Code P0505 Will not Clear
CF Veteran
Joined: Aug 2010
Posts: 8,357
Likes: 102
From: Canton, MI
Year: 1999
Model: Cherokee
Engine: 4.0
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.
The '99 PCM part number should be 56041492AC (I think it is up to rev AI now). What PCM part number do you have installed?
Newbie
Joined: Mar 2018
Posts: 16
Likes: 0
Model: Cherokee(SJ)
Here's a diagram of the IAC circuit and a test you can do of the IAC itself.
Attachment 311850
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:
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.
Attachment 311850
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)
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.
Newbie
Joined: Sep 2010
Posts: 11
Likes: 1
From: SoCal
Year: 1999
Model: Cherokee
Engine: 4.0
Seasoned Member
Joined: Jul 2018
Posts: 419
Likes: 59
Model: Cherokee (XJ)
I've read through this post and responses. I took it off-site and read that the code will latch if there's a vacuum leak on the manifold that the PCM cannot correct by positioning the IAC. It assumes the IAC is bad and latches the code but in reality there's a vacuum leak that's the real culprit.
EDit: Troubleshooting - clear the code, then isolate all vacuum taps on the manifold (with a good IAC). See if that keeps the code from re-latching. Then begin reattaching the vacuum lines one at a time to see which one latches the code. My 98 had a leaking brake booster.
EDit: Troubleshooting - clear the code, then isolate all vacuum taps on the manifold (with a good IAC). See if that keeps the code from re-latching. Then begin reattaching the vacuum lines one at a time to see which one latches the code. My 98 had a leaking brake booster.
Last edited by Idunno; Sep 27, 2020 at 03:12 AM.
Junior Member
Joined: Jul 2023
Posts: 34
Likes: 8
From: Fallon, Nevada
Year: 1993
Model: Cherokee (XJ)
Engine: 4.0 300K miles
After a number of heat cold cycles the IAC body could warp, change shape. I have seen the special screws actually bottom out, and the body then cannot squeeze the gasket,,,, and you have vacuum leak ...
I removed mine, carefully filed the body FLAT again, then added a small flat washers to give the screws a chance to flatten the gasket.... my leak went away.
I removed mine, carefully filed the body FLAT again, then added a small flat washers to give the screws a chance to flatten the gasket.... my leak went away.
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