DIAGNOSING THE BATTERY, CHARGING & IGNITION SYSTEM

After reviewing the vehicle's wiring diagram, the PO700 was most likely set due to the PO123 DTC since the signal wire from the TP sensor goes to both the PCM and the TCM. By looki

Well, you know the problem area, now what? You turn to your repair information and there is a diagnostic flow chart for the problem. Great!! Following the flow chart and going from "branch to branch" (sometimes referred to as a trouble tree chart), you come to the end of the diagnosis and staring you in the face are the words you dread to see... "Replace the PCM (Powertrain Control Module) and recheck." You look at the words in total disbelief and just can't bring yourself to this end result. You are hesitant, and for GOOD reason!! What if you buy a replacement PCM and the problem is still there? Maybe you have been faced with this dilemma before and the experience still haunts you to this day. The first question you may have is "How can the flow chart be wrong and misleading?" Well, maybe this article will shed some light on that question.

It seems that I get more than my fair share of "nightmare problems" at my shop, and the one in this article I felt I just had to share with you. The day started like any other, the first appointment on the book was a 1998 Jeep Cherokee 4.0L six cylinder with the Check Engine light on and a severe hesitation. At the same time, I was performing an equipment review on AutoTap's OBD II Scanner for PC's. Great timing!! I connected the AutoTap OBD II scanner, and found that there were two stored DTC's:

PO123...Throttle/Pedal Position Sensor/Switch A Circuit High Input.
PO700...Transmission Control System Malfunction

After reviewing the vehicle's wiring diagram, the PO700 was most likely set due to the PO123 DTC since the signal wire from the TP sensor goes to both the PCM and the TCM. By looking at the TP sensor output on the AutoTap system, it was obvious that I indeed had a problem with the TP circuit

Complete article here:

http://www.autotap.com/articles/Vehicle_Diagnostic_Flow_Charts_Explained.html

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