How to set the Rich/Lean mixture on a Classic 900.

This explains how to set the lean rich mixture on LH systems where there is an adjustment screw on the Air Mass Meter.

High View (Anders reference)

How LH works.

The AMM controls 100% of the fuel flow; the O2 sensor about 25%. The O2 sensor works in a very narrow range. You 'coarse adjust' the AMM to get the mixture near right, and then the O2 sensor takes over. If the AMM is set too rich or too lean, the O2 sensor doesn't have a chance to work. The adjusting screw changes the resistance by 40 ohms/ half-turn, clockwise increases resistance (richer), counterclockwise decreases resistance (leaner).

Using an analog meter with a setting for a range of 0-12v or just over, connect the test meter (voltage) leads as green= positive, red/grey as negative and adjust the AMM screw until you get an evenly paced swing on the meter's needle between 0v and 12v. If your analog voltage meter jumps backwards under zero, just switch polarity.

One caution though with using that test socket, if the O2 sensor is not working properly, it will not send the pulsing signal to ECU and subsequently, the ECU will not send a pulsing signal to the test socket.

If you get steady 0V, the fuel mixture is too lean. If you get a steady 12V, it's too rich. When the Voltage is going all over the spectrum in an even speed, the fuel mixture is right. It's a very quick test to confirm that the ECU, AMM and O2 sensor are working properly.

The voltage showing on the meter is more of an end of the line reading in the sense that it is the representation of what the ECU is telling the injectors to do based on the info it receives from any component that sends it info. This also represents the correct small fluctuation between rich and lean that LH 2.2 exhibits at warm idle when tuned correctly and shows up as the small 50 or so variation in RPM.

If the basic setting is correct, the meter needle will swing back and forth between both ends of the scale. Note: The movement of the needle will not be rapid and there may be a pause between each movement. Dial in the AMM until the signal is swinging properly between the two ends of the scale, spending roughly equal amounts of time at each end over a minute or so of fully-warmed-up idle. If I can't get the AMM adjusted properly, I replace the O2 sensor.

Notes on Anders Method

When perfect, the voltage will swing between 0 and battery voltage at test plug during steady load acceleration (not full throttle though) but any movement of throttle will alter the swing until steady load is achieved again.

What the Saab Master tech's where CMyles used to work found out was that you can adjust the CO screw all day long on a defective AMM and it will never be right (because it's busted). The adjustment can be performed successfully on a good AMM of course but rarely needs to be. So have a spare, known good, AMM on hand.

Bentley is incorrect, BTW, when it tells you to disconnect the O2 sensor for AMM adjustment.

If you adjust the fuel mixture at that socket to where the volt meter fluctuates evenly between 0 and 12 V, and then disconnect the O2 sensor and measure the voltage coming out of the O2 sensor signal wire. You will discover that the signal voltage is at a steady low voltage (around 0.1V).

Only when you hook the signal wire back up to the ECU harness will it start to fluctuate between 0.1V to 0.9V. That's the setting/mapping of the ECU.

The Ohm reading on AMM will never tell you if the AMM is bad or not. It is other things inside the AMM that goes bad and is not traceable with an Ohm or Voltmeter.

original source

Thank you notnoel

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