Can a cracked flexplate mimic a failed CPS? - JeepForum.com
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post #1 of 6 Old 08-07-2019, 04:04 PM Thread Starter
camillescott
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Can a cracked flexplate mimic a failed CPS?

This is a question I started wondering about today. If the CPS measures rotation from the flexplate, could a cracked flexplate mimic a failed CPS? Basically, I'm imagining that cracks near the bolts would cause a wobble along the plane of the plate that would increase in severity between the center and edge. This would cause the plate to move slightly further away from the sensor during a portion of its rotation, which could cause the signal to weaken.

I got to thinking about this with my XJ, as I'm about to replace the CPS (again), and I'm quite sure I have a slightly cracked flexplate that I've been putting off repairing. It took me a while to accept that the CPS had failed again, and a lot of forum searches ensued; I found a lot of threads going back through the years with intermittent issues similar to my own (sudden temporary cuts in power at speed; unsteady idle; cutting out entirely at idle; but no issues starting), most of which were fixed by a new CPS, but some of which never seemed to be solved and turned into abandoned threads. Given how relatively common a cracked flexplate is, along with complete or partial removal of the transmission to repair it being a daunting task for many people, I have to imagine it's an issue that is often ignored or overlooked, and could explain some of those mystery issues.

Then again, if the flexplate is warped enough to affect the sensor readings, it might be damaged enough that it's impossible to ignore. Thoughts?


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post #2 of 6 Old 08-07-2019, 05:07 PM
CJ7-Tim
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I can't answer your specific question, but I do know that cheap crappy Chinese made parts that most auto parts stores sell, some that even come with a "Lifetime Warranty", are poorly manufactured and/or made from inferior materials. They are often out of specification, or even failed, right out of the box. The ones that are not faulty many times will have a short service life before they fail. Always buy top quality replacement parts and genuine Jeep engine sensors. Numerous threads detail long and frustrating searches for a "problem" that ended up being cured simply with genuine Jeep repair parts.

Theoretically, a warped or cracked flex plate could damage the CPS.

Buy a genuine Jeep CPS, and inspect your flexplate for cracks. Flex plate replacement is not very technical at all, just time consuming.

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post #3 of 6 Old 08-07-2019, 05:28 PM
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you could measure the flex plate for run out. would be rather simple with a dial indicator on a mag base. HF sells them cheep enough.

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post #4 of 6 Old 08-07-2019, 05:47 PM Thread Starter
camillescott
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Quote:
Originally Posted by CJ7-Tim View Post
I can't answer your specific question, but I do know that cheap crappy Chinese made parts that most auto parts stores sell, some that even come with a "Lifetime Warranty", are poorly manufactured and/or made from inferior materials. They are often out of specification, or even failed, right out of the box. The ones that are not faulty many times will have a short service life before they fail. Always buy top quality replacement parts and genuine Jeep engine sensors. Numerous threads detail long and frustrating searches for a "problem" that ended up being cured simply with genuine Jeep repair parts.

Theoretically, a warped or cracked flex plate could damage the CPS.

Buy a genuine Jeep CPS, and inspect your flexplate for cracks. Flex plate replacement is not very technical at all, just time consuming.
All good points, I don't disagree with you on any of them. Time is what's kept me from doing mine so far.

Quote:
Originally Posted by 222Doc
you could measure the flex plate for run out. would be rather simple with a dial indicator on a mag base. HF sells them cheep enough.
Good point. I'm not that worried about whether this is what's happening in my case in particular, just whether it's possible. Now that I think about it, I have a friend who has both a dial indicator and an old oscilloscope, who also happens to love over-engineered explorations... seems like a fun weekend project!
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post #5 of 6 Old 08-07-2019, 06:13 PM
jay-h
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I think your theory is definitely plausible run-out could be intermittent

But I'd definitely replace the flexplate regardless. If there are cracks, it's a disaster waiting to happen.

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post #6 of 6 Old 08-08-2019, 07:27 AM
V8GCZJ
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CPS reads the rectangular holes in the flex plate, It would have to be way off to the point of not running. CPS(or wiring) can be damaged during an engine swap so it's wise to remove it. I think your issue is the CPS or possibly the cam sensor. Are you sure the CPS is the issue? As said stay away from the junk the auto parts stores sell(especially BWD) only use genuine mopar or Delphi ones. If you can get a scanner that reads live data this will show if the CPS is the issue.
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