2003 suburban my daily driver, trans failed at 208,000 miles (torque converter failed). Everything else still looked good, I did a full rebuild with new stock converter, high energy frictions, molded pistons, sonnax .500 boost valve, transgo separator plate, beast sun shell, TransGo shift kit/valve body rebuild kit, new shift solenoids and harness (GM), transgo 1-2&3-4 accumulator, corvette servo, and a kit to remove the pwm lockup (may have been a Sonnax kit, don’t recall). This was all done 6 years ago (82,000) miles. Trans has been flawless and I love the smooth crisp shifts but it does have a problem. About 6 months after the rebuild it started having a slip in first. Almost felt like it was in second gear for a second then shifted down to first and took off. This occurred about once every 6 months then gradually increased to once a month. By this time it had about 60,000 on it so I dropped the pan and it all looked good, new filter topped off the fluid and it went back to only happening about once every 6 months. Fast forward 20,000 miles and it started happening once a month again, then once a week. Then I had surgery and it sat for 2 weeks. After sitting it started happening almost every takeoff after it had warmed up. I was worried it may be causing damage and I was restricted on what I could do after surgery so I put a bottle of Lucas in and that helped. It now is very intermittent, sometimes I can drive it and it won’t happen at all, sometimes it may occur 4 takeoffs in a row then not occur after that. Issue happens in D and 1, have not noticed it in Reverse. When it does happen in drive a few times it has fumbled the 1-2 shift. Trans fluid is still bright red, doesn’t smell burnt. I did hookup a pressure gauge and will post the videos of that. Any ideas what could cause this? I really thought my line pressure was dropping causing it but that’s not the case, if anything it appears to be higher when the issue occurs.

 

Is it actually slipping or taking off in 2nd gear and then immediately down shifting into 1st?

 

hard to tell but it isn’t your typical high rpm slip and it does propel the vehicle forward, just is sluggish (like taking off in 2nd) and the transition from slip to grab is smooth, no felt downshift. But it really doesn’t feel like a true slip. Taking off in second does feel about how it does when it acts up but I did try taking off in second and immediately pulling it down into 1st and you can feel that downshift. The more I think about it the more the pressure seems normal based on behavior. If it was taking off in 2nd then it would require more torque which should call for more line pressure, when it drops back into 1st, less torque so less line pressure. This is assuming it is taking off in 2nd. Maybe if the 1-2 shift valve or solenoid was sticking it would cause this and would be smoother than a manual 1-2 downshift?

 

It could be that the AFL valve and AFL valve bore in the valve body is worn, but I can't say for sure. Hopefully Frank or someone can step in and confirm this.

 

If it turns out to be the AFL valve, Central Valve Bodies sells a slightly oversized AFL valve that will drop into your valve body that will hopefully take care of your problem.

 

I would say you are correct on the AFL that or one of the solenoids is leaking a little. I would inspect the solenoids then though I have not used one yet get one of the .005 oversized AFL VALVES for it, Unless he wants to just buy a VB or get the reamer set up and all from sonnax or transgo.

https://globaltransmissionparts.com/...xoCp4UQAvD_BwE

I am gonna order one of these to do a couple test with soon, The idea seems simple quick and good.

 

It did something today it had not done before. Taking off from the stop sign was normal, no slip, and then it fumbled the 1-2 shift. Previous times that it fumbled the 1-2 shift have only occurred after a slip taking off. I’ve had no issues with 2-3, 3-4, or lockup. There’s got to be something wrong with the 1-2 solenoid or valve.

I still have my old shift solenoids, I had no shifting issues before the rebuild so I think I may drop the pan, pull the 1-2 solenoid, check the 1-2 valve for free movement and then install my original 1-2 solenoid and see if that makes a difference.

 

They are actually .001 oversized

Even though AFL lineup might vac test ok, I still put one in every build I have done. They are cheap enough

 

OK I was not sure I thought i saw someone say .005 , On the site just says GENTLY OVERSIZED lol . Either way seems like a quicker alternative to reaming and all if works well.

 

Yeah, it's only about a .001 oversize.

 

Saturday I dropped the pan and nothing looked abnormal. I pulled the 1-2 solenoid and pressed the 1-2 valve with my finger and it seemed to move freely in the valve body. I found my original shift solenoids and swapped in one of those. Installed a new filter, gasket, and topped off the fluid. Driving it the 1-2 shift seems a bit smoother and so far no slip, but last time I did a fluid/filter change it helped so not sure the solenoid fixed anything, maybe just the higher viscosity of new fluid? I tested the solenoid I pulled out by blowing through it and applying battery voltage to it and it engaged every time and sealed perfectly, so it didn’t seem to be defective unless it was only failing in certain scenarios. My plan is to just drive it and if it starts slipping again I’ll pull the pan and install the oversized AFL.