Speedometer intermittent, transmission shifting erratic, voltage drop in heater and 12v Aux plug?

MHILL58

MEMBER
2001 JEEP GRAND CHEROKEE
4.7L
V8
4WD
AUTOMATIC
210,000 MILES

I have been trying to chase this issue for some time, I have replaced the speed sensors and checked the main connector with these systems and found no corrosion, I have noticed the voltage from the Aux plug, when the speedo is working normally the voltage is its normal 14v when the speedo drops to zero and stays there the voltage drops to 11.8 and the heater seems to get colder and at times the vehicle will stall but will restart immediately. I’ve been pulling my hair out trying to track this issue down, also only the speedometer is acting up. The rest of the gauges are accurate and working and when the speedometer does work the transmission shifts normally.

Saturday, January 25th, 2025 AT 8:14 AM

3 Replies

KEN L

MASTER CERTIFIED MECHANIC
50,191 POSTS

It sounds like the PCM is the problem, I would try to scan it to see if any codes are stored.

This guide can help:

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

Here is where the PCM is so you can swap it out for a preprogrammed unit you can find by searching Google or Ebay. Here is the charging wiring diagrams as well, there is a small chance it could be the cluster shorted out I need to tell you. Check out the images (below). Please let us know what happens.

Images (Click to make bigger)

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Saturday, January 25th, 2025 AT 4:55 PM

MHILL58

MEMBER
2 POSTS

I put my scan tool on the vehicle last night and found codes P0500 for speed sensors signal and P0601 for internal control module Memory. I looked up the code on all data and found that it is pointing to the PCM malfunctioning. I have found rebuilt PCMs. I will try when I can afford it. I will post my findings when I install it.

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Sunday, January 26th, 2025 AT 7:07 AM

KEN L