[Diy_efi] Digifant (was:This turbo assist stuff...)

[Bernd Felsche]

On Thu, Jan 09, 2003 at 10:43:23PM +0800, Mike wrote:
> At 10:55 AM 1/3/03 +0800, you wrote:
> >If you think that odd - schematics for earlier cars show a 10A fuse
> >to power the ECU relay coil; and nothing else! Looks like the ECU
> >was originally powered entirely via the ignition switch and somebody
> >though better of it, putting a relay in to take the bulk of current.

> >But the coil is still running via the ignition switch!

> >Maybe that's deliberate - the extra cable length (inductance) may be
> >being used to isolate the ECU from voltage fluctuations induced by
> >the coil.

> Nop, its the same on my car. The reason seems to be the ECU releases
> the relay when its done, but the ignition can be 'killed' ad therefore

Not on Digifant. At least the version in my car.
The ECU relay is switched in parallel with the ignition power.

> there isnt a 'hold' time. The other reason is the ECU is then
> compatible with the hot wire AFM burn requirement. Inductance at

Doesn't make sense. Not economically speaking and the design intent
of Digifant - to be a low-cost digitally-controlled engine
management system (i.e. Motronic's cousin from the wrong side of the
track). Hot-wire was even more expensive than the flap-type meter.

> those frequencies is not an issue IMHO. My schematic has a
> capacitor, I am speculating yours is in your 'bosch amp' - which
> if old may be marginal causing erroneous early and overly
> sensitive determination of engine knock.  The ECU may also have an
> eeprom which may need an update time after ignition off, mine
> holds for about 12 seconds after ignition removed, but thats
> mostly for AFM burn, though there is a small eeprom in the
> bosch/hitachi ecu.

No such EEPROM on mine. Later ones did have it (G60 engine), but
AFAICT they use a power-fail circuit to initiate a save of any data
that needs to be saved.

> Onw thing which I dont understand with your setup given the point
> you mentioned that your ECU occasionally does a knock sensor test
> to drive the ignition excessively advanced to test the sensor etc.

Yep. How else can you check it's working?

VW and Bosch describe the process in their Digifant and
Motronic-related SAE papers and literature dating back to about
1986.

> How does the ECU determine the difference between a slight knock
> and a severe knock or are you implying it just doesnt care ?

Of course it cares; otherwise the odometer wouldn't have gone this
far without a major engine rebuild. :-)

Slight knock conditions were at the time being recognized as not
being damaging to engines and usually resulted in higher torque,
especially at low to moderate engine speeds.  The way to detect
slight knock is to back off the timing until no knock is detected
and then incrementally advance until knock is just detected.

i.e. suck it and see. :-)

-- 
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