Itinerant Air-Cooled

I have several AFM units that I want to evaluate. Specifically,they are the Bosch 020
L-Jet system air flow metering boxes. Bentley specifies a range of acceptable resistances as measured between various pins on the integral plug. Six pin-outs for early L-jets and seven for later models. Here is my confusion. For one AFM the acceptable resistance range between pins 7&8 is from 100 to 500 ohms. Between pins 6&9 the acceptable range is from 100 ohms to 300 ohms. How can this be? It would seem that any ohm set between these numbers would give rise to wildly different voltages sent to an ECU looking for a specific voltage matched to a specific air flow. Or maybe the ECU is only looking for voltage differentials, so any starting point and hence an initial specific resistance is immaterial. Or maybe it doesn't use that paradigm.
The Bentley illustrations of the inside circuitry of the air box are not particularly helpful. I have read that laser surgery was performed at the factory to set (tweak) various resistances on the circuit board. Did they actually have focused lasers for such industrial purposes in 1975 and up? BTW..all my air boxes were within spec (whatever that means).
Any enlightenment appreciated. Iciclebob.

Psucamper wrote: For one AFM the acceptable resistance range:
pins 7&8 ... 100 to 500 ohms.
pins 6&9 ... 100 to 300 ohms.

How can this be? It would seem that any ohm set between these numbers would give rise to wildly different voltages sent to an ECU looking for a specific voltage matched to a specific air flow. Or maybe the ECU is only looking for voltage differentials, so any starting point and hence an initial specific resistance is immaterial. Or maybe it doesn't use that paradigm.

Icicle Bob? I invite you to apply your considerable brain power and dogged determination to answering these questions exactly through whatever reverse engineering or intuitive leaps of understanding you embark upon or are graced with.

These AFMs have tormented me mightily, and every time I think I understand them, some field practicum or another yields confounding results.

I see a variable resistance between 7 and 6, this agrees with my feeble mind so far.
I see the specified temperature resistance between 6 and 27, fine.
Now, just for the heck of it, 7 and 27 show above resistance, continuity broken if you pick up wiper contact

I see a non-variable resistance between 7 and 8 that twitches between track segments as wiper is moved, circuit is dependent upon wiper contact with board.
Same as above for 7 and 9

So who is supplying the discrete board segments with voltage that must pass through a bunch of discrete resistances that are picked up and brought home via a nice variable resistance read off 6 and 7?

I believe 7 & 8 and 6 & 9 are discrete circuits with specific resistance values. But how doth the variable communication between 7 & 6 ? And why does 36 (!) have communication with 6?

Help me, Bob, I can't sleep.

Psucamper wrote: The Bentley illustrations of the inside circuitry of the air box are not particularly helpful. I have read that laser surgery was performed at the factory to set (tweak) various resistances on the circuit board. Did they actually have focused lasers for such industrial purposes in 1975 and up? BTW..all my air boxes were within spec (whatever that means).
Any enlightenment appreciated. Iciclebob.

I would be curious to know if anyone sees a different part number of the board itself across different applications, like Volvo, Mercedes, BMW, so we can see if the board resistances are different, or can just spring/wiper/mixture bypass screw adjustments allow the use of one board?

Bob, I have done the following gross matrix adjustments:

A = light spring vs lean wiper @55 11.8
B = heavy spring vs rich wiper @55 11.8

and my fuel mixture numbers did not follow any sort of logical linear progression to either side.

Let's consider the following scenario where variables are:
a) engine rpm signal (#1 wire to ECU)
b) resistance signal from AFM

Drive up a hill from 70mph down to 40 mph with accelerator held steady at midpoint let's say:
a) rpm signal drops frequency (injector pulses correspond 1:1)
b) resistance signal remains constant (steady throttle gives steady airflow position of the wiper)
Fuel map dictates that the fuel supply increases as engine drops into torque curve . . .
Discuss
I hate physics, can I go play now?
Colin

Colin, Thank you for the reply regarding the mysteries of AFM operation. I will see if I can correlate some additional resistance readings with data already taken. However,i am not overly hopeful. I do have a spare 6 pin box from a 1974 T4 car (no external T1 temp sensor). I'll ohm it and see if there are any differences that might be helpful. Cogita! Bob

Psucamper wrote:Cogita! Bob

. . . 1975, 1975 was the year that L-Jetronic was introduced to all Volkswagen automobiles.

So,
steady accelerator,
engine loses rpm on gradual hill,
mixture is supposed to richen as rpms drop into torque band,
but we would observe the wiper slowly moved clockwise (lean) across the board as airflow diminished.

Does the ECU do a rpm signal vs airflow **analysis** and increase injector timing?

Colin