No Start Jeep Liberty

Michael Mobile Technician Clinton, Utah Posted   Latest   Edited  
Case Study
Electrical
2007 Jeep Liberty Limited 3.7L (K) 4-spd (42RLE)
P0700 - Transmission Control System (MIL Request),
Crank / No Start

I was called out to program a Jeep Liberty after a PCM replacement. After VIN correction, programming and adaptation, the original problem of no-start continued. Since the customer was buried in this job, I wanted to do a few quick tests to see if I could pin down the problem. The first thing I tried was adding some fuel to the intake stream. The vehicle ran for a short time then quit. So I was able to determine that the ignition timing was at least close enough to run and the spark system was functioning. I pullled up a schematic to see how the injectors worked. A 12V supply came to the injectors by way of the ASD (Auto Shut Down) relay. The PCM grounds the circuit to fire the injectors. Since the problem was identical between two PCMs, I deduced that the injector drivers would be an unlikely cause. My next step was to use bi-directional testing to verify the power.

diag​.​net/file/f7g4il4xx…

diag​.​net/file/fjhtqzul5…

Hmmm, something seems wrong here. Besides the vehicle eating my backprobe, this voltage does not seem right. A quick re-check of the diagram and it is verified it should be 12V. I pulled the relay from the engine fan and installed into the ASD slot.

diag​.​net/file/f51o2ib6p…

Much better. There must be some knarly thing in the relay to cause this.

diag​.​net/file/f2bbozvgj…

It looks normal. Something definitely is not. With the ASD relay changed the vehicle started and ran. The P0700 persisted. Changing back to the original PCM fixed the P0700. Bad rebuilt PCM. That never happens. At least the customer can return the PCM as a defective part.

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Interesting
Eric Owner/Technician
Edgerton, Wisconsin
Eric Default
 

Whatever happened to testing before replacing parts? I wonder if the shop will learn anything from this, of course I know they won't. I will bet that the next 10 Chrysler/Dodge/Jeep crank, no-starts will get a new ADS relay as part of their "testing" process.

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Anthony Technical Support Specialist
Kirkwood, Pennsylvania
Anthony Default
 

Hi E: They wouldn't be alone though I'm not sure that I can hold it against them. rockauto​.​com/en/catalog/jee… napaonline​.​com/en/p/ECHAR534 Guido

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Eric Owner/Technician
Edgerton, Wisconsin
Eric Default
 

Guido, Way back in the early 70's both my grandfather and father impressed upon me the importance of knowing how a system works. They told me, if you know how and why a system works you will always be able to figure out how and why it failed. So far that simple statement has served me very well. Eric

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Anthony Technical Support Specialist
Kirkwood, Pennsylvania
Anthony Default
 

Hi E: I don't disagree with you. But unless a valid test can be performed in much less than .1, swaptronics is becoming the game plan of the day. Guido

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Michael Mobile Technician
Clinton, Utah
Michael Default
 

Eric, I have tried really hard not to judge to harshly. At times I have to be called back to do some process again. Pinion factor on Dodge trucks has bit me a few times. I tell myself that it happens to all of us. A missing ground, blown fuse, wrong part. In this case there were no circuit codes for the injectors. A noid light may have even lit up with 5 volts.

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Eric Owner/Technician
Edgerton, Wisconsin
Eric Default
 

Michael, Having been the go to shop for hard to fix vehicles for many years I stopped judging a long time ago, just do the job and move on to the next one. It still bothers me that a shop will spend hours or days on a job that takes 10 minutes of studying the SI and 15 minutes of testing to find the problem, and they will do the same thing over and over. I shake my head at the "fixes" some…

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Tim Mechanic
Blytheville, Arkansas
Tim Default
 

"what makes it worse is the guesses given as replies by techs who should know better" I am guilty of this. To be fair, I can't access si while not at work, but I guess I can wait to comment until I am A. At my employer's location and B. Have the time to look up the information and study it. Maybe I should not comment? Please don't take this the wrong way. I am being sincere. I just don't want…

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Tim Mechanic
Blytheville, Arkansas
Tim Default
 

Good find Michael! I hope all is well with you.

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Michael Mobile Technician
Clinton, Utah
Michael Default
 

Thanks Tim, Hope to see you soon!

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Jürgen Owner
San Cristóbal de las Casas, Mexico
Jürgen Default
 

Strange enough the engine started with some fuel in the intake as the coils are fed (or not) by the same ASD relay....

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Michael Mobile Technician
Clinton, Utah
Michael Default
 

Hi Jurgen, Sorry about quoting your name incorrectly, I don't know how to do umlauts on my keyboard. I see now looking at the entire diagram both are served by the ASD relay. Is it possible with the 5V loaded voltage it was enough to drive the coils but not injectors? Not sure on that one.

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Stuart Mobile Technician
Blue Island, Illinois
Stuart Default
 

Thanks for sharing​.​Isn't it something what that ASD actually controls and when they fail the different issues that come about?And a solid proper diagnosis saves alot of headaches and moolah.

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Nolan Technician
Nassau , Bahamas
Nolan Default
 

I look forward to your case studies very much . Your case studies give me great insight for my diagnostic process i just wanted to get that out there . I love learning new ways of testing and you always has something new or way of going about it . Thank you and please looking forward to more of your time .

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Geoff Diagnostician
Lahaina, Hawaii
Geoff Default
 

Good to see you back, telling the tales of the daily action, Mike. I always wonder on stuff like this what exactly they thought was wrong with PCM. I know 15 years ago it was just a "magic box" to me...I think too many guys are still at that point, despite all the advances in training, and resources available since then.

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Zachary Mobile Technician
Austin, Texas
Zachary Default
 

Thanks for your post Michael. I appreciate you sharing your experiences always. You saved that shop's bacon for sure.

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