Hard shift 1st to 2nd with auto
Thread Starter
Instructor
Joined: Dec 2014
Posts: 241
Likes: 17
From: Poway CA
Hard shift 1st to 2nd with auto
My 2001 base coupe, after thoroughly warming up, shifts hard from 1st to 2nd. The rear diff was recently changed from the stock 2.73 to a 3.15, and the PCU was programmed to recognize the new gear ratio. The car is also giving the P1870 code - 'transmission component slipping' message on the DIC. It also seems to take longer than usual to shift from 1st to 2nd when cold, but it is not a hard clunk of a shift like when it is warm, and by warm I mean after a 20 minute drive on the freeway at 70MPH. Thoughts?
Racer
Joined: Oct 2018
Posts: 458
Likes: 29
From: Bellingham, wa.
No mater what when you change rear diff ratios it will amplify a sort of hard shift to a harder one but if you don’t thik it’s that maybe a fluid change would be in order, cheep try/fix and if that doesn’t help then you know the awnser��gl
Thread Starter
Instructor
Joined: Dec 2014
Posts: 241
Likes: 17
From: Poway CA
It did not have a hard shift before the change. It seems too coincidental, but this may be the issue.
https://www.gmtruckclub.com/forum/th...-shifts.54475/
https://www.gmtruckclub.com/forum/th...-shifts.54475/
Drifting
Joined: Feb 2008
Posts: 1,596
Likes: 67
From: Bahama North Carolina
You stated that you had the pcm programmed to recognize the gear change. Did you have the transmission set up at the same time. A lot more needs to be addressed than simply changing the ratio. Line pressure, shift points, torque converter setting and more all need to be changed to function properly.
Safety Car
Joined: Mar 2005
Posts: 3,756
Likes: 1,215
From: Wisconsin
You stated that you had the pcm programmed to recognize the gear change. Did you have the transmission set up at the same time. A lot more needs to be addressed than simply changing the ratio. Line pressure, shift points, torque converter setting and more all need to be changed to function properly.
TCC slip table and the WOT shift MPH value need adjustment, you will damage your transmission if the proper adjustments haven't been made
Thread Starter
Instructor
Joined: Dec 2014
Posts: 241
Likes: 17
From: Poway CA
How are those adjusted? Is it all computer related, or are there mechanical adjustments that need to be made. The work was done by my regular mechanic, but he is not a transmission specialist. I am thinking about taking it to my local trans shop, which is very good.
Thread Starter
Instructor
Joined: Dec 2014
Posts: 241
Likes: 17
From: Poway CA
Are those available to the consumer, or only to shops? Do you think it's likely my transmission shop would have access to those? I apologize for my ignorance; I am not a mechanic but I am very interested in learning more about my car.
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Safety Car
Joined: Mar 2005
Posts: 3,756
Likes: 1,215
From: Wisconsin
They are available to the public, they are sophisticated programs not easy to learn, you should look for a performance shop familiar with Corvettes, not something you will find at the local transmission shop
Drifting
Joined: Feb 2008
Posts: 1,596
Likes: 67
From: Bahama North Carolina
You will have to find a shop that has experience doing this to get it correct. Most good tuners with Corvette experience will know what to do, as feeder82 stated, your local trans shop will not have a clue.
Race Director
Joined: Apr 2007
Posts: 11,152
Likes: 890
From: South Western Ontario
If you did want to try software like HP Tuners yourself, you could likely find a factory tune for a 3.15 geared car and copy the transmission parameters from it into your PCM.
The transmission slipping when cold and then shifting hard once warmed up isn't a great sign that it's healthy.
The code being set likely results in full line pressure so the shifts will be harsh. Is the code set and the CEL on when you're getting these hard shifts?
The link you posted talks about up to 2000 model year vehicles so it's not your issue.
The transmission slipping when cold and then shifting hard once warmed up isn't a great sign that it's healthy.
The code being set likely results in full line pressure so the shifts will be harsh. Is the code set and the CEL on when you're getting these hard shifts?
The link you posted talks about up to 2000 model year vehicles so it's not your issue.