Torque Converter doesn't lock up in 4th (but locks up in 3rd)

[mitchb]

Vehicle:
- 2007 GMC Savana
- 6.0L / 4L80E / 3.73 rear end
- 69,000 miles. I purchased it at 43,000 miles, all xway miles from 43-69k.
- Trans oil full, not burnt, but slightly dark. Was last changed at 45,000 after I purchased vehicle. Trans fluid was slightly dark when I bought the vehicle as well.

- Trans operated perfectly normally up until it stopped locking up.
- No trailer tow, no overloading. Van is used to transport my motorcycles and mountain bikes.


Hello Everyone,

Transmission is not locking up in 4th gear, causing p0894 code if it driven on the xway. RPM on freeway is approx 300 higher than it should be. I have a scangauge so I know the exact RPM.

Harsh 1-2 shifts after code is thrown (as expected).

Clearing the code results in normal shifting, including torque converter lockup in 3rd gear. I didn't check to see if the trans is locking up in 3rd once the code is thrown.

With the codes cleared the transmission doesn't even try to lockup in 4th gear, even at light throttle and slight downhill grade. Lockups in 3rd gear are perfectly normal.

 

I grabbed a code scanner and read the p0894 code. I cleared the code then disconnected the battery and let the vehicle sit overnight. Yesterday I hooked up the battery and went for a drive. Everything is 100% normal EXCEPT no torque converter lockup in 4th gear.

I don't know why the torque converter locks up in 3rd but not 4th - any ideas out there? I would assume it is a mechanical problem, that some sludge broke free and is preventing a valve from opening or closing, but why does it lock up in 3rd tho?

 

Thanks!

[mitchb]

Start of problem:
The transmission was working perfectly normal from home to the xway (8 miles). It was definitely locking up in 4th gear. Upon entering the xway the right lane was backed up, so I went wide-open-throttle to merge into traffic. WOT occurred from creep speed to probably 30mph. No issues whatsoever, vehicle shifted normally. Keep in mind that I drive like a grandma, over the past 25,000 miles I've probably gone WOT less than 5 times. After several miles on the freeway I noticed that I was at 2300rpm, not 2000rpm like I'd expect to be at. After I exited the xway (10 miles of xway) I started to get harsh 1-2 upshifts. 2-3 upshift was OK. No check engine light was thrown, but I work right off the freeway so I only got 3 harsh shifts before arriving at work.

[mitchb]

I checked the brake light switch, that's not the culprit either.

 

I was hoping to get a "Since you did a 1-2 shift at WOT the line pressure went to max which can make the <xxxxx valve> stick closed , just drop the valve body and pop the valve out and clean it" kind of response....

 

Anyone?

[oldscsc]

It's hard to list possible mechanical culprits without hydraulic diagrams to look at. Its possible that a check ball is stuck in one position, but I don't know that I'd drop the valve body on that hunch.

 

The first thing I would do is to get a scan tool on the van and drive it. When driving at highway speed, command the TCC on via the scan tool. If it works, you've likely got a sensor issue since we know the converter itself functions as intended. If it doesn't, you probably have a mechanical issue.