- Subject: Transmission fluid entering PCM through wiring harness
- Unit: Mercedes 722.6
- Essential Reading: Rebuilder, Diagnostician
- Author: Wayne Colonna, ATSG, Transmission Digest Technical Editor
Capillary action is defined as the movement of a liquid within the spaces of a porous material due to the forces of adhesion, cohesion and surface tension. A simple example would be dipping a paper towel into a glass of water and watching it climb up the towel. If the towel is long enough, eventually the force of gravity will be too great for the water to overcome.
So what does all this mean to you? Intermittent codes for speed sensors or solenoids, or both, with vehicles that use the 722.6 transmission. As a result you may go the TCM to do some electrical checks (see figures 1 and 2) only to find that transmission fluid has worked its way up the wiring harness and filled the computer (see figures 3, 4 and 5).
There is your capillary action. And this will happen even if you have good seals on the vehicle harness and pass-through sleeve in the case to the conductor plate (figures 6, 7 and 8).
One method to prevent a