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Discussion Starter · #1 ·
My Ascent Limited has about 19000 miles on it. Early on I had the air conditioning repaired (easy warranty fix and no arguments from dealer or SOA, no problems with it since). I have had the Transmission valve body replaced (warranty). The harness resistance was tested at the time and indicated it was within spec so no replacement was warranted. I did not have any symptoms with the transmission at any time (lights came on which started the process for the valve body). I have had all the recalls and service campaigns taken care of (I had no symptoms). A couple of months ago my dash lights lit up, including the oil temp light but I could not get a photo or video of it. The lights cleared so I was just going to monitor both the lights and vehicle symptoms. No symptoms and no lights came on until this weekend. Lights came on as I was about to leave on a weekend trip (of course). I did capture the lights with video other than the first iteration (which had the trans light go on). I noticed the check engine light came on and captured that on video as well. After work I stopped by my dealership to have them record the issue in their notes, take a look at the video and see if there were any codes that could be read. The service advisor checked the codes himself (just a simple handheld) and found two codes. PO700 - transmission control system malfunction light request. The other code was PO842 - Transmission fluid pressure sensor/switch (A circuit low).

The service advisor took it upon himself to call down to his mechanic to get his take. The mechanic told him the harness would need replacement under warranty even though no specific code for the harness was recorded. They knew the history and knew I had all the updates prior to this code being thrown. It was going to take one to two weeks to get the harness in and installed (along with my already schedule oil change).

The service advisor asked me if I wanted to leave the car now. I said sure thing and he proceeded to pull out the keys to a 2020 Ascent. I transferred my personal tools and gear to the loaner and was out the door in less than an hour from my arrival.

I had always expected the harness would need to be replaced since it was rare that the vale body would go without the harness. I am glad it went out now and very pleased that the code was captured. I could not have asked for a more responsive dealership (Golden, CO). I have been very happy with the Ascent and as long as the harness is replaced prior to any damage of the CVT I have no concerns. I will be exploring the testing protocol for this to make sure the CVT would not have been damaged.

I have a good relationship with the service advisor despite the fact I did not purchase the vehicle at this dealership (I have had it serviced there). My suggestion is for any issue that arises is to keep your cool, gather as many facts as possible to assist in any diagnoses, be reasonable and choose a quality dealership. The partnership will more likely than not work out just fine.
 

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Interesting in good and bad ways. Good because it seems with the recall your car was able to detect that the hydraulic pressure sensor was going out of range and gave you early warning (earlier than a squeal and a destroyed transmission that is). So the TCU update did its job of throwing a bunch of codes.
The bad part is that we were hoping that the update was also going to apply some kind of correction if that sensor went a little out of range, to extend the life of the harness. It seems our speculation was wrong, harnesses may fail randomly at any minute.
 

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Discussion Starter · #3 ·
Interesting in good and bad ways. Good because it seems with the recall your car was able to detect that the hydraulic pressure sensor was going out of range and gave you early warning (earlier than a squeal and a destroyed transmission that is). So the TCU update did its job of throwing a bunch of codes.
The bad part is that we were hoping that the update was also going to apply some kind of correction if that sensor went a little out of range, to extend the life of the harness. It seems our speculation was wrong, harnesses may fail randomly at any minute.
I do want to investigate further on whether there is a chance the CVT might have been damaged due to the length of time the harness may not have been working. I do not know enough about the interactions and look forward to Robert's time being freed up to provide some clarity. At this point I am not worried, but I intend on learning more about all of it. The dealership called me today now that they have had the vehicle for one day. The mechanic apparently took the time to look into it further (probably a deeper scan) and confirmed his initial suspicion that the harness would need replacement. The transmission needs to be taken out (or dropped) in order to replace the harness so it takes a full day.
As you noted I never had the squeal or poor transmission performance. Once I learn more I will decide whether I should ask SOA for some extended warranty.
 

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If your harness was really bad, then, theoretically, the stall test would have caused a chain slip, or, your driving would have. You may (or may not) have noticed the chain screeching (depending on how loud your music is), but, usually, the car will detect it (though not always).

In the event a chain slip was logged, or they could replicate one on your harness, they'd have replaced both CVT and harness.

My guess is, based on what you shared with me, and the diagnostic and repair instructions, they either detected a harness anomaly during testing, or found a stored code for such, but found no stored codes for a CVT chain slip and could not cause a chain slip.

The Ascent has a decent chain clamping pressure safety margin. If the harness reports back something wrong, but that "something" is within a range that the TCM maintains a clamping pressure sufficient to prevent chain slips, then, the CVT will be fine and only the harness would need to be replaced.

If the harness resistances are impacted to the point where the data received by the TCM causes it to set the wrong clamping pressure (too low), then, the chain may slip (if both load and torque are maintained during that time).

With the clamping pressure safety margin, it's not surprising that they'd diagnose the CVT to be fine and the harness to be bad. There have been other instances of that.
 
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