TASC Force Tips
- Author: Mio Cundl
Shift problems caused by loss of line pressure
The 722.6 transmission went into production for use in 1996-model-year vehicles and is still being offered in 2009. The 722.6 is used in Mercedes, Porsche and Jaguar vehicles and in a variety of Chrysler products, where it is referred to as NAG1.
NAG is an acronym derived from the German for new automatic gearbox, the number 1 being the first version. In the U.S., NAG1 units include the W5A580, W5A380, W5A330 and W5J400. In the W5A series, the last three digits indicate the maximum input torque in Newton meters. The W5A330 has the widest gear ratio; the others have the same ratio. J400 identifies the unit as a Jeep application.
You can identify the origin by the following process: If the unit has only a pin stamp on the driver side, or a combination pin stamp and bar-code tag, it was manufactured in Germany. If it has only a bar-code tag, it was manufactured by Chrysler.
The Sprinter, Crossfire and Mercedes units require specific fluid because of calibration issues; the other Chrysler applications use ATF4.
This article focuses primarily on drivability issues related to the electro-hydraulic control unit or valve body. This gearbox was one of the first five-speeds with a new concept in load-sensitive working pressure and overlap clutch control. The converter was also unique because of its multi-disc torque-converter clutch (Figure 1).
All these advances require precise hydraulic control, which leads us to the problems discussed here. One of the drivability complaints involves engine-speed flare on a 2-3/4-5 upshift or a neutral flare on a 5-4/3-2 downshift. During upshift or downshift under load, line pressure (referred to as working pressure in the 722.6) must be responsive. The outer bore of the working-pressure regulator valve wears and allows the pMod solenoid-signal oil to leak past the boost end of the valve (Figure 2). As the casting wears, working pressure does not increase with engine load and the flare or loss of gear occurs.
In 1998, many of the high-mileage cars we repaired under Mercedes warranty had the same problem: 2-3 flare causing the vehicle to go into limp mode. After several inspections we discovered the link between the working-pressure bore wear and the shift problems. We believed that the lack of hardness of the valve-body material was the cause of this common failure. We designed a bushing to correct the leakage, using a harder material that would allow it to last longer when paired with a high-speed valve. The valve is a long, one-piece design, and when it is leveraged on the larger end, it also loads into the bore on the smaller inboard end. Both of these wear conditions can be discovered by either a vacuum test (Figure 3) or a visual inspection. Figure 4 shows a worn bore on the right and a repaired bore on the left.
The 1-2 /4-5, 2-3 and 3-4 overlap sleeves are also in the pMod solenoid-signal circuit (figures 5, 6 and 7). It is common for them to wear on the inside diameter and to leak at the outside diameter as well. Wear at any of these sleeves reduces pressure at the working-pressure valve. These sleeves can be vacuum tested prior to removal or cleaned and inspected for wear.
Mio Cundl is the owner of D.B. Automobile in Berlin, Germany, and a member of the Sonnax EURO-TASC Force (Technical Automotive Specialties Committee), a group of recognized industry technical specialists, transmission rebuilders and Sonnax Industries Inc. technicians.
