Chrysler Repair: follow up to low idle, powertrain control module pcm, powertrain control module

Question
Hi Kevin I told you about 4.0 grnd cherokee 2004. About a low idle problem that happens periodicly after it is warm. The rpm will drop from 700 to 500 rpm and shakes the steering and car. I have once cleaned the IAC and checked out the throttle body like you sugested ( it was clean as can be ) . What is my next step . Should i replace the IAC fist or the Tps ? What does the TPS exactly do and what do i have to watch out for when I change it .I also have read the you may have to restart the vehicle a couple of times to get the proper idle ? Remember the low idle situation is a periodic thing  with no codes.I will change one so what do you suggest Thanks Greg

Answer
I found this TSB for a PCM update a dealer will need to do this.

NUMBER: 18-008-04
GROUP: Vehicle
Performance
DATE: February 24, 2004
This bulletin is supplied as technical information only and is not an authorization for repair. No part of this publication may be reproduced, stored in a retreival system,
or transmitted, in any form or by any means, electronic, mechanical, photocopying, or otherwise, without written permission of DaimlerChrysler Corporation.
THE SOFTWARE WILL BE AVAILABLE ON TechCONNECT ON MARCH 02, 2004.
SUBJECT:
Flash: Low Idle Speed Induced Drone and/or Intermittent Part Time Transfer Case Lamp
OVERVIEW:
This bulletin involves selectively erasing and reprogramming the Powertrain
Control Module (PCM) with new software.
MODELS:
2004 (WJ) Grand Cherokee (Domestic Market Only)
NOTE: This bulletin applies the vehicles equipped with a 4.0L engine (sales
code ERH) and built prior to March 06, 2004 (MDH 0206XX)
SYMPTOM/CONDITION:
The vehicle operator may experience a drone-like sound that occurs when the vehicle
engine is running at idle speed. The PCM automatically adjusts engine speed, while at
idle, dependent upon various engine loads (i.e transmissions, air conditioning compressor,
etc.). The variation in engine idle speed, due to various engine loading combinations
while at idle, may vary the intensity of the low speed idle drone.
The vehicle operator may notice that the instrument cluster transfer case part time
indicator lamp may illuminate intermittently. This condition may occur only on vehicles
equipped with a Selec-Trac® (NV-242) transfer case, and only when either 4 (HI)
Part Time or 4LO transfer case positions have been selected.
The new PCM software sets the engine idle speed at approximately 744 rpm
(engine fully warm), and addresses the software induced intermittent transfer
case part time indicator lamp illumination.
DIAGNOSIS:
Using a Scan Tool (DRBIII®) with TechCONNECT or the appropriate Diagnostic Procedures
Manual, verify all engine systems are functioning as designed. If DTCs are present record
them on the repair order and repair as necessary before proceeding further with this bulletin.
If the vehicle operator describes or experiences one or both conditions,
perform the Repair Procedure.