Engine-Carb Swap: 4L80E Driving me Loco - Upshift Blues

[Medic0893]

No problem. Electricals include the harness, shift solenoids, electronic pressure control solenoid and pressure switch manifold assembly.

MLPS = manual lever position sensor...Here's some more detailed info:

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Your downshifting issue is likely TPS, especially if it's aftermarket as they can be tricky to get dialed in. It can also potentially be your 3-2 control solenoid but I'd def rule out the TPS first before swapping in another solenoid...But if you do have to drop the pan, I'd replace everything electrical/electronic down there.

Appreciate the info. Baseline info: the trans was rebuilt about 20k prior to the carb swap. Always ran and shifted fine. I’m not a big believer in coincidence…

Initially I used the HGM Acculink TPS with no luck. I recently installed the Holley TPS but didn’t get a chance to dial it in. It runs and shift fine (actually better than the HGM) through all gears in surface streets, but same issue on the highway. I read the signal should be 0.45 - 5.0 for the lever sweep… sound right? I did replace the input speed sensor after reading it might help. Initially I thought that was my solution, but over time it seems not to have made the difference I thought it would. I’m gonna back-pin the signal wire and see what it’s putting out. When I did that with the HGM the numbers were crazy (nothing less than 0.75) HGM was worthless, no support unless you buy their $1000 computer.

 

[Medic0893]

Two Speed sensors on transmission and tps are two common reasons for defaulting to third when at speed. Worth checking data from the ecu on what it thinks tps data is at that speed and then reference to the data table for that computer on what it has set. On early computers with 4l80e after about 80mph it is set to allow lock up in 4th gear even at 100% throttle

Great information and I appreciate it. I did replace the input sensor. Initially I thought it solved the issue but alas no. I didn’t do the output sensor as I read it only controls the Speedo signal which is dead accurate. It’s a 92 so no OBDII port. I have to get a cable for my scan tool for OBDI.

 

[yevgenievich]

I had issue with one of the speed sensors beig noisy(rear) on 91. But that one also had a 3rd sensor on a tcase. My strategy was replace one, and then once that did not fix it used the sensor that I took out to replace the second. I still have a file from 91 regarding shift table and will check later nonight what the tps numbers are vs speed vs gear

 

[yevgenievich]

TCC Engage table. There are also two modes, one for when shifter is in drive and one for shifter in overdrive. When in drive, tcc is enabled in 3rd gear. With 350tbi in a v2500 to maintain 80+mph it was almost equivalent of putting a brick on the pedal and letting it cruise.

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[Medic0893]

No problem. Electricals include the harness, shift solenoids, electronic pressure control solenoid and pressure switch manifold assembly.

MLPS = manual lever position sensor...Here's some more detailed info:

xc_hide_links_from_guests_guests_error_hide_media

Your downshifting issue is likely TPS, especially if it's aftermarket as they can be tricky to get dialed in. It can also potentially be your 3-2 control solenoid but I'd def rule out the TPS first before swapping in another solenoid...But if you do have to drop the pan, I'd replace everything electrical/electronic down there.

Finally got to dial in the Holley TPS. It was off a little. Now its 0.52 to 4.53. It was initially around 0.38 at idle, so not that far off. Results: No improvement, actually seems worse in that it isn't resetting as easily with a rolling restart. Still down shifts and won't upshift without a reset of the ignition.

 

[Medic0893]

Well... I gave up! After a year of trying this and that, I took it to someone with an old OBD1 computer. Not a handheld jobber, "The Brick". Turns out the problem is coming from the PCM/TCM. It is just not giving the right commands. Good news is the transmission is working perfectly. So... I bit the bullet and purchased the US Shift Quick 2 TCM. My hopes are high, but my gut says "No dancing yet". Once I get it in I'll update the thread. Thankx to everyone for their help and suggestions.