Trans Shifting Hard

c4racer · August 26, 2008

I have a 2001 2500 suburban with the 8.1L / 4LE80 combo.

I drive it maybe once a week on average. It has 90K miles on it.

Yesterday I took it out in the morning and the trans shifted very hard. Felt almost like when an old school non PCM controlled tranny lost the vacuum connection or something. So I parked it for 15min. then when I got back I checked the level - which was perfect, and the fluid looked good - smelled normal. Then when I drove it again it functioned completely normally - shifted smoothly like normal - no issues.

Seems like the PCM or electronics were doing something wrong on the first drive, then went back to normal after a re-boot cycle. If it were mechanical, seems to me it would not just go back to 100% normal after a restart when the trans was at the same temp and all.

Anybody have this happen before in a GM vehicle, or know what it could be related to?

dewfpo · August 26, 2008

I have a 2001 2500 suburban with the 8.1L / 4LE80 combo.

I drive it maybe once a week on average. It has 90K miles on it.

Yesterday I took it out in the morning and the trans shifted very hard. Felt almost like when an old school non PCM controlled tranny lost the vacuum connection or something. So I parked it for 15min. then when I got back I checked the level - which was perfect, and the fluid looked good - smelled normal. Then when I drove it again it functioned completely normally - shifted smoothly like normal - no issues.

Seems like the PCM or electronics were doing something wrong on the first drive, then went back to normal after a re-boot cycle. If it were mechanical, seems to me it would not just go back to 100% normal after a restart when the trans was at the same temp and all.

Anybody have this happen before in a GM vehicle, or know what it could be related to?

If the atf has never been replaced, that's the first thing I'd do.

DEWFPO

c4racer · August 26, 2008

I have a 2001 2500 suburban with the 8.1L / 4LE80 combo.

I drive it maybe once a week on average. It has 90K miles on it.

Yesterday I took it out in the morning and the trans shifted very hard. Felt almost like when an old school non PCM controlled tranny lost the vacuum connection or something. So I parked it for 15min. then when I got back I checked the level - which was perfect, and the fluid looked good - smelled normal. Then when I drove it again it functioned completely normally - shifted smoothly like normal - no issues.

Seems like the PCM or electronics were doing something wrong on the first drive, then went back to normal after a re-boot cycle. If it were mechanical, seems to me it would not just go back to 100% normal after a restart when the trans was at the same temp and all.

Anybody have this happen before in a GM vehicle, or know what it could be related to?

If the atf has never been replaced, that's the first thing I'd do.

DEWFPO

I believe it has been once, but I have that on my to-do list anyway. I have the fluid and filter, just waiting to find some time to actually do it!!

HDdave · August 27, 2008

Throttle position sensor... It happened to my 01 6.0/ 4l80e. My gears would rev out a lot higher and it felt like I had to force it to shift by playing with the throttle, and then when it did shift it would just slam the next gear. It felt like I was driving a standard shift and just dropping the clutch after shifting

c4racer · August 28, 2008

Throttle position sensor... It happened to my 01 6.0/ 4l80e. My gears would rev out a lot higher and it felt like I had to force it to shift by playing with the throttle, and then when it did shift it would just slam the next gear. It felt like I was driving a standard shift and just dropping the clutch after shifting

It shifted at the normal RPM point, just that it shifted very hard - so not exactly the same symptom. I will take it for a spin in the morning to see if it does it again.

irengineer · August 30, 2008

Throttle position sensor... It happened to my 01 6.0/ 4l80e. My gears would rev out a lot higher and it felt like I had to force it to shift by playing with the throttle, and then when it did shift it would just slam the next gear. It felt like I was driving a standard shift and just dropping the clutch after shifting

It shifted at the normal RPM point, just that it shifted very hard - so not exactly the same symptom. I will take it for a spin in the morning to see if it does it again.

Sounds like the trans was operating at high line pressure for one key cycle. This is a likely diagnostic action if a diagnostic malfunction has been observed. There should be a code stored that would define which malfuntion was observed as well as freeze frame data that "might" help explain "why" the malfunction occurred.

c4racer · September 5, 2008

Throttle position sensor... It happened to my 01 6.0/ 4l80e. My gears would rev out a lot higher and it felt like I had to force it to shift by playing with the throttle, and then when it did shift it would just slam the next gear. It felt like I was driving a standard shift and just dropping the clutch after shifting

It shifted at the normal RPM point, just that it shifted very hard - so not exactly the same symptom. I will take it for a spin in the morning to see if it does it again.

Sounds like the trans was operating at high line pressure for one key cycle. This is a likely diagnostic action if a diagnostic malfunction has been observed. There should be a code stored that would define which malfuntion was observed as well as freeze frame data that "might" help explain "why" the malfunction occurred.

Ya - that makes sense. I need to get some body ECM codes out anyway, because I have some issue in the ABS or brake system that causes the light to go on. Functions normally at least. I have been driving it every morning for the past week and it has not happened again.

cvandegna · March 30, 2009

Ya - that makes sense. I need to get some body ECM codes out anyway, because I have some issue in the ABS or brake system that causes the light to go on. Functions normally at least. I have been driving it every morning for the past week and it has not happened again.

Did you ever figure out if you were getting a code for this? Mine started doing this occasionally. I can't attribute it to anything in particular - seems completely random.

Am about to change the ATF (including filter change, TC flush), replacing the pan with a deeper B&M and swapping the factory cooler with a B&M plate cooler. Am hoping that this will help things out, and help keep things from overheating in the TX heat.