Most of the Mercedes-Benz vehicles that come to our shop for Mercedes transmission service are equipped with the 722.6 automatic transmissions. This type of Mercedes transmission employs 5 forward speeds that are entirely controlled by a computer that employs adaptive shift learning. In the past, it was common for a Mercedes transmission to experience major gear train breakdown, results of which led to a jumble of broken internal workings. The most evident indications of gear train failure are loss of movement in forward or reverse gears and grinding/whining noises when a gear was engaged. The noise may happen with or without the vehicle in motion. Gear train failures have been significantly reduced in later Mercedes transmission models thanks to improvements in design.
TRRS Shifter Switch Electrical Malfunction
The gearshift selector found in the center console of Mercedes-Benz is coupled with a complex switch that sends electrical signals to the transmission computer controller. This switch is not adequately protected against conductive water based liquids that may enter through the center console area. Spilling your favorite beverage or leaving the sunroof open during a cloudburst can ruin the TRRS shifter switch, usually resulting in transmission malfunctions. This occurs because unlike oil based liquids and materials which do an adequate job of insulating electrical current, water based liquids are highly conductive. This is why the insulation on electrical wires is made of plastic, a petroleum oil based material. The TRRS switch cannot be repaired or bypassed, so replacing the switch will is required to restore proper function, allowing the correct signals to reach the Mercedes transmission computer controller through the wiring harness.
Transmission Computer Electrical Problems
All 722.6 Mercedes transmissions experience many electronic problems; however, there are major differences between the earlier and later 722.6 transmission models. These differences play a big role in the probability of cause of failure as well as the cost of repair. Electronic problems are still very prevalent even in the late model units, but these issues are relatively inexpensive to repair, because they can be addressed without removing and disassembling the transmission. The three electronic problems mentioned below account for 85% of the electronic problems we see in Mercedes-Benz transmissions.
Electrical Valve Body Conductor Plate Sensors
A defective transmission conductor plate is generally diagnosed by the speed sensor codes it produces when it has failed. This conductor plate is located on top of the transmission valve body, which can be accessed from under the vehicle. Repair of this component is not feasible, so the manufacturer, available from the Mercedes dealership, must replace the part with a new one produced. The replacement of the conductor plate requires removal of the transmission valve body. When the valve body is removed, the conductor plug must also be removed, and should always be replaced with a new one.
Conductor Plug Leaking Fluid into Wiring Harness and Computer
The conductor plug often leaks fluid to the external wiring harness that is attached to it. When this occurs, the leaking fluid can get drawn up through the wiring harness and can accumulate in the transmission control module. This is called wicking action, like a wick for an oil lantern. Oils, including transmission fluid, are not good conductors of electricity; however they can wreak havoc when introduced into sensitive electronic equipment, such as microprocessors. To correct this problem, the conductor plug must be replaced, the wiring harness must be cleaned of excess transmission fluid, and the transmission control module should be disassembled and sprayed clean with electrical contact cleaner. In most cases the transmission control module will revert back to proper operation, but in some cases it must be replaced.