VW Passat Harsh downshift to 1st under light throttle
Good afternoon all. This is my first post to this forum and it's a good one. This vehicle was towed in by my client with a complaint that the vehicle lunges in parking lots and slow traffic. After several test drives I have been able to duplicate the symptom with regularity. Under normal to heavy acceleration this vehicle shifts fine and has good power. When under light acceleration (@10-25% throttle) the vehicle jolts really hard during the 2-3 upshift. The first time it happened to me I thought I had ran over a 3 foot deep pothole, the jolt is very harsh. My thoughts are that there is either a bind or an unintended downshift to first gear. I captured some scan data showing the event and the data just isn't making sense to me. I am following the theory that when the transmission needs to disengage the B1 clutch and apply the K-3, something is preventing the K-3 from apply and results in a shift to 1st gear. The scan data I collected shows a rapid upward spike in RPM during the event, the current for N90 dropping slightly (indicating switching off) and the current for N283 rising switching on, which should begin the transition to third gear.
During normal shifting under normal to heavy acceleration, the scope data shows that in 1st gear both N90 and N283 are cycled at about a 50%. During 1st to 2nd upshift the TCM decreases N283 on time gradually until it is nearly off and leaves the N90 on at @50%. That holds until it is time for the 2-3 upshift and the TCM gradually increases the N283 on time while gradually decreasing the N90 on time until they are both cycled at nearly the same rate (@35% on time.) Then to complete the shift to 3rd the TCM turns off N90 and ramps N283 to about 50% duty cycle.
Under light throttle the problem occurs during the 2-3 upshift. When the TCM gets both N90 and N283 to the 30%-40% duty cycle it fails to make a transition and reverts to turning N283 off again.
I have looked at other inputs such as throttle %, accelerator pedal position, map, load and others to see if there was some input that may be causing the problem, but I came up with nothing. I have done a few valve body repair and replacements on this style unit before but never for this type of symptom. I'm going to follow up with cleaning the throttle body for the P3078 per TSB, but I doubt it has anything to do with the issue. Has anyone else ran across this? Any ideas on what other conditions may cause this issue before inspecting the valve body and solenoids?
In the event that someone runs across this symptom in the future, I am updating the findings on this vehicle. The transmission oil pan was removed in order to complete a visual inspection of the valve body and wiring. There was a significant amount of brass fragments in the pan and the transmission fluid was burnt. We suggested to our client that the transmission would need to be removed and…