On Thu, 22 Oct 2009 10:17:58 +0100, Dave Plowman (News) wrote:
It's working well but has an interesting side effect. There is an OBC
which calculates fuel consumption etc which is now wildly out. It worked
by counting the time one injector was open. The new system uses PWM with
feedback current limiting to drive the injectors which obviously confuses
it.

Hmm, interesting. From your description, it sounds like you need something
to convert PWM to a proportional voltage, then a second stage to
essentially carry out voltage to frequency conversion (giving pulses of
fixed width but varying frequency according to variation in the injector
pulse width - a VCO)? Possibly with some sort of adjustment between so you
can fine-tune things...

In the early 90s I'm sure there were circuits in the Maplin catalogue to
do both the PWM-analogue and the VCO part, but of course the catalogues
got less comprehensive over the years (and the one I'm remembering is
unfortunately stuck in storage).