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

Bernd Felsche bernie at innovative.iinet.net.au
Fri Jan 3 03:06:16 GMT 2003


On Fri, Jan 03, 2003 at 01:01:51AM +0800, Mike wrote:
> At 10:45 PM 1/2/03 +0800, you wrote:
> >> I and other engineers can probably be safe to say they can
> >> guarantee that placing a good suppression cap from ignition +ve
> >> near your coil (if this is the positive ignition supply) to
> >> ground (also near your coil) wont do any damage but will
> >> improve dramatically the AC circuit your ECU and ignition amp
> >> is seeing !

> >The ECU doesn't see any of that.

> Huh !?  Are you saying the ECU and coil are driven from separate
> supplies, dont think so - you have 'one' battery dont you and
> one alternator ?

Well, sort of. It's quite odd, really!

But I was referring mainly to the coil drive voltage.

The ECU is powered via relay contact direct from the battery bus
(30) on central electric; the main fuse/relay panel. The ignition
amplifier for some reason, is not powered via that relay, but
separately from the ignition-on bus (15) in the central electric;
and it gets there by detour through the ignition switch. (15) of
course drives the ECU relay coil.

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.

After all, the ECU and ignition amp are right next to each other and
it'd be unusual for VW not to simplify electrics (saving on wiring)
by powering ECU and ignition through the one relay and wire. They'd
had 6 years' practice building Mk2 Golfs by the time mine was built
and a year on the "new" electrics introduced in 1989.

> ECU +ve comes from ignition +ve, maybe through a relay, one side
> of your coil is switched from ground or switched from ignition +ve,
> hence the ECU does and will see disturbance on its supply voltage
> and via AC coupling will also see  noise on its signal input
> lines depending on how well they are loomed...

If the quality (?) of the connectors is anything to go by, the loom
isn't brilliant. Then again; Audi seems/seemed to have the uncanny
knack of doing even worse with the same style of connectors - maybe
they had somebody from BMC designing their looms. :-)

Nevertheless; ignition and ECU looms are well separated. There are
only two direct connections; one via "chassis" (actually to battery
-ve terminal, each unit via its own chassis wire) and the other
being the ignition trigger signal.

Again, individual chassis wires connecting to the same point, from
the co-located ECU and amplifier to the battery over a metre away,
seems to indicate an attempt to keep the parts' supply voltages
somewhat electrically separated.

> >No. All the other Digifant Golfs do that when you don't give them
> >enough Octane. It's a feature.

> Well the only direct solution is either reprogram the old ECU,
> get another ECU from a later model that didnt have this problem
> which is compatible or throw it away firmly and get an after
> market ECU.

The "problem" is an artifact of how the ECU adapts to varying ONR.

I think I can do better than an off the shelf after-market.
Integrated fuel and ignition control - not just sharing the same box
- is one approach.

-- 
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