Nu, Phi Kappa Phi in EE/CS,  O so long ago.
>
>
>
>