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Car OBC

I have an old Rover SD1 fitted with a Smith's OBC which gives MPG, etc. The injection system is Lucas/Bosch and the OBC works by adding one injector circuit open time against distance travelled. And is pretty accurate.

But I've changed the EFI to a MegaSquirt. This drives the low impedance injectors directly - ie no resistor pack - and limits the current using a flyback circuit. They are also driven PWM rather than straight DC as on the old system.

This confuses the OBC. But not always. It sometimes gives an instantaneous reading which could be correct - sometimes one which is obviously not.

I can't find a schematic for the Smith's unit so can only guess at the input. And dunno whether it's the flyback current limiting or the PWM which is upsetting it.

Any suggestions for an interface which would sort things?

--
*Change is inevitable, except from a vending machine.

    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.
Reply to
Dave Plowman (News)
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Anyone recommend a better newsgroup to try? I realise it's more a design than repair thing.

--

*Whatever kind of look you were going for, you missed.

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW To e-mail, change noise into sound.

Reply to
Dave Plowman (News)

A gated pulse train to simulate what the original saw?

Reply to
JeffM

Forgive my inverted thinking. What you need to turn a pulse train into a something resembling DC is an integrator. TV vertical sections use those gizmos. You need to put some numbers to this before proceeding.

Reply to
JeffM

Thanks, Jeff. Can you clarify what's needed spec wise - I'm more of an audio sort of person. ;-)

--
*Funny, I don't remember being absent minded.

    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.
Reply to
Dave Plowman (News)

Dave Plowman (News) wrote:

As stated:

Got an oscope?

You need an RC time constant that will integrate this a bit. In a TV, the integrator must combine the closely-spaced stuff during the vertical sync pulse interval to make one fat pulse. Your task is similar.

If you make the time constant *too* long, the intervals between cylinders will be joined up;

**that** is what you need to figure out how to avoid.

Surely you've seen pictures of how a power supply filter slurs one (sinusoidal) peak into the next. Similar deal there.

You need to characterize what you have and/or do some trial-and-error runs.

RC filters are common in audio as well. I already mentioned power supply filters and their similarity to this application.

Reply to
JeffM

Dave Plowman (News) wrote:

As stated:

Got an oscope?

You need an RC time constant that will integrate this a bit. In a TV, the integrator must combine the closely-spaced stuff during the vertical sync pulse interval to make one fat pulse. Your task is similar.

If you make the time constant *too* long, the intervals between cylinders will be joined up;

**that** is what you need to figure out how to avoid.

Surely you've seen pictures of how a power supply filter slurs one (sinusoidal) peak into the next. Similar deal there.

You need to characterize what you have and/or do some trial-and-error runs.

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RC filters are common in audio as well. I already mentioned power supply filters and their similarity to this application.

Reply to
JeffM

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