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So my 02 suburban with a built 4L60 decided to lose 4th gear the other day. Transmission only had 15k miles on it and has had zero issues before. I started my investigation with the 4th gear servo because everything else worked normal. I found the factory washer, clip and sonnax washer broken. I installed a new factory washer and clip along with the factory 4th gear servo. Went for a test drive and everything was back to working just shifted a little softer. I called the shop that built it and also sent my photos to Sonnax and they both said they thought there was a pressure issue. I am no engineer but have been a certified master technician for a long time but specialized in imports for most of my years. I've since changed my career path and gone to working on heavy equipment and big diesel trucks. I like to think things through and what they are saying doesn't make any sense to me. I would have thought that an increase in pressure would have caused some noticeably hard shifting before it broke anything. I just recently cleaned out the Sonnax servo and reinstalled it with a new washer. Transmission is back to shifting like it did when it was new. So my question is what could have caused this kind of damage without making the trans shift hard? I literally just left for work one day and it didn't shift into OD. It was fine on my way home the night before. I've built several manual transmissions but don't really do much with autos. I knew enough to know that if 2nd and 3rd were fine that the band and 3-4 clutch pack should have been fine. My best guess with the limited amount I know of how this works is that the Sonnax servo has 2 pistons inside and if one got stuck it could have broken the washers the way it did. At this point I'm just trying to keep it from happening again.
Do you know what the shop did to the transmission to increase the line pressure, if anything? There's different "shift kits" out there most rebuilders will use that can do this, but I've read some users complain about harsh shifts. Drilling oversized holes on certain valve body passages or parts can also be done to adjust pressures.
I read of a guy that had his glovebox pop open every time his transmission shifted into a certain gear. Sounded painful to me. I certainly wouldn't want that in a daily driver.
However, you are saying the transmission shifts ok, or a little softer now. So not sure what to say.
May be worthwhile to have the transmission shop check the line pressure via the test port and gauge to verify actual pressures. Otherwise the issue is not fixed and may cause the same issue down the road.
I don't believe there is a band issue as 2nd works perfect with a slight bump when shifting. I never changed the pin but measured the stroke before it went back together and was in spec per the Sonnax instructions. I remembered that I had flashed the ecu and changed the shift points to shift at a higher speed which seemed to make it shift a touch firmer. I decided to change it back to stock and after driving it a few days the shifting seems even better. I'm guessing when I changed the speeds that I also needed to change the pressures associated with the different speeds but am not sure. It was supposed to be built to withstand some abuse and was built by one of the most respected 4L60 builders in the business. I'm going to leave it alone until I get my cam and tune finished. Worst case scenario I'll put the stock one back in and have this one freshened back up. It has a lifetime $450 rebuild associated with it.I've tuned plenty of turbo 4 cylinder import cars with aftermarket ecu's but the stock ecu game isn't for me. I'm just going to leave that to the pro tuners. When I get caught up I'm going to check pressures and see what happens. I got on it pretty hard last night just fooling around and it almost barks the tires going into 2nd WOT. I don't normally drive it like that but wanted to see what it would do.
11-03-2023, 07:47 AM
4L60 Issue
