2012 Jeep Liberty APP Codes

Sam Mobile Technician Michigan Posted   Latest   Edited  
Case Study
Driveability
2012 Jeep Liberty Jet 3.7L (K) 4-spd (42RLE)
Multiple App Codes
App High
App Low
No Throttle Response

This is a typical Monday. Nothing has gone right. My son does programming for me, and he is on his 2nd trip to a customer that has installed a USED PCM for no 5 volt reference to APP2. My son does not typically do diagnostics work. The nature of programming has made him a decent trouble shooter. I almost never doubt the information I have been given by him. He has never worked on cars professionally except for me. So when he calls and says this shop is approaching 20 hours diagnosis on this jeep, and it is littered with new parts. The owner just wants this thing fixed. It is the end of the day and I decide it is a good opportunity to teach my son to get to the next level. And the checkbook is open.

Upon arrival I determine my son is correct no 5 volt to APP2. (2 minutes in). I pull a schematic and discuss an approach with him. The 5 volt line is shared by several components. I decide to cut the 5 volt line at the PCM. Instantly I have 5 volts out of PCM. Doesnt need a PCM. Lets find our thief. I explain we need to unplug the EGR, MAP, CMP, fuel tank pressure sensor and the transmission Variable line pressure sensor one at a time and determine who is stealing our 5 volts. My son says I will get under Jeep while you do the "easy ones". 

I unplug all of the underhood components previously stated 1 at a time with no changes. My son finds and unplugs the variable line pressure sensor on the transmission and we have 5 volts. AHAH. So I explain to him leave it unplugged lets do the throttle learn and make sure it revs up. Sure enough after a ECU reset and a code clear the ETC learns and everybody is happy. Well except the shop owner. 

I bring you this case study because here is the case of a technician that had no direction, did not understand what was happening and asking the shop owner to bill the customer 20 hours of diagnostics time. In less than an hour with an inexperience tech and a mobile diagnostics tech a clear path to completion has been determined. No extra parts. 1 wire that needs repair. I am not bragging by any means I was merely surprised I used the tech's schematic, and volt meter. He had the tools just no understanding. I offered some training and recommended some classes and got the typical responses. Shop owner wont pay and I am not working all day and then going to class. OK. See you next time. I attached the schematic. Be interested if someone has a different approach. I was not a fan of cutting the wire. I saw it as a method to make sure the PCM could send the 5 volts. 

I also wanted to add the tech had followed the procedure that the manufacturer had laid out for the codes(also attached). and was so vague he had no idea where to go. 

Have a great day

Sam

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Helpful
Albert Owner/Technician
Pennsylvania
Albert Default
 

No I'm not a fan of cutting wires either. When I find a 5v ref missing my first though is almost always bad ground once I check the grounds then I move on to check what shares the 5v ref then do as you did unplug them one at time to find where as you said the thief is. I'm smh wondering why the tech would not take your offer for training.

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Sam Mobile Technician
Michigan
Sam Default
 

I always offer and rarely accepted. Maybe they are afraid

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Albert Owner/Technician
Pennsylvania
Albert Default
 

Afraid of what? I'm always up for leaning more

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Justin Mobile Technician
Utah
Justin Default
 

I would bet that tech has been to too many after school specials where he got a cold piece of pizza and a sales pitch disguised as training. I know I have.

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Sam Mobile Technician
Michigan
Sam Default
 

Justin, I don't think that is the case. It could be.

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James Owner/Technician
Indiana
James Default
 

My pizza has always been warm. We are lucky to have great training in northwest IN, John Thornton & Ken Zanders to name a couple.

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Marlin Technician
Oregon
Marlin Default
 

I don't think that is much of a reason. Tech who pursue excellence find … classes, and are not deterred by cold pizza. I hope the CTI guys don't read that. ;-(

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James Owner/Technician
Indiana
James Default
 

Joke Marlin, it was a joke!

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Marlin Technician
Oregon
Marlin Default
 

I don't cut the wire in those cases; I much prefer to disconnect the harness from the PCM, remove that terminal, then connect the harness. Then I can backprobe through the connector to verify output voltage with a light to ensure that it isn't just trace voltage. I can also power the circuit with a light, making it easy to see when I eliminate the short. The harness usually has a hard bend just…

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Rudy Technician
California
Rudy Default
 

Nice. I'll cut the wire. Its easily repaired and can allow one to move on quickly to the next phase of testing.

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Victor Owner/Technician
Florida
Victor Default
   

Was the variable line pressure sensor replaced to complete the repair? Electrical trouble shooting states that a shorted sensor can either pull reference voltage high or low.

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Sam Mobile Technician
Michigan
Sam Default
 

I do not do the actual repairs. I diagnose and advise

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Victor Owner/Technician
Florida
Victor Default
 

I guess that makes you a repair consultant then​.​Good job.

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John Diagnostician
California
John Default
 

Hi Sam. Good case study. I too have used the wire cutting method to find the problem area. I just wanted to offer another option that I have had success with: I disconnect the connector from the module that supplies the 5v reference. I then use a sensor simulator to apply 5v to the circuit that is of concern. I monitor this 5v supply with a voltmeter or graphing multimeter. If my supplied 5v is…

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