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Automatic Idle Speed (AIS) Motors and Sensors, and Idling

The automatic idle speed motor (AIS motor) and code 25

This section written by Bob O’Neill and Bob Lincoln.

AISThe automatic idle speed motor is common to most fuel injected vehicles. It is a small electric motor, controlled by the engine computer, which lets a measured about of air into the engine at idle (when the driver’s foot is not on the gas pedal), thereby controlling the idle speed.

Turbocharged 2.2 liter engines without intercoolers (Turbo I) engines from 1984 to 1987 used a pull-through throttle body and an AIS motor with a planetary gear system to open and close the speed control valve. This AIS motor is controlled by the logic module, which uses two drivers (in 1984, the logic module used two sets of drivers). 

Throttle Body Injection (TBI) 2.2 and 2.5 liter engines from 1986 and newer, as well as 1988 and newer Turbo I engines and 1987 Turbo II engines, used a four-terminal AIS motor.  This newer style used a brushless stepper motor to move the pintle, which controls the amount of air needed to regulate idle speed.  Four drivers were used to control two field magnets, which controlled the stepper motors; fields were energized one at a time at switching polarities to rotate the pintle shaft a quarter at a time.

With either system, the logic module checks the motor drivers to monitor their function. If the logic module does not detect any voltage when the driver is on, it “knows” there is an open driver circuit or a short in the wiring. Likewise, if the logic module sees a voltage when the driver is off, there is a short in the driver circuit. In either case, code 25 is triggered. (In 1988, in addition, not being able to set the targeted idle speed after several attempts will set code 25.)

Paul F. Schikora's experience - AIS motor

I had the Dodge 2.2 Turbo I engine with a 3000 RPM idle problem. The AIS controls the idle speed by letting air bypass the throttle plate. All it is is a valve that opens up to let additional air into the throttle body. It has a semi-circular opening, with a semi-circular shutter that opens or closes to allow air to bypass the throttle plate.

The shutter is designed to rotate 180 degrees, and then it hits a stop to keep it from rotating any further. The shutter sits inside a circular sleeve, and the sleeve is what's turned by the motor. The sleeve holds the shutter snugly enough to turn it when it's free to move, but the sleeve can continue to move when the shutter has stopped, so the motor won't burn out accidently.

Not only is the shutter designed to rotate, it is spring loaded, so it can be pressed down into the motor housing. Doing so makes a closed shutter effectively useless, because air can bypass it. I believe the spring is there to allow air bypass even if the shutter is closed, if there is enough of a pressure differential in the throttle body to force the shutter down into the housing and let air bypass it, perhaps to avoid other damage.

There is one flaw with this design -- when the shutter is depressed, it can be moved past its stops. If that happens, and the shutter then pops back up, it gets locked 180 degrees out of phase. When the computer tries to close it, it is actually opening it. It will keep trying to close it until it opens full up, and then your idle is racing at 3000 RPMs. The computer keeps trying to close the shutter, but since it's wide open, there's not enough pressure differential to force the shutter back down so it can again go past the stops into its normal position.

This is apparently what happened to mine. For all I could tell, the motor worked fine. Applying voltage to it opened and closed the shutter, only I couldn't tell that it was out of phase, opening when it should close and vice versa.

What compounded my problem was the fact that the shutter was gunked up with carbon and sticking in the sleeve. So it got stuck down and stayed there, allowing it to pass the stops. Eventually, it popped out enough to lock into an out of phase position. That's when the problem wouldn't go away. I finally figured it out when I noticed a rattle in the motor, and that the shutter was moving up and down slightly as I shook it. I was so fed up at that point that I ignored the service manual advice to the contrary, and took apart the AIS. I didn't ruin it -- it's a pretty simple design. I cleaned the parts and got the shutter to where it wasn't binding in the sleeve anymore. I then reassembled it (praying I had it in phase, which I did) and everything was fine (if it still raced, I would have taken it apart and reassembled 180 degrees around).

After "fixing" the problem, I went for a test drive. Everything was OK for a few minutes, but then the racing returned! Back to the garage where I found the following (warning -- more conjecture coming). In the top of the throttle body, there are two more ways for air to bypass the throttle plate. One is an orifice which is controlled by a set screw (idle set screw?). A while back I had apparently closed off the orifice -- some carbon gunking didn't help either. Running the engine at various speeds created the pressure differential needed (which couldn't be compensated for by the closed off orifice), and the AIS shutter passed its stops again. I backed the screw off, re-fixed the AIS, and got the engine running again. I then sprayed some carb cleaner into that orifice. No problems since then!

The second air bypass is a small hole in the side of the throttle opening just above the plate. It had some carbon buildup too, so I sprayed it out as well. (Yes, I know, I should use throttle body cleaner. I just didn't have any handy.)

