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Adjusting the AFM

Adjusting the AFM


Hunt Dabney

Craig Van Steenburgh wrote:

Hunt, I saw your message about how to test the 02 sensor in the jag. I was curious how mine would fare since it has been a few years and miles since I changed mine. I was getting a .925 volt reading that didn't change much when the engine was revved up. If anything, it would drop to about .880 volts and then come back to .925. Does this seem to indicate a bad O2 Sensor or should I adjust the AFM? Where did you get the info on how to test the sensor? Does this apply to all O2 sensors? Thanks.

It sounds to me like you either have a leafing cold-start (or normal) injector, or the AFM is misadjusted. I ommitted some critical info from my earlier O2 post - it's in here somewhere... A mechanic friend let me borrow a Lambda Sensor tester that he has had since the 80s, when 1 wire O2 sensors were in common use. I reverse engineered it to get the info that I posted. I borrowed it because I had replaced my AFM, and was getting terrible mileage. The behavior that you described sounds like the AFM is misadjusted. Here's my version of how to adjust it: With the lid removed from the AFM there are two possible adjustments: the flap spring tension, which is the clock spring, and the wiper position, which is a small slotted screw on a black plastic assembly, all of which moves with the flap. The spring tension controls the slope, e.g., does the mixture stay relatively constant over the powerband, or does it go more rich or lean as there is more airflow. If the slope results in constant mixture, then the wiper offset controls the absolute mixture, e.g., you can set it richer or leaner with this. I left out a critical piece of information from my earlier post: to perform the measurements that I described, the system needs to be operating open-loop. This means that you take the measurement from the O2 sensor, while it is disattached from the cable to the ECM. Otherwise, the ECM will try to correct everything that you do and readings will be meaningless. To adjust the AFM, with the system operating open-loop and the engine warmed up... start by idling.

  1. Loosen the clamp that retains the clockspring, (mark its position so that you can put it back if you need to!)
  2. if the voltage reading is too high (as in your case), increase the spring tension so taht the AFM flap closes more, indicating less air. This will result in less fuel being metered. (This is very sensative.) Once it settles around .6 volts (it will actually fluctuate about a center value - .55 - .6 volts is a good target) secure the adjuster there.
  3. Now, rev the engine. The voltage should bounce rapidly to full rich, then full lean, then come back near the original setpoint. Increase engine RPM to around 2500. Note if the mixture goes ricg (higher voltage) or lean, or stays about the same. Play with the spring tension to try to level this response out, although it may result in being too rich or lean overall. It is not necessary to get this perfect - just try to flatten it out as best you can.
  4. Now adjust the wiper so that the sensor is indicating back around 0.55 - 0.6 volts (with the regular fluctuations previously mentioned).

If you think it looks about right, tighten the setscrews, stop the engine, reattach the O2 sensor to the ECU (and attach the meter again), then start the engine. The ECU should regulate the voltage from the O2 sensor to around 0.55 - 0.6 volts with some rythmic fluctuation. If you increase engine RPM, it should adjust back to that range when RPMs stabilize. If you release the throttle, it should drop to a very low voltage, then come back up to the previous setpoint. Now test-drive the car. Given that you have a feel for how to play with mixture, you can tewak it like you would an SU carb to get the driveability how you like.


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