88 951 Air Flow Meter Troubleshooting

[bdhtree]

Some background: I have been having some trouble with power cut out on my 951 under full throttle. It happens at all RPMs (even from idle, and not under load) if I open the pedal more than ~75% it cuts power similar to what many describe overboost feeling like. I have read many threads on this forum for troubleshooting and I am slowly working through potential problems.

I am getting no blink codes, but verified my light works by pulling TPS connector
I have checked and double checked all my vacuum connections
Tested the TPS and replaced with a "known good" unit just to be safe
Currently checking the AFM as the next step.

I took the AFM off the car and observed the output voltage through the entire swing of the barn door per the procedure on Clark's garage and noticed a voltage drop right about the middle of the travel (using a 9v battery as the power source, that makes the drop happen at about 3.9v).

The problem: Thinking I had found the issue I ordered a "known good" replacement AFM, but after installing it on the car it is having the exact same symptoms. Just to be thorough I did the testing procedure on the new one and it is seeing a voltage cutout at the exact same spot as my "bad" original one.

My question: Do all the AFM's have this small hiccup in voltage when tested, or should it be completely smooth increase in voltage? Did I just get unlucky enough to get one that had failed the exact same way as mine?

Any other troubleshooting suggestions for this cutout? Seems to be directly related to pedal position.

Thanks in advance for the help.
-Brian

[944 Ecology]

Should be a completely smooth increase in voltage.

Here is a test for the TPS:

http://tinyurl.com/zszd8hn

[Techno Duck]

The AFM wiper arm can be reoriented as a fix. The wiper arm makes physical contact on a ceramic wiper track. The wiper track will wear out in these sections especially in areas where the wiper track stays for a long time; like idle or 5th gear cruising. So i think yes, its entirely possible that you got two that are bad in the same spot.

This is is a great write up on the operation of the AFM and towards the bottom explains how to reorient the wiper arm.

Air Flow Meter (AFM) - from "The 944 Motronic DME" by FR Wilk

[bdhtree]

Originally Posted by Techno Duck

The AFM wiper arm can be reoriented as a fix. The wiper arm makes physical contact on a ceramic wiper track.

I did reorient the arm out of the grooves on my original bad AFM and no matter where I made the arm contact the ceramic track the voltage drop was the same. That is what lead me to purchase a new AFM. When the new one had the failure in the same spot I was thinking maybe it wasn't related to the grooves in the ceramic after all.

Not sure what else besides the grooves could cause this voltage drop. Maybe I need to check my voltmeter and make sure it isnt broken at that reading level...