Back on the Bench - September - 2018

Common Mercedes 722.9 Problems

Like most other transmission, as the years go by, the 722.9 is hitting the bench more and more often. And, over the past few years, we’ve seen some common problems among them.

As usual, the simplest things can lead to major problems in the 722.9. The majority of the failures we’ve run into are caused by the O-rings on the tips of the solenoids deteriorating, leaking fluid, and not properly stroking valves (figure 1). We’ve seen the O-rings cut, some missing, and the clutch they operate usually burns.

The solenoids themselves rarely fail, but, like anything else, they have the potential to. As for the Mechatronic unit, if there are no electrical codes set, no water contamination, no signs of overheating, and no visual damage, consider letting the customer choose whether he’d like to reuse the old one or install a new one.

On the early 722.9s, we’ve seen multiple bearing failures between the K1 drum and stator support. Mercedes has an update for this that requires replacing the stator and drum (figure 2). So far, we haven’t seen this bearing fail on the later stator and drum. It wouldn’t be a bad idea to replace them during a normal rebuild: It may be a little expensive now, but it’d be more expensive to replace them under warranty.

Molded pistons have a tendency to fail, just like in any other transmission. The most common one is in the K1 drum (figure 3). When this happens, the transmission will usually drop into neutral going into third. While the B1 and B3 rarely have any issues, consider replacing all of them during a rebuild.

The valve body is subject to a variety of problems. We recommend completely disassembling it on every rebuild and checking for broken springs. We’ve seen some springs break in the middle and still stroke the valve. And if you’re only picking the valves, this could make you think the springs are good.

Valve bores sometimes wear, so vacuum checks are always a good idea. One area to pay particular attention to is the working pressure regulator bore; a worn bore can lead to burnt clutches and overheating.

Check valves sometimes stick, causing various issues with shifting and even TCM cooling. Check valves 1 and 2 (figure 5) balance pressure from the clutch regulating valves to TCM cooling pressure. If these check valves aren’t functioning properly, they can cause the TCM to overheat, creating shift-related issues after the transmission warms up.

Extended overheating can cause the TCM to fail and develop circuit codes. Like all others, the two rubber checkballs can get damaged, stuck in the separator plate, or completely disintegrate. Keep a checkball location sheet beside you during disassembly.

In the pump, later models have been updated from a bushing to a bearing (figure 6); the converter hub wasn’t updated until later. With the old-style hub and new bearing, the hub or bearings would often pit, causing a whining noise. During a rebuild, it’s a good idea to update the pump and make sure your converter builder is sending you a converter with the later-style, hardened neck.

The 722.9 is one of the easier transmissions to rebuild. But, like most transmissions, the 722.9 requires a great deal of attention to the details. Taking the time to check those details can prevent seeing this one back on the bench.

For disassembly tips, check out Bill Brayton’s YouTube video, 01 Transmission Disassembly Procedures, Mercedes 722.9.