The problem Don is alluding to is the failure of the connector plug seal on the conductor plate connector at the transmission. If this seal fails it allows transmission fluid to wick up the spaces between the wires in the vehicle harness. Eventually the fluid ends up at the transmission controller in the black box under the hood. The fluid drips down into the transmission controller which destroys the controller. This is $$$. Prevented with a $15 part when you change the fluid.

The other side to the story is a failure of the conductor plate itself. The shift solenoids (6 of them), a pair of speed sensors, and a fluid temperature and level sensor sit in this plate. It is constructed similar to the tail lights, with strips of metal in plastic to make up the circuits.

The usual failure mode here is one of the speed sensors quits. When this happens, the transmission controller does not know that the transmission is doing what it is being told. The controller compares the input and output speeds and based on what gear it told the transmission to go to, knows whether the shift happened successfully or not. If it does not solve correctly, then it goes into limp home mode because it assumes the transmission has a mechanical problem that would be exacerbated by continued operation.

Another problem is that one or more of the solenoids stops making contact with the signal wires in the plate. When this occurs, then the transmission cannot execute one or more shifts. The solenoids work in combination for the various shift sequences. Again this causes codes to be thrown and most likely limp home mode.

All of this gets recorded in great detail but it can only be read out by STAR diagnostics.

The codes can be read by a generic OBDII reader but they group many different individual STAR codes under one generic OBDII code. So if you plug in the OBDII reader and get a code in the P07xx range (the standard range for the transmission) then you know there is a transmission problem. However you will need to get a read with STAR to really know what is going on. Also you cannot erase transmission OBDII codes with a generic reader, they have to be cleared using STAR.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech