The BAC valve is (unlike the other two valves) a variable valve whose opening is controlled by a duty cycle signal from the computer. This signal is a square wave with a variable duty cycle. The duty cycle of a square wave is the percentage of the time that the signal is in the "on" state vs. the total time to complete a cycle. The higher the duty cycle, the more power is transmitted to the device. This is depicted in Figure 1. The computer can increase the opening of the BAC valve, and thereby the engine speed, by increasing the duty cycle of the signal it sends to the BAC valve.
All three of these valves are controlled by "open collector" transistor outputs. In this configuration, the transistor is a simple switch connected between the device and ground. The circuit and the equivalent circuit are depicted in Figure 2. When the transistor gets a positive signal voltage from the computer, it conducts and the circuit is completed through the solenoid to ground. This creates an opening force on the valve.
In the case of the BAC valve, the switch is opening and closing 125 times per second (125 Hz). This means that a complete cycle takes 8 mS. With the AC off, the transistor is "on" for about 2.4 mS which gives a duty cycle of 30%. When the AC is turned on, it requires a significant amount of aditional power from the engine. As many 2nd gen owners have experienced, if something isn't done to increase the air available at idle, the engine will die. The ECU senses that the AC is on by monitoring the signal from the AC compressor clutch. When it is engaged, the "on" time of the signal to the BAC valve is increased to 5.2 mS, which brings the duty cycle up to 65%. More power is transmitted to the BAC valve and the idle speed increases to compensate for the power loss to the compressor.
The idle adjust screw appears to control the quantity of air that flows through a bypass passage around the BAC valve.
If you notice a significant dip in your idle speed when you turn the steering wheel, something is wrong in the Air Supply Valve circuit. Given the location of the thing, it is going to be a bear to replace.
Notice the warning in the manual "Do not tamper with the adjust screw." Hah!
Barrington Daltrey and I have discussed the possibility of creating an add-on circuit which takes the approach of monitoring the engine speed directly and changing the opening of the BAC valve accordingly. This would have the affect of compensating for any change in load (whether it be AC, power steering, or that killer stereo and alternator you want to install). I believe in this idea enough that I have started designing such a circuit. More on that in the future.
Anyway, all these components provide plenty of things to go wrong with the your car's idle quality.
2. Check the Throttle Position Sensor setting. A variety of instructions for doing this are contained in the FAQ. My personal favorite is the two light bulbs. After setting it, rev the engine a couple of times to let everything settle back into place, and re-check it. You may even want to take the car for a quick drive and re-check the setting. An incorrectly set TPS seems to be the most frequent idle culprit. Make sure you get it right.
If you are still having problems, you probably have some kind of component failure. The tests now get a little more sophisticated.
3. Turn off the ignition. Remove the connector from the BAC valve and check the resistance of the BAC valve solenoid. This is done by connecting an ohm-meter across the BAC valve terminals. Care must be taken not to let the probes touch each other or you will get a false reading. You should get somewhere between 10 and 20 ohms. The manual says 10.7 to 12.3 ohms, but it's really not that picky. What you don't want is near zero ohms (solenoid is shorted) or very high resistance (solenoid is open-circuit.) If you have a shorted or open-circuit solenoid, you will need to replace the BAC valve. You might want to try disassembling it just on the off chance that it is repairable.
4. With the connector still removed, and the engine still off, place a 10k ohm, 1/4 watt resistor across the terminals of the connector. This is easy to do, simply insert the resistor leads into the connector. Make sure that the leads of the resistor do not short against each other or anything else. Turn the ignition on, without starting the car, and measure the AC voltage across the resistor. You should get a value between 6 and 7 volts after it settles down. A voltage lower than 5V indicates a likely ECU failure. Go here for more information. If you get the correct voltage, move on to the next step.
NOTE: This is a good chance to check that the conductors in the connector are firmly seated. Mine came loose, slid down, and shorted out, causing ECU failure. More on that in a moment. At the very least, if they are sliding around, they aren't making good contact. If you find that the conductors are loose in the connector, press the wires from the back of the connector towards the front. They should snap into place and not slide back. If they do slide back, you will need to either get a new connector, or epoxy them in, like I did. It works. (Be careful not to get epoxy on the connector surfaces!)
