Date:         Thu, 10 Apr 2003 09:58:11 -0700
Reply-To:     laurasdog@WEIRDSTUFFWEMAKE.COM
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Steve Delanty <laurasdog@WEIRDSTUFFWEMAKE.COM>
Subject:      More rant on grounds and vanagon syndrome
In-Reply-To:  <OFBF38805F.D3ECB8EC-ON85256D04.00529C38@lvs.dupont.com>
Content-Type: text/plain; charset="us-ascii"; format=flowed

Jay wrote:
>Grounds and clean connections are the key. I have added extra body to
>engine ground wires, as well as  wires from the alternator and the AFM
>casting.  All the connections are cleaned and assembled with di-electric
>grease.

Grounds, grounds, grounds...

I believe that part of the problem is that VW engineers violated
one of the most important rules of good grounding tecniques
by grounding the ECU and some of the sensors to the body of the
vehicle and the O2 sensor (and who knows what else) at the engine.
Good design practice would dictate grounding ALL of the EFI components
including the ECU to the engine ONLY. (and most modern EFI systems
go out of their way to do this)

Grounding the ECU at the body and O2 at the engine means that the
ANY resistance (and there is always some) in the ground strap from
engine to body will cause an error voltage to be added to the output of
the O2 sensor.

Consider the following facts for your amusement...
Under "normal" conditions the voltage output of the O2 sensor is
approximately 0.5 volts. A voltage output of 1.0 volt would indicate an
EXTREMELY rich condition, and a voltage of 0.25 volts would be
extremely lean, so it's obvious that small voltage errors can make a pretty
big difference between what the O2 sensor sees and what the ECU *thinks*
the O2 sensor sees.

When the alternator is producing current, the ground return path for said
current back to the alternator travels through the ground braids. Since these
braids have a finite resistance, there *will* be a voltage drop across them.
How much? Well, if the alternator is producing 30 amps, and the ground
braid resistance is 0.01 ohms, then the voltage difference between the
body of the car (where the ECU is grounded) and the engine (where the O2
is grounded) will be 0.3 volts, with the body voltage being positive
relative to
the engine. (that is the engine is 0.3 volts more negative than body)
So... this 0.3 volts introduced by grounding errors is subtracted from the 0.5
volts that the O2 sensor puts out, and suddenly the ECU only sees 0.2 volts !!!
Holy Cr@p!  We're way to lean says the ECU, and cranks the mixture up to
rich as hell..

The problem is worsened by the fact that the DC offset voltage caused by
charging current is only an average, and an error of 0.3 volts will also have
an AC component riding on it caused by the 3-phase alternator output, which
causes the peak voltages to be much higher. Also, "Load dump" (which is brief
high voltage spikes caused when electrical loads are suddenly removed)
causes very short duration but high current spikes to pass through the
ground braids and I saw brief voltage peaks well in excess of 1 volt between
the body and engine when I was measuring. This means that the O2 sensor
input to the ECU actually goes to a voltage BELOW ground (negative with
respect to ground) periodically.

I think the ECU just can't cope with this BS on the O2 line and eventually
goes into "stupid mode" until the power is reset.

Some folks have said that disconnecting the O2 sensor made their vanagon
syndrome go away. Yep. Makes sense to me...

I believe that what REALLY needs to be done, is to find ALL the wires that
ground the EFI system to the body (ECU and all sensors) and remove these
grounds from the body and ground them ALL at the engine only. Only in
this way can all error voltages caused by charging current passing through
imperfect engine to body ground straps be eliminated

Since our digifants, don't do it right, the next best thing is to add plenty
of extra heavy duty grounding, preferably directly from the alternator to some
body point removed from the EFI ground locations. There will always be
*some* error voltage present on the O2 line this way, but you can probably
reduce it to an insignificant amount.

And again, the noise suppression capacitor on the back of the alternator
is an important item. It not only helps remove some of the 3-phase AC
component off the charging voltage, but it helps absorb the brief voltage
spikes caused by alternator load dump.  Get one if ya ain't got one!

So there. That's what *I* thnk about digifant and it's silly ground system...

Happy motoring!
Steve
'86 Westy