Strange Behavior post-LS Swap

Joined
Sep 14, 2010
Messages
111
Location
Colorado Springs
Website
www.summitwise.com
Details:
In February of this year, after saving for over five years, I took my truck in to a reputable shop for an LS3 swap. Went with the 6L80 transmission as well to have a couple more gears available. Nine months later and I'm working through an extremely pesky issue that continues to befuddle the best technicians in the land. My truck is a triple-locked '94 with 245,000 miles on the body.

Situation:
About 40% of the time the shift from first to second gear causes a lurch or stutter. It's experienced as a firm shift, what then feels like a release, and then it grabs and goes. Calling it a stutter is not out of the question. I've performed a series of tests across every possible permutation and believe it to be torque related. Issue occurs more often when going uphill and/or turning at a gradual rate of acceleration. It happens less at flat grade and rarely when descending. Outside temperature, engine temperature, and type of terrain does not matter. Higher shift points are smooth as silk. My father-in-law has the exact same cruiser, took it to the same shop, performed the same service, and has no issues whatsoever.

Solution efforts thus far that have proven ineffective:
  • Front and rear axles serviced (tangential but good to be aware)
  • Myriad of changes to engine and transmission programming following inspection of data.
  • Examination of transmission wiring to confirm proper wiring.
  • Replacement of the transmission controller.
  • Transfer Case Rebuilt
  • Full replacement of entire transmission which was ~4 months new at time of replacement.
  • Rear U Joints replaced.
We are now considering the viscous coupler as a potential means of causing the torque reverb back through the transmission. Other than that, we are all out of ideas and I'm beyond desperate. Does anyone have some ideas on what might be causing the issue?
 
Joined
Sep 19, 2011
Messages
422
Location
Auckland, New Zealand
Assuming you're running a stock computer, have you checked all the settings in the transmission settings? There's loads of configurability in there that could have gone funny.
some examples:
 
Joined
Nov 9, 2012
Messages
13,508
Location
Olathe, KS, USA
Details:
In February of this year, after saving for over five years, I took my truck in to a reputable shop for an LS3 swap. Went with the 6L80 transmission as well to have a couple more gears available. Nine months later and I'm working through an extremely pesky issue that continues to befuddle the best technicians in the land. My truck is a triple-locked '94 with 245,000 miles on the body.

Situation:
About 40% of the time the shift from first to second gear causes a lurch or stutter. It's experienced as a firm shift, what then feels like a release, and then it grabs and goes. Calling it a stutter is not out of the question. I've performed a series of tests across every possible permutation and believe it to be torque related. Issue occurs more often when going uphill and/or turning at a gradual rate of acceleration. It happens less at flat grade and rarely when descending. Outside temperature, engine temperature, and type of terrain does not matter. Higher shift points are smooth as silk. My father-in-law has the exact same cruiser, took it to the same shop, performed the same service, and has no issues whatsoever.

Solution efforts thus far that have proven ineffective:
  • Front and rear axles serviced (tangential but good to be aware)
  • Myriad of changes to engine and transmission programming following inspection of data.
  • Examination of transmission wiring to confirm proper wiring.
  • Replacement of the transmission controller.
  • Transfer Case Rebuilt
  • Full replacement of entire transmission which was ~4 months new at time of replacement.
  • Rear U Joints replaced.
We are now considering the viscous coupler as a potential means of causing the torque reverb back through the transmission. Other than that, we are all out of ideas and I'm beyond desperate. Does anyone have some ideas on what might be causing the issue?
Does it still do this if your CDL is locked?



I would lean more towards a torque converter going bad or not achieving lock-up. This could be electrical or hydraulic.
 
Joined
Apr 14, 2016
Messages
2,364
Location
Oregon
We are now considering the viscous coupler as a potential means of causing the torque reverb back through the transmission

Pull the front drive shaft and lock the transfer case to eliminate that possibility. It's conceivable that the front drive flanges are slipping.

I think my friend had the 6L80 in his Silverado. When it went out unexpectedly, I did some research and found out that the torque converter solenoid is pulse-width modulated in order to give smoother engagement and I think it works as low as 2nd gear. How is your transmission controlled? Maybe it's only a 6L80*E* thing and completely irrelevant here...
 
Joined
Sep 14, 2010
Messages
111
Location
Colorado Springs
Website
www.summitwise.com
Assuming you're running a stock computer, have you checked all the settings in the transmission settings? There's loads of configurability in there that could have gone funny.
some examples:
We replaced the ECU with a Chevy ECU. The transmission has a built in computer. We've analyzed the data six ways to Sunday and worked with a myriad of different configs. Changing some of the data here and there made improvements but never fully resolved the shifting issue.

Does it still do this if your CDL is locked?



I would lean more towards a torque converter going bad or not achieving lock-up. This could be electrical or hydraulic.
From the shop owner who is intimately involved in the resolution of the issue:
The torque converter doesn't lock up until higher speeds and wouldn't be a factor in the 1-2 shift. I went back and double checked some of the old data logs and it's definitely unlocked in these low gears.
Pull the front drive shaft and lock the transfer case to eliminate that possibility. It's conceivable that the front drive flanges are slipping.

I think my friend had the 6L80 in his Silverado. When it went out unexpectedly, I did some research and found out that the torque converter solenoid is pulse-width modulated in order to give smoother engagement and I think it works as low as 2nd gear. How is your transmission controlled? Maybe it's only a 6L80*E* thing and completely irrelevant here...
This is exactly what we are going to do. Thinking back, the main difference between my truck and my father-in-laws from a front axle perspective is 1) double cardan ds and 2) front locking diff. While those two things shouldn't be causing this issue, it will be good to eliminate that as a possibility.

I'll drop the front ds, lock the CDL, and report back.
 
Joined
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Messages
111
Location
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Website
www.summitwise.com
Installed the CDL switch and tested various scenarios with CDL engaged. Issue persists.

When compared to my father-in-laws swap, performed at the same shop with the same cruiser (both '94's) and parts, it's night and day. His is buttery smooth under all conditions. He also gets considerably better gas mileage which leads me to a possible culprit.

When we first took delivery of our trucks my father-in-laws gas cap had been drilled to let out extra pressure. Mine had not and we took the trucks to 12,500'. My fuel pump began SCREAMING so I removed the gas cap to let out pressure. The cap flew off and then the tank hissed for over five minutes.

My fuel pump (Walbro GSS340 Intank Fuel Pump 255LPH High Pressure (Universal) then began whining and then, after ~60m of driving, screaming. We replaced it with a new one and it continues to get loud, but not as loud as before.

While the data is inconclusive it seems like a MAF replacement could be a good next step. From there, an analysis of the torque converter as a culprit. Agree?
 
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