NOTE: Do not perform the following test if you obtained a reading of 0 ohms (short circuit) when you measured the resistance of the solenoid above. Something will get *very* hot.
5. The BAC valve may be stuck. Using jumper wires, connect 12V across the solenoid leads, being extremely careful not to create a short. If you didn't get an open-circuit reading when you measured the resistance of the BAC valve, the valve should click. If it is stuck, doing this a couple of times should free up the valve. I would also recommend removing the BAC valve from the intake manifold (this can be accomplished without removing the water hoses connected to the valve) and spraying WD-40 into it. Work it a few more times with 12V. If you can't get the thing to click this way, replace the BAC valve.
If you got to this point and you are still having idle problems, the problem is not in the electronic idle control system. Here are a few other possibilities and a few ways to check them:
1. Vacuum Leak
Vacuum leaks are frequently the cause of rough idle. A leak ruins your idle by allowing un-metered air to enter the intake manifold, creating a lean air-fuel mixture. Leaks typically develop as a result of vacuum or pressure hoses coming loose or failing, or gasket failures.
Vacuum leaks are frequently diagnosed and located by spraying some kind of flammable substance around the engine. When the flammable substance is sucked in through the leak, the lean condition is temporarily rectified and the engine will speed up. As can be imagined, spraying flammable substances around hot, running engines is a potentially very dangerous practice. It may best be left to qualified service personnel. "Poof, no eybrows!!" I don't even want to suggest flammable substances that can be used here.
A safer method that I have sometimes used successfully is to use a mechanic's stethoscope or a piece of vacuum tubing stuck in your ear. Use the other end to probe around the engine, listening for a hissing or whistling noise. I have successfully found both vacuum and exhaust leaks using this method.
2. Leaks in Intake Hoses
Leaks in the intake hoses after the Mass Airflow Sensor can result in bad idle in exactly the same manner as the vacuum leak. Again, the problem is unmetered air entering the engine. The best way to find these is simply by inspecting all of the components between the MAF and the throttle body.
On a TII, the problem is frequently the hose that connects the MAF to the turbo inlet. This hose has a tendancy to split, which lets air pass. A very small split can cause pretty big problems. Splits can sometimes be fixed with some RTV, but you need to be careful not to let the stuff get into the turbo. Also, make sure you use sealant that is O2 sensor safe. If you can afford it, or have access to a good used, hose, replace it.
3. Obstructed Exhaust System
At least one Rx7 list contributor has had problems with his idle as a result of plugged catalytic converters. I don't know how to check for this. If you do, let us know, and we'll put it here.
For those of you who are handy with electronics, the ECU fix is
to replace the BAC valve output transistor. On my ECU (which is an N332),
the transistor is T801, which is the last power transistor in line to the
left of center of the ECU (on the bottom board.) If you have a different ECU,
you can find the correct transistor by tracing the circuit with an ohm-meter,
starting with connector pin 2Q (see the diagram in the shop manual). This pin
is connected directly to the collector of the output transistor. (If you do
this, please send me the info so I can get it in here for everyone else.) My
transistor was an NEC
D1309, which I couldn't cross reference. Since it is only wired as an open
collector switch, though, close is good enough. I used a readily available
TIP120 (NPN Darlington) and it worked great. Just desolder the old one and
solder in the new one, and you're in business. The failure? Severe
degredation of Hfe from heat. In my case, this was caused by the shorting
of the BAC valve connector conductors. Another TeamFC3S member contracted
what we believe to be this same problem after letting a service shop do
a regularly scheduled tune-up! On two different cars! This suggests a
potential problem in the diagnostic procedures used.
Yet another TeamFC3S member had a burned out resistor in the output section.
I don't yet know which resistor it was. I have traced out the circuit and
can't figure it out. However, replacing the resistor cured his problem.
For the rest of you, I am going to write up a detailed repair procedure that
you can follow, complete with pictures. It could take me a little while, so
please be patient. I am also working on a write-up of the bolt-on fix
mentioned above.