Transmission bucking
Thread Starter
Instructor
Joined: Apr 2012
Posts: 224
Likes: 29
St. Jude Donor '12
Transmission bucking
This is not short. Would love some help or opinions / a direction. I had the following done to my stock 2004 vert base
Pfadt aci
Pfadt y long tube headers x pipe and high flow cats
Moved from 2.73 to 3.42”s
Full Dyno tune
This is a daily driver but wanted some kick.
Now when I drive 50 to 60 miles at 79 to 75mph and get off at exit / come to a stop. When I start to drive again the tranny slams into 2 nd gear. I mean neck whipping back breaking snap into second gear. If I stop the car and give it a few minutes (almost like letting it cool off) the car drives like normal again. Shifts right thru to 2nd as expected. I live in Fl and it can get pretty hot but this never happened before I had the work done. Does the tuning / shift points need to be adjusted or am I looking at something more serious.
This seems to happen every time I go for an extended ride. I drive her 20 miles each way to and from work morning and evening at 70 mph and never an issue. May be early in the day and early evening. Cooler outside temps? Just know it’s happening, happened today and I’m at a loss.
Thanks for any help or suggestions.
Pfadt aci
Pfadt y long tube headers x pipe and high flow cats
Moved from 2.73 to 3.42”s
Full Dyno tune
This is a daily driver but wanted some kick.
Now when I drive 50 to 60 miles at 79 to 75mph and get off at exit / come to a stop. When I start to drive again the tranny slams into 2 nd gear. I mean neck whipping back breaking snap into second gear. If I stop the car and give it a few minutes (almost like letting it cool off) the car drives like normal again. Shifts right thru to 2nd as expected. I live in Fl and it can get pretty hot but this never happened before I had the work done. Does the tuning / shift points need to be adjusted or am I looking at something more serious.
This seems to happen every time I go for an extended ride. I drive her 20 miles each way to and from work morning and evening at 70 mph and never an issue. May be early in the day and early evening. Cooler outside temps? Just know it’s happening, happened today and I’m at a loss.
Thanks for any help or suggestions.
Burning Brakes
Joined: Feb 2013
Posts: 904
Likes: 296
From: Sacramento, California
Possible problem with the 1-2 accumulator piston getting stuck, bad throttle position sensor, or problem with 2nd gear solenoid. These are just a few possibilities, but there could be something else failing in the trans.
Steve
Steve
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Thread Starter
Instructor
Joined: Apr 2012
Posts: 224
Likes: 29
St. Jude Donor '12
It did throw a code I am going to check that tonight. Im always driving her and you need to have key on car not running I'm always leaving for work at 7AM or getting home at 6 so I need to check that code tonight. Only seems to happen when jot.
Fluid was changed at 66K when all work was done, there is 72K now
Fluid was changed at 66K when all work was done, there is 72K now
Race Director
Joined: Apr 2007
Posts: 11,150
Likes: 890
From: South Western Ontario
It's highly unlikely to be the tune if both good and bad shifts happen with the same amount of throttle and shift at about the same speed. At the same throttle and rpm the shift should be repeated the same. Same throttle/rpm for the shift would mean you'd be in about the same operating area of the shift pressure table which means the same shift pressure which means it should shift the same way.
I'd tend to agree it's more likely to be something like the 1-2 shift accumulator or the main pressure control solenoid sticking. A failure and the resulting code could have the PCM defaulting to maximum line pressure as well.
I'd tend to agree it's more likely to be something like the 1-2 shift accumulator or the main pressure control solenoid sticking. A failure and the resulting code could have the PCM defaulting to maximum line pressure as well.
Thread Starter
Instructor
Joined: Apr 2012
Posts: 224
Likes: 29
St. Jude Donor '12
Pulled the code PCM-0894 Only happens after driving the car extended mileage at 75 to 80 MPH. Never happens on local driving or short highway driving 25 mile commute 2 times a day / never happens.
And I've used this car for trips to Nashville and Corvette Museum from Florida and never had an issue before the upgrades.
And seems to reset itself after the car is shut down and rests for 2 or 3 minutes and restarted. just odd and aggravating. Thanks everyone for the responses
And I've used this car for trips to Nashville and Corvette Museum from Florida and never had an issue before the upgrades.
And seems to reset itself after the car is shut down and rests for 2 or 3 minutes and restarted. just odd and aggravating. Thanks everyone for the responses
Drifting
Joined: Sep 2009
Posts: 1,862
Likes: 594
From: Prescott, Arizona
When changing from 2.73 to 3.42, it is necessary to reprogram the PCM for new shift points, both at part and full throttle up and down shifts as well as downshifts at idle while coming to a stop. If this is not done properly (from what I can tell there is some art to it), the car may exhibit unexpected shifts, too hard or too soft.
The PCM looks at engine RPM, speed, throttle position, manifold pressure and other inputs. Speed comes from the rear drive reluctor wheel which stays the same regardless of rear drive ratio. If the PCM is not reprogrammed, it will see RPM/ speed signal inputs that are out of pre programmed limits, causing erratic shift behavior.
I would get with a tuner specializing in Corvettes and have them check PCM programming to see if it is appropriate for the 3.42 rear drive.
On the West Coast, I had A&A Corvette in Oxnard reprogram the PCM on my '01 A4 after a rear drive ratio change from 3.15 to 3.73. It took some time and a test drive to confirm correct shifting and eliminated inappropriate shift points left after a less experienced tuner attempted the reprogram.
The PCM looks at engine RPM, speed, throttle position, manifold pressure and other inputs. Speed comes from the rear drive reluctor wheel which stays the same regardless of rear drive ratio. If the PCM is not reprogrammed, it will see RPM/ speed signal inputs that are out of pre programmed limits, causing erratic shift behavior.
I would get with a tuner specializing in Corvettes and have them check PCM programming to see if it is appropriate for the 3.42 rear drive.
On the West Coast, I had A&A Corvette in Oxnard reprogram the PCM on my '01 A4 after a rear drive ratio change from 3.15 to 3.73. It took some time and a test drive to confirm correct shifting and eliminated inappropriate shift points left after a less experienced tuner attempted the reprogram.