Bohdan Bodnar: EGR solenoid

[Responding to a 1985 Shelby Charger that starts and runs like new with the EGR vaccum line disconnected; reconnect it-and it stumbles-and never a Fault Code.]

EGR is never operational at idle. If ported vacuum is present, the throttle is sticking open. The symptoms also present another problem: why is the EGR solenoid permitting control vacuum to be present under all conditions? EGR should be present only when the engine is hot (exception: 2.5 liters TBI engine -- EGR on a cold engine is compensated for by the firmware). I suspect either an electrical malfunction (is solenoid off or on when the engine is cold?) or incorrect coolant temperature sensor data. The former is more probable than the latter since the usual ECT sensor failure mechanism is to increase resistance (caused by corrosion) and thus fool the computer into seeing an abnormally cold engine ( ==> EGR is off; symptom: engine knock and increased engine temperature).

If the connections to it are corroded, it will always permit EGR to be on. This will result in the engine taking an abnormally long time to warm up and, probably, drivability problems when the engine is cold.

Bohdan Bodnar: AIS motor 

I've been having an idling problem with my '88 Caravan, 2.5L, non-turbo, 141K miles, automatic. On cold start-up. it idles quite high; around 2000 RPM or so. After it warms up for a couple of minutes, it starts to cycle between about 1000-2000 at about a 4-5 second rate. After if fully warms up, it stops the cycling business, but is still idling slightly high; around 1200 or so. I found that the PCV elbow had a big crack, so I replaced that and all of the other hoses down near the throttle body. Then, I noticed that the 4 wires going to the AIS motor were all cracked. I spliced in some wire and put heat shrink tape on. Put it all back together and it's still idling funny (high and cycling up and down). I disconnected the AIS motor connector completely and it ran normally.

A stepper motor has two windings. When one is pulsed and the other isn't, the motor will turn a a bit in one direction. When the second is pulsed but the first isn't, the motor will turn a bit in the opposite direction. If a known number of pulses are applied from the time the stepping motor is in a known position, the computer will know *exactly* how many degrees the motor has turned.

 The AIS has a pintle valve connected to the motor. Every time the motor turns N degrees, the valve is either extended or retracted a certain amount. More retraction => more airflow => higher rpm.

If you have the wiring diagrams for your vehicle, locate the section which has the powertrain control module and its I/O; there, you'll find the AIS connections. Unplug the AIS and measure the resistance of each winding -- they should be the same. With the connector plugged back in and the engine running, both windings should be pulsed periodically -- you can check this by backprobing with a multimeter. On the other hand, if you want to throw away about $80, you can always buy a noid light set which will inform you, via fancy flashing LEDs, whether or not stepping pulses are being applied.

Your high idle rpm suggests that either the computer's seeing an abnormally low coolant temperature at startup, you have a bad vacuum connection to the MAP sensor, canister purge is on when it should not be, or your fuel pressure's too high. The "hunting" implies an a/f mixture mismatch.

Here's what I'd do:

  1. Borrow a scan tool and measure the CTS output on a cold engine -- it should match the ambient to within 1 degree Fahrenheit (yes, it's that accurate). I would not be surprised if the computer reads a MUCH lower temperature. In this case, disconnect the CTS's connector and *liberally* spray the connector's pins and the CTS's pins with television tuner cleaner, let the cleaner soak for about 1/2 minute, and then reconnect the connector. Turn the ignition key off and then on -- you'll notice a higher temperature reading. Do this a few times and you'll finally get a CTS reading close to the ambient.
  2. If you have a bad MAP vacuum hose connection, the computer will set a diagnostic trouble code (it looks for a drop in intake manifold pressure after the engine starts).
  3. Canister purge should be on ONLY when the engine's operating in closed-loop. The easiest way to check the control line is to T-tap a vacuum gauge into the (vacuum) control line at the canister and see if you have a vacuum reading with the engine cold. If you have a reading, turn the engine off, disconnect the solenoid's connector, and liberally spray it with television tuner cleaner (do the same for the solenoid's pins). Reconnect and see if the vacuum "goes away." Also, be aware that the vent filter on the solenoid will deteriorate with age -- I replaced mine with a piece of crankcase breather filter material. If the canister is purging when the engine's in open loop, you'll run a richer mixture than you should and this will raise your rpm (within limits).
  4. The fuel pressure on your vehicle's engine, as measured at the intake feed, is to be 14.5 psi +/- 1 psi (I don't remember what this converts to in kPa). If it's too high, shut the engine off, disconnect the fuel return line and tap a LONG hose from the return line to a fuel can. Put the ignition key in "run" but don't start the engine -- if the pressure is now normal, the return line's clogged.

— Bohdan Bodnar

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