I found this, but still wondering what "signal" would come out of the ECU to indicate the car is running lean…would it be earth, or open circuit? Or ? I wouldn't think the Turbo+ control module would be looking at a frequency…I assume it would just kick off if it detects a lean signal.
- Next to the idle speed test point pigtail is a green and white pigtail. This is your carbon monoxide or mixture test point, which measures the oxygen sensor output. Using Volvo special tool part #9995280 (approx. $35) plug one end of the test LED to the fender well connector and the red connector to the positive terminal of the battery. A logic probe can be used in place of the Volvo special tool or if you are handy build the circuit below. Remove the tamper proof plug from the air mass meter by drilling two 1/8" holes in the plug about 3/16" deep and removing the plug with needle nose pliers or snap ring pliers. This will reveal the mixture adjusting screw. Adjust the AMM until the LED on the tester blinks on and off at equal intervals. Congratulations! You have just set your engine's base idle specs. Note: when the tester LED is off it represents a lean mixture, when it is on the mixture is rich.
LH 2.1 & 2.2 Notes. [Art Benstein/John Sargent] The LED adjustment procedure presumes everything affecting mixture is in the ball park to begin with. Stuff like vacuum leaks, fuel pressure, injectors will throw the mixture off more than that pot (screw adjustment) will correct. If the O2 sensor isn't working, e.g. clogged with soot from rich running, the LED test point will not switch. It is actually monitoring the output of the first circuit inside the ECU that handles the O2 sensor signal, so the flashing indicates the switching between lean and rich that occurs while the ECU is maintaining the correct mixture. Because AMMs are slightly different in response, an adjustment is given on the AMM to adjust the ECU to match it up until LH2.4 when the ECU became automatically adaptable to the slight variations in sensor output. A better procedure to use in replacing AMMs would have you measuring the voltage on the yellow lead (LH2.2) before you adjust or disconnect the old one. Then you could at least put the pot back where it was if needed or set the new one to the same value before installing it. You could measure the resistance, but it is much easier to backprobe the voltage as you set the pot-- it varies from zero to about 2.7V-- then you don't need to guess where you ran out of adjustment. The ends are very hard to feel. Turning the AMM adjustment screw CounterClockWise gives a lean mixture, so turning the screw ClockWise gives a rich mixture. The mixture setting method using the LED tool gives a slightly lean mixture compared to the more accurate exhaust gas analyzer probe. Turn the AMM adjustment screw 2-3 turns CW (rich) to put the mixture adjustment in the right range.
Instead of using the test port, you can put a multimeter (with minimum impedance of 10 megohm for digital units) on the O2 sensor output. You should get an oscilliting voltage from about 0.1 to 0.9 volts. If your O2 sensor is generating a fairly even duty cycle between .1 and .9V, the system is in closed loop and the mixture is correct. The test point, if you are on it (pin 22 on the -544 ECU) is an open collector output protected by a 511 ohm series resistor-- pretty hard to damage externally. You don't really need the 750 ohm resistor, and its value certainly isn't critical to the function; just protects the LED if you drop the ECU end of the probe on a ground. The LED just shows you what the ECU sees as crossings of stoich point (which you will see graphically if youI use a 'scope on the oxygen sensor lead and set the crossing for 450 mv) with a set of comparators a bit on either side of that midpoint. That having been said, it can take more than a few turns on the AMM pot screw to adjust it, and some just won't adjust no matter what.
