Date: Wed, 16 Nov 1994 10:50:39 CST6CDT
Sender: Vanagon Mailing List <vanagon@vanagon.com>
From: "Dan Houg" <fairwind@northernnet.com>
Subject: Air flow meters.
Here's some info on airflow meters, for better or worse.
I made a $180 mistake a couple years ago by convincing myself my
airflow meter was bad ( I had 'bogging' problems and 'low' [read
typical] fuel mileage). I had taken the ohm meter readings per the
tech manuals and the values jumped around when moving the air vane.
I thought , "oh, its bad" and willfully paid for a new Bosch unit
from the dealer. I checked the new unit, and its values also varied
in a non linear fashion as I moved the air vane. I checked another
van with similar results indicating a 'bad' unit. Engine
performance/fuel mileage/drivability varied not a wit with the new
unit. ( I kept asking my wife- 'it seems a little better, doesn't
it?'). Anyway, as the saying goes, "Wisdom comes from experience.
Experience comes from mistakes."
On my quest ( and after my purchase) I called a place in Minneapolis
specializing in high-end German cars. They said the *only* way to
check out a suspected airflow meter was to put an oscilliscope on the
voltage output lead of the airflow meter and move the vane while the
harness is connected and the key on. A good meter will give a smooth
voltage change on the oscilliscope screen-- a bad meter will result in skips and jumps in the
displayed voltage. They faxed me an internal wiring diagram for a
4-lead airflow meter (manuals sometimes show an internal diagram of
the 6-lead ones). Here's the pin breakdown on a 4-lead meter as
found on at least '83-'85 Vanagons:
Pin 9 - Input
Pin 8 - Reference voltage
Pin 5 - Ground
Pin 7 - Wiped output (the one that varies with the vane movement)
So, attach the oscilliscope to Pin 7 (by back-probing thru the
harness) and a ground.
I did have an airflow meter fail on a Toyota. The symptoms were an
extremely sharp engine cut off at very specific rpms. Other rpms ran
flawless. The dealer said it was a $550 item so I took mine apart to
see if it was repairable (prompted by the dealer assertion that there
was no way one could repair one). Everything looked good with no
burned spots on the carbon resistors so I took a pencil eraser and
gently cleaned the carbon track. Then I used piece of paper card-stock to
clean the underside of the wiper. Put it all back together, resealed
the plastic lid with silicone sealer and have had a perfectly running
airflow meter for the last 50,000 miles! Somenone had posted a
repair by elongating the mounting screw holes to present a different
portion of the carbon track, this sounds like a good idea too,
especially if the wiper has worn a groove in the track.
Airflow meters DO go bad, the individual carbon resistors and/or the
carbon track can burn out. But I'm sure suspect of the diagnostic
procedure using an ohm meter.
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