09_05 Tech Speaking

Out of Sight, Out of Mind

Often-overlooked parts cause shift problems in 4L80-E transmissions

By Wayne Colonna

Technical Editor

From time to time the ATSG tech line receives calls concerning 4L80-E transmissions with a reverse and/or third-gear problem. First thought may be compromised piston seals in the direct-clutch drum, or center-support rings. Perhaps even the center-support feed bolt is loose or cracked.

Another cause that often is overlooked is stuck or mispositioned No. 7 or 11 checkballs (see figures 1 and 2). This situation is easy to overlook, as the checkballs are inside the valve body; out of sight, out of mind.

The problem usually occurs after a severely damaged unit was rebuilt without a thorough cleaning of the valve body. Foreign debris can collect and lodge in such a manner that the ball is literally jammed or cannot position itself correctly, causing the reverse/high-gear complaint.

We also have experienced similar third/reverse problems involving a valve body that was cleaned properly but during installation the rebuilder placed the retaining roll pin into the valve body so that it is in the groove of the bore plug (see Figure 3). This mispositions the bore plug and checkball. When positioned correctly, the roll pin should be visible from the end of the bore (see figures 4 and 5).

Technically Speaking

Figures 6 through 9 provide brief hydraulic functions of the No. 7 and 11 checkballs in third, reverse and manual low.

Take note of one other important detail: You will notice that the No. 11 checkball sleeve is fitted with an O-ring that faces the 3-4 shift valve (see figures 2 and 10). This O-ring seals solenoid-A signal oil. If this O-ring is damaged, hardened or missing, the transmission will fall out of fourth gear or will have no fourth gear. We are accustomed to seeing this condition caused by wear in the bore of the actuator-feed-limit valve, a problem with the shift-solenoid feed filter and O-ring behind the manual valve, or mechanically failed shift solenoids. However, one clue that could point you to this O-ring as the cause of this problem is that at times, when you come to a stop you will have a sensation of a sticking governor giving you second-gear starts, and when you step into the throttle the transmission bumps down to first.