Date:         Tue, 07 Jan 1997 16:57:25 -0800
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         Tonya Pope <galba@premier.net>
Subject:      Re: Re(2): Rough running when Hot

Charles,

The problem with the air mass meter (not the original harness) is that
it gives intermitten bad signals to the computer.  After a while, these
bad signals build up and really confuse the computer.  The 'magical
harness' that VW added simply 'conditions' the signal so that those
intermittent drops/surges (I'm not sure which one it was) don't actually
reach the computer.

>From your most recent postings though, I too question how long it takes
to make it mess up.  If it is a very regular, short, time increment,
look at that O2 sensor.  I believe that the computer ignores it at
startup until it has had time to heat up.  Those extra 2 wires are to
heat it up, by the way.  However, because the signal is in such a small
voltage amount (under 1v), it must be hot to give a good reading. The
computer is suppose to ignore readings out of the expected range (i.e.
too high or too low), but yours could be giving it a reading in the
range but not accurate.  If restarting is all that is necessary (no
cooling off period), the computer has to be getting a bad signal which
is reset when you turn it off and back on.

As far as your questioning how the grounding strap can help, the
computer is looking at fairly minute changes in voltages from its
sensors.  The voltages aren't a definitive amount, they are relative to
a ground.  They are simply the 'difference' between two points.  Believe
it or not from the definition I'm going to give you, I'm an electrical
engineer.  But I'm going to explain it so that it makes sense (although
theoretically, it might not be scholarly accurate or in the right
terms).  Say the computer's ground is .2v higher than a sensors ground. 
If the sensor is sending a signal that, to the sensor, is 1v, the
computer is only going to see .8v.  The computer interprets this as a
completely different situation than what the sensor was trying to tell
it.  Likewise, if the sensor has a ground that is .1v higher than the
computers, a 1v from the sensor will look like 1.1v to the computer. 
Adding grounding straps can stabilize the ground to where everything has
an equal one.

Wow, was that long winded or what?

If you can give a better description of how/when/what temp it
misbehaves,  and how/when/what temp it acts right, it'll help all of us
help you better.

It took almost a year to get mine working right, I hope you have a lot
better success!!  But then, I didn't join the list until I was 9 months
into my saga, so you've already got a BIG headstart!

Good luck!
Tonya
87 Vanagon GL Wolfs
74 S. Beetle