ViribusUnits

Does anyone have a primer on how to modify the L-jet afm?

I just got my new used one. Now that the car seems to run like a top, I had the courage to take my old one apart. After all, it didn't work right anyways, what can I ruin...

A short time of careful prying later, and I've got the plastic cap off. To my suprise, I didn't break it. Boy, this thing is simple!

OK, here's somethings that I noticed. There are two copper contacts that go between the center rotor and the varable resister plate. In several places I can see where the copper contacts wore through the black part of the resister plate, and into the white plastic material below. For some reason, the copper contacts don't seem to be at right resting place. When the flap is wide open, they are completely off the resister board. When it's at full closed, they don't follow the twin tracks all the way up to where they stop. Nothing seems to be bent, but there is an adjustment screw that is all the way at it's limit. <<Looking at the probably origional glue to hold the screw, I'm 99% sure someone modifyed the adjustment of the screw.>>

I guess I found why I had a rich idle. The contacts where adjusted so that they were always slightly richer than the air flow demanded. What I can't figure out now is how it passed the service's mannuals resistance checks. I guess that explains the throtal possion sensitive, non igniton related, miss that I had. I wasn't sure if it was realy there or not, because I could not get that much of an air flow. This was because of my rev. limiting ignition related miss.

On looking at the glue on the inside of the unit, there apperes to be two layers of it. I'm begining to susspect someone else messed with the unit, trying to get the car to run. Darn PO. <<that borken glue to hold the adjustment screw in place also says someone screwed with it.>>

Does anyone know of a place that makes a replacement for the little plastic resister plate? I've got a perfectly good AFM, except for the resister plate. That thing can't be that rare, can it?

PS, thanks to Branden's parts car, I seem to have a fully fixed, speed demon 928S! It's a wounder what a good AFM, and good igniton box will do! My mixture seems to be spot on now, no adjustments, and my igniton doesn't seem to cut out any more. I'm happy!!! No more getting beat up on my mini vans. <img border="0" alt="[jumper]" title="" src="graemlins/jumper.gif" /> Soccers moms beware, I can take you now!

Thanks to the board for help trouble shooting the problems with me. I wouldn't have realy known where to go without ya'll folks. Hum, anyone figured out how to clone Wally yet? I'd buy one...

Thanks again.

(Edit)

PS, has anyone tryed modifying the AFM by pulling the copper contact arms in just a bit, or out, just a bit, so they don't follow their old tracks. I noticed that the reister plate is made like a pie chart, will this change the resistance?

Thanks again.

(End Edit)

_Thomas_

Hi

actually I have done that for my BMW 635 CSi which has got a similar AFM (Bosch 0280 203 002). Read instructions below.

Cheers Thomas

Air Flow Meter Pointer Repositioning and Repair - Barry Jay Burr &lt;justbarrys@snip.net&gt;

Correcting Idle Fallout due to air flow meter carbon band wear on Ljetronic and Motronic models:

The problem of intermittent stutter at mid throttle, failure to hold idle, and the engine reponding when the throttle is depresses as if it were out of gas very often is due to wear on the carbon band within the Air Flow Meter.

The dealers will say replace the AFM. Thatcan cost you a few hundred dollars or more. Instead of that, try thefollowing procedure. My old 84 318 still had the original AFM at228,000 miles because of doing this procedure several times, and I'm pretty sure that AFM is still in action, now somewhere out in Minnesota:

Remove the black plastic lid.Notice the carbon dust on the pointer tracks on the carbon band.VERY GENTLY, ever so lightly lift up the pointer from the band using a small bladed screwdriver or thin hook tipped nose pliers.Blow the dust away, compressed air is preferred. If you don't have a compressor, see below.

To verify band wear or loss of pointer tension is the problem, holdthe pointer at about the place it would be at idle. If you are notfamiliar with this, try about a centimeter from the resting end. Thentry to start the car. If just by pressing down on the pointer at the right spot gets the car to hold idle, then you have found the cause of your problem.

Now to regain tension at the pointer:

1. Notice the end of the copper pointer arm where it joins thepivot.

2. Notice where the arm is a double V shape, with an open slot between the inner and outer sections of the V

3. Press down on the arm, only on one side of the V, just away from the pivot until you just feel a little chance of it bending a bit. Nowdo the other side of the V, again just away from the pivot.

4. Pressing down GENTLY on the tip of the pointer with a fingertip, move the pointer forward and back across the range of movement of thec arbon band.

5. START YOUR ENGINE.

You may not get it exactly right at first, but once you get the ideaand can see exactly what you need to do, a little more or less pressure exerted on either side of the pointer arm will restore full contact over the full range of throttle motion.

You may then need to adjust the pointer position a tooth leaner or richer because of the slightly different resistance at any given point. This is done by moving that big black gear a tooth forward or backward.The gear retainer is easily lefted by a screwdriver. Be sure to put a dab of white-out or something to mark the original position so you can go back to initial setting if you need to.

The goal of this bending and repositioning of the arm and pointer is to both:

A. restore tension to the arm so it presses down sufficiently on the carbon band again and B. reposition the pointer ever so little further inward or outward so that it is now rubbing across an unworn part of the carbon band. You may then need to adjust the pointer position a tooth leaner because of the slightly different resistance at any given point.

and a good seminar on how it all works:
<a href="http://www.firstfives.org/faq/ljet/jetronic.pdf" target="_blank">http://www.firstfives.org/faq/ljet/jetronic.pdf</a>

BC

I remember when I first got my 924S, that I read somewhere that the spring could be "weakened" so that the barn door would open MORE for less air, thereby richening the mixture, and in the reverse, strengthened, so that the door would be more closed, etc. The important thing here is that you really have to have an 02 sensor hooked to a readable meter to see the differences. Also, that black cover is not easy to reseal, and if you use anything with silicone, it will corrode everything in there.

I'm glad my car will live on in others. The money from this car will live on in my 89S4 5spd.

ViribusUnits

Sweet.

I was thinkging about useing a pair of plyers to bend a crook into the copper followers, and see if that didn't fix my problem. I apperes I'm not the first one with that idea. Cool. :-)

Thanks.

BC

By "spring" I mean that the tension can be changed. Moving that arm, however, as I recall, is not such a good idea.

Paul Bloomberg

Let's see more spring tension requires more air to push the flapper open to the same position. Which would lean the mix. Tighter (clockwise) is leaner. Which in my case picked up a couple of hp on the dynojet.
All the adjustments and troubleshooting #'s w/pictures are on FR Wilks site. Including a update for early models.
BTW adjusting the pointer without a dyno is pissing in the wind.....
Paul

John..

The unit is nothing more than a rheostat with tension adjustment and adjustment over the starting and ending points.

There are two copper pickups, because each one has its own resonant frequency. With two of them you always have good signal back to the brain.

It is a crude, but very effective device.