HELP!! Programming Issue WE Think
Thread Starter
Pro
Joined: May 2002
Posts: 537
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From: Conway SC
HELP!! Programming Issue WE Think
A little base history first - '94 Corvette Coupe, automatic with Dana 44, 3:45 gears, LT4 hot cam package with 1.65 roller rockers and the GM beehive springs, BFG 275 DRs, dyno tested at 307 RWHP, best to date at 12.426 @ 110.8 MPH w/ 60' time of 1.696.
In late Fall of 2005 the transmission started to refuses to up shift from lst to 2nd under full accelertion and would run up against the rev-limiter. This issue ocurrs if the shifter is in either "Drive" or in 3rd. Since this was the original trans we figured a tranny issue and had it totally re-build using updated parts wherever possible by a good racing transmission shop. The shifting issue continued into this Spring at the MRI test n' Tune and the Nationals in Richmond. Tranny comes out and builder goes through it and fines nothing amiss. Travel to E-town to run first CC event and the shifting issue is still present starting with the second pass - on the first pass the Corvette runs perfectly but on passes two and three the Corvette will not shift. Transmission shop takes the tranny out again and goes through it after pumping 60 quarts of fluid through the tranny and changing filters twice. Street test the Corvette and it shifts just fine. Back to E-town to run second CC event. Shifting issue returns for the first of three passes made that day but on the fourth pass of the day the Corvette runs perfectly. Take a fifth pass and the shirfting problem returns,
Now we begin to think the issue might be a PCM issue and not the tranny so the program is checked: Rev-Limiter is set at 6100 RPM and first to second shift point is set at 37 MPH - just to make sure the PCM is re-flashed with these settings. Back out on the street the Corvette is tested under acceleration and shifts just fine, back to E-town for the weekend and of course the Corvette will not shift from 1st to 2nd when under a race condition. I do not know what to do next.
This is a '94 so there is no shifter cable to adjust all shifter and gear selection commands are given electronically. We are only guessing at this point but plan to lower the shift points MPH setting in the PCM at 3 MPH at a time to see if that will allow the Corvette to shift. Any suggestions on what we should be looking for in the programming would be very helpful as at this point in time nobody believes the issue is within the tranny.
Thanks for any suggestions
In late Fall of 2005 the transmission started to refuses to up shift from lst to 2nd under full accelertion and would run up against the rev-limiter. This issue ocurrs if the shifter is in either "Drive" or in 3rd. Since this was the original trans we figured a tranny issue and had it totally re-build using updated parts wherever possible by a good racing transmission shop. The shifting issue continued into this Spring at the MRI test n' Tune and the Nationals in Richmond. Tranny comes out and builder goes through it and fines nothing amiss. Travel to E-town to run first CC event and the shifting issue is still present starting with the second pass - on the first pass the Corvette runs perfectly but on passes two and three the Corvette will not shift. Transmission shop takes the tranny out again and goes through it after pumping 60 quarts of fluid through the tranny and changing filters twice. Street test the Corvette and it shifts just fine. Back to E-town to run second CC event. Shifting issue returns for the first of three passes made that day but on the fourth pass of the day the Corvette runs perfectly. Take a fifth pass and the shirfting problem returns,
Now we begin to think the issue might be a PCM issue and not the tranny so the program is checked: Rev-Limiter is set at 6100 RPM and first to second shift point is set at 37 MPH - just to make sure the PCM is re-flashed with these settings. Back out on the street the Corvette is tested under acceleration and shifts just fine, back to E-town for the weekend and of course the Corvette will not shift from 1st to 2nd when under a race condition. I do not know what to do next.
This is a '94 so there is no shifter cable to adjust all shifter and gear selection commands are given electronically. We are only guessing at this point but plan to lower the shift points MPH setting in the PCM at 3 MPH at a time to see if that will allow the Corvette to shift. Any suggestions on what we should be looking for in the programming would be very helpful as at this point in time nobody believes the issue is within the tranny.
Thanks for any suggestions
Melting Slicks
Joined: Apr 2001
Posts: 2,090
Likes: 214
From: Ca.
Especially considering how intermittant the problem is without any programming changes in between-
I would first datalog the runs and see if the pcm is losing/getting incorrect input for mph. A partially broken/pinched wire.....or sensor....simple things first.
I would first datalog the runs and see if the pcm is losing/getting incorrect input for mph. A partially broken/pinched wire.....or sensor....simple things first.
Le Mans Master
Joined: Jan 2003
Posts: 8,596
Likes: 19
From: Portland Oregon
It could be a programming issue, but normally if it is the problem would be more consistent. Check your WOT shift vs RPM tables. You may want to lower them about 100 RPM or raise your rev limit 100 or 200 RPMs. It could be that the two are so close that if the shift doesn't occur just right and it bumps the rev limit it won't shift.
