ignition coils?

[Phil Hunter]

Howard Chu wrote:
[snip]
> While browsing thru the archives for this list I
> noticed a discussion on replacing an AFM with a MAF.
> This has been on my mind too, since this Mazda KL
> engine also uses a pretty restrictive volume
> measurement sensor. The MAF and VAF transfer
> functions are pretty much completely opposite in
> slope. I decided to try just going MAF -> ADC ->
> EEPROM -> DAC. No processor/controller of any kind,
> just a fixed mapping in the EEPROMs. Unfortunately
> this setup doesn't take temperature into account,
> and the stock ECU uses the IAT to convert the VAF
> input to mass. I started looking at analog
> multipliers to take this into account, but the
> IAT's output is only linear for part of its range,
> which tells me I need another EEPROM to map that
> out, and a CPU of some kind to do the compensation.
> I've left things alone for the time being; I think
> it will make more sense to just reprogram the stock
> ECU to accept a MAF input. Does anyone have any good
> info on programming a Mazda OBDII ECU?
[snip]

Assuming you are refering to a post I made, I've got
to clarify: the ADC/EEPROM/DAC thing was to explain
the concept only, while it may seem simple to
implement it's not, "the devil's in the details" and
there are a lot of 'em. Basically you need to take
into account the settling and access times of the
components using some kind of sequencer to sample the
input, let the ADC settle, enable the EEPROM and let
it's output settle, then enable the DAC and let
it's output settle. It can be done, but I hope you see
it's a lot more work than first appearances. If you
don't sequence things, you'll get a lot of spikes and
noise on your output. You can filter it, but then your
response time can suffer, may or may not be an issue
since you're imitating a mechanical device (right?)
which is relatively slow responding compared to an
electronic device.

I'd say you'd be better off using some kind of micro
to do the sequencing, lotsa choices here, some have
built in ADCs to simplify things even more, leverage
the work the chip designers did for you. The Basic
Stamp series are easy to program if that's your
concern, a LOT of info and examples are available,
even filling books, they can read resistance using an
RC network (timing) but I don't think they have an
ADC so that'd have to be external. Their speed is
relatively slow but in this case may not matter like
I said above. Another limit is they do not do
interrupts, newer ones have a kinda pseudo-interrupt
capability. http://www.parallaxinc.com

With regards to temp. compensation, at a concept level
you could have 2 ADC's into the EEPROM, if the temp
ADC drove the upper address lines, it'd be like having
temp-comp. tables.

As for the MAF vs. MAP discussion, outa my league, I'd
say listen to Bruce. Being more familiar w/ the MAF
idea myself, I'd probably consider a hybrid/evolution
using both, eventually phasing out the MAF.

phil



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