pomsct

I have a 1999 Jeep Cherokee Sport 4L with Automatic trans. When I bought this car it had a bad rear O2 sensor and was showing a P0505 code. I replaced the O2 sensor and the Idle Air Control motor, all codes cleared except the P0505. I can clear the code with the key on and engine off then start the engine and it will not set the code but as soon as you turn the ignition switch off and back on it will throw the code even before you start the engine. I have replaced the PCM and have checked all the wiring with a meter and also have stripped all the tape and conduit off the wiring harness from the IAC to the PCM and I can find no problem. There does not seem to be the correct voltage going to the IAC. Is there another circuit internally in the PCM that can affect this circuit? I have tried all the tricks I can find on the internet, and nothing has worked. everything else on the car is working fine. I have read many post on the internet of people having the same problem but I haven't seen anyone post how they fixed it.

VTJeep

Well theres a 50/50 the IAC you bought is faulty.. Probably not the issue, but its common. Grounds are all clean right?

pomsct

I cleaned the grounds just to make sure but I didn't find anything bad. I still have the original and have tested the ohm readings on it and I don't see anything wrong with it, but it does the same thing with either the old motor or the new one. On my 96 Cherokee, if I pull the IAC connector off and turn the key to the on position I get 12 volts on two of the pins, but on the 99 I only get 5 volts to one of the pins. I don't know if they work differently in these two years.

VTJeep

I THINK (dangerous ground) that the 5v signal is correct.. Needs verification though.. Frustrating. The o2 is just reporting info, and should help indicate the issue, not cause it.. What other codes did you have? Whats the back story on new PCM? replaced the TPS at all?

pomsct

I only had the down stream O2 senor fault and the p0505 fault when I started working on it. The old owner had replaced the up stream O2 sensor before I bought it. I haven't replaced anything else yet as I'm not getting any codes and everything looked OK when I had a Snap on SOLUS scanner hooked up to this car and watched live data on all the sensors and they all seem to be working properly. The SOLUS scanner said the IAC circuit was shorted, I have unwrapped all the wires and can't find a problem in the harness. I replaced the PCM thinking that the circuit must be shorted internally. But I still have the same problem. If I clear the code all I have to do is cycle the ignition switch off and back on and the code returns immediately. Other than not having a fast Idle when cold the car runs fine.

VTJeep

We need Ken or Crusier or Ronin.. I'm useless beyond this point.

VTJeep

We need Ken or Crusier or Ronin.. I'm useless beyond this point. Hang on..

CCKen

The IAC motor has 4 wires with 4 circuits. Two of the wires are for 12 volts and ground to supply electrical current to the motor windings to operate the steppermotor in one direction. The other 2 wires are also for 12 volts and ground to supply electrical current to operate the stepper motor in the opposite direction.


With 5 volts on one pin you have a sensor reference voltage leaking in somewhere, or something else is wrong.


Did you open the fuel injector wiring plastic manifold to inspect the wiring inside?


Did you check the wiring harness loom where it goes past the end of the fuel rail to see if the fuel rail has chafed through to the wiring inside the loom?

pomsct

I have separated the wires from the IAC plug all the way to the PCM plug and found no damage. I have checked the wires for continuity and shorts to ground and found no problems.

CCKen

Here's a diagram of the IAC circuit and a test you can do of the IAC itself.





Here's a description of how the IAC works (From the 2000 FSM):


IDLE AIR CONTROL (IAC) MOTOR—PCM OUTPUT

DESCRIPTION

The IAC stepper motor is mounted to the throttle body, and regulates the amount ofair bypassing the control of the throttle plate. As engine loads and ambienttemperatures change, engine rpm changes. A pintle on the IAC stepper motorprotrudes into a passage in the throttle body, controlling air flow through the passage. The IAC is controlled by the Powertrain Control Module (PCM) to maintain the target engine idle speed.

OPERATION

At idle, engine speed can be increased by retracting the IAC motor pintle and allowing more air to pass through the port, or it can be decreased by restricting the passage with the pintle and diminishing the amount of air bypassing the throttle plate.

The IAC is called a stepper motor because it is moved (rotated) in steps, or increments. Opening the IAC opens an air passage around the throttle blade which increases RPM. The PCM uses the IAC motor to control idle speed (along with timing) and to reach a desired MAP during decel (keep engine from stalling).

The IAC motor has 4 wires with 4 circuits. Two of the wires are for 12 volts and ground to supply electrical current to the motor windings to operate the steppermotor in one direction. The other 2 wires are also for 12 volts and ground to supply electrical current to operate the stepper motor in the opposite direction.

To make the IAC go in the opposite direction, the PCM just reverses polarity on both windings. If only 1 wire is open, the IAC can only be moved 1 step (increment) in either direction. To keep the IAC motor in position when no movement is needed, the PCM will energize both windings at the same time. This locks the IAC motor in place.

In the IAC motor system, the PCM will count every step that the motor is moved. This allows the PCM to determine the motor pintle position. If the memory is cleared, the PCM no longer knows the position of the pintle. So at the first key ON, the PCM drives the IAC motor closed, regardless of where it was before.This zeros the counter. From this point the PCM will back out the IAC motor and keep track of its position again.

When engine rpm is above idle speed, the IAC is used for the following:

• Off-idle dashpot (throttle blade willclose quickly but idle speed will not stop quickly)
• Deceleration air flow control
• A/C compressor load control (also opensthe passage slightly before the compressor is engaged so that the engine rpmdoes not dip down when the compressor engages)
• Power steering load control (2.5 engine)
  

The PCM can control polarity of the circuit to control direction of the stepper motor.

IAC Stepper Motor Program

The PCM is also equipped with a memory program that records the number of steps the IAC stepper motor most recently advanced to during acertain set of parameters. For example: The PCM was attempting to maintain a 1000 rpm target during a cold start-up cycle. The last recorded number of steps forthat may have been 125. That value would be recorded in the memory cell so thatthe next time the PCM recognizes the identical conditions, the PCM recalls that125 steps were required to maintain the target. This program allows for greatercustomer satisfaction due to greater control of engine idle.

Another function of the memory program, which occurs when the power steering switch (if equipped), or the A/C request circuit, requires that the IAC stepper motor control engine rpm, is the recording of the last targeted steps into the memory cell. The PCM can anticipate A/C compressor loads. This is accomplished by delaying compressor operation for approximately 0.5 seconds until the PCM moves the IAC stepper motor to the recorded steps that were loaded into the memory cell. Using this program helps eliminate idle-quality changes as loads change. Finally, the PCM incorporates a “No-Load" engine speed limiter of approximately1800 - 2000 rpm, when it recognizes that the TPS is indicating an idle signal and IAC motor cannot maintain engine idle. A (factory adjusted) set screw isused to mechanically limit the position of the throttle body throttle plate.

Never attempt to adjust the engine idle speed using this screw.

All idle speed functions are controlled by the IAC motor throughthe PCM.

pomsct

I have checked the resistance of the original and the replacement IAC motor winding's and they are both reading 46 to 47 Ohms and no shorts or grounds. How common is it to have the IAC circuit fail in the ECM? I find it odd that the original ECM had no 12 volt output to the IAC and the replacement ECM would have the same failure.

CCKen

Seems to me there is high resistance, as in open, in the wiring from the PCM to the IAC.


If you don't have a pinout of the PCM connector let us know.

pomsct

The 4 wires from C1 to the IAC have 1 to 2 ohms of resistance, they are not shorted to each other or to ground. I checked them when I firs started trouble shooting this problem and have since removed all the tape and plastic conduit and separated the wires from the harness. I don't belive there is a wiring problem from the PCM to the IAC. I'm looking for another possible cause such as a sensor or another circuit that could fail and cause a this circuit to burn out or malfunction causing the P0505 code. There is a wire on pin #12 of C1 that is identified as Idle actuator and I don't know where it goes or what it its purpose is, it stays in the wiring harness that goes along the firewall behind the engine. I have read posts on line about the air bag circuit causing this problem but I haven't perused that yet, I was hopping to find someone that has solved this problem before. Looking around on the internet it seems like many people have had this problem.

CCKen

I believe that is for the police package XJ's, where it has a switch on the aux switch panel for activating the idle-up to around 1,000 RPM, and is only operational in Park or Neutral. It is the "Extended Idle" switch.


The switch provides a ground to the PCM. The ground is G107. on the far right side of the dash.


Do you have this switch on your aux switch panel (police XJ)?

pomsct

No I don't have a switch. It was not a police model. I just noticed the wire and thought it was worth investigating. This problem is very frustrating and I;m looking for anything. Maybe I', wrong but it seems that the PCM has a problem. I can plug a PCM I have laying around that is out of a 96 XJ and I get the proper voltages to the IAC, Seems like the wiring is OK.