The Air Flow Meter (AFM) - from "The 944 Motronic DME" by FR Wilk ©2001
All procedures and descriptions herein were created by me over a four year period 1998 to 2002.
The Air-Flow Meter (AFM), also know as Air-Flow Sensor (AFS), measures the volume of air flowing into the engine. The AFM is two sensors in one, air-flow and air temperature. AFM measures volume in units of cubic meters per hour (m3/h). The DME requires air mass. The difference being air density. Within the AFM is a sensor that measures air temperature. Air density and then air mass is calculated by the DME and the proper level of fuel is metered.
Inside the AFM
The Early 944 AFM
DME supplied voltage UV and the AFM output UP. The ratio of UV/UP is used by the DME. The early DME voltage was typically 12V unregulated (8V minimum). Both voltages UV and UP are constantly being measured by the DME.
The output is non linear with respect to flap angle. The output is linear with respect to air-flow rate. This complex function is designed into the ceramic substrate and its resistor array.
When filling in the appropriate air-flow rate, we have:
The Late 944 AFM
The design of AFMs changed with the late 944 AFMs. There is no longer any intelligence to its output. This makes it much easier to design and cuts the cost. On the down side, it uses more resources of the CPU. The output is linear with the angle of door opening. If the door open full has a UP=4.5V, then the door half open will have a UP=2.25V. A quarter open, UP=1.13V, etc.
In the late AFM, the voltage being supplied by the DME UV has changed to a regulated 5V ±0.5V. This voltage is now very stable. The early DME voltage was typically 12V unregulated (8V minimum).
Testing the AFM's Potentiometer
You can not use an ohmmeter to test an AFM's potentiometer. It will jump about meaninglessly. You must test it as it is being used. You have to force a voltage UV and measure the output voltage UP. This can be done with the AFM in the car and the ignition on. Here is a simple bench test setup.
Bench testing an AFM. Sophisticated test equipment consisting of a 9 Volt battery, battery connector, and two 0.1 inch female blade connectors (hard to find). Cost, under $2. The output voltage UP must rise smoothly as the door is opened. UP must never decrease as the door is opening not even for a moment. This is called non-monotonicity and can drive your DME into oscillations.
Testing the AFM's Air Temperature Sensor
An ohmmeter is used to measure the resistance of the temperature sensor. This is a room temperature measurement and the values are in the tables below. With an ohmmeter, measure between pins 6 and 22 (AKA 1 & 4). If the resistance is out of tolerance, you will have to get another AFM. Spare parts are not available. The temperature sensor can be moved from another AFM but it is easier to just buy a good used AFM.
The early and late AFM sensors appear to be the same part but are specified in different ways.
Refurbishing your AFM
The deep black grooves in the wiper track is from normal wear. The red groves are drawn to demonstrate the new track over virgin material.
The goal is to repair the damaged potentiometer. The potentiometer is made up of the wiper running along a resistive wiper track on the ceramic substrate. Moving the wiper assembly so that it runs along a fresh unused material on the wiper track. Observe the relationship of the top of the shaft and the top of the wiper assembly. They are usually about the same height. You will need a 7mm wrench to loosen the bolt on the wiper assembly retainer. Never touch the Phillips screw holding the wiper in place as this will move your angular set point.
North: Moving the wiper assembly down on the shaft will move the wiper contacts north of the previous wear grooves. This is the simplest to do.
South: Moving the wiper assembly up on the shaft will move the wiper contacts south of the previous wear grooves. This is a little more work. First, remove the 4 screws holding the connector in place. remove the connector. Loosen the bolt on the wiper assembly retainer. The entire wiper assembly will pull off of the shaft. Do not worry about the alignment, the shaft is slotted so that it will go back to the exact same shaft angle. Bend the wiper as shown. Replace the wiper assembly on the shaft leaving it higher on the shaft. This will move the wiper contacts south of the damaged area. Replace the connector.
Update Your 1984 AFM
In the late 1980s, BOSCH started using a redundant wire from the connectors bridge to the wiper and the wiper. Probably to reduce noise and contact resistance at the pivot point. On a few that I tested, the voltage drop was unusually high. The red wire, above, simulates the BOSCH installation . The blue ties are to take pressure off of the solder joints. I recommend making this modification if ever you have the black plastic cover off of the AFM.
Changing the Spring Tension of an AFM
Don't do it! Never change the tension of the spring on your AFM. You will destroy the calibration of your AFM. The AFM is laser trimmed to a fine tolerance by BOSCH. If you do, have a good insurance policy.
from 'Automotive Electric/Electronic Systems ©Robert BOSCH GmbH, 1988'
" Air-flow measurement
potentiometer of the air-flow sensor is of the thick-film type, with a ceramic plate base.
Its high-resistance wiper track as well as the two contact rivets on the wiper itself are
of an extremely wear-resistant material. The resistance are connected to the wiper track
by very narrow conducting segments, the so-called data points. These data points terminate
under the wiper track and thus define the specified voltage steps. In the Motronic, the
wiper-type potentiometer is constructed so as to ensure that there is a linear
relationship between inducted air quantity and slider output voltage. The high
temperatures and abrupt temperature changes that always prevail in the vehicle engine
compartment have no negative effects on the behavior of the thick-film resistors. In
addition, in order that potentiometer aging and temperature fluctuations do not affect the
accuracy of the measurements, the control unit evaluates only resistance ratios. An
adjustable bypass diverts a small quantity of air past the air-flow sensor flap.
Variations in this bypass air quantity, which is independent of the sensor-flap position,
are applied as the basis for correction of the air-fuel mixture for idle adjustment. In
the Motronic system, the control unit switches on the fuel pump by way of an external pump
relay; for reasons of safety, no fuel is pumped if the ignition is on and the engine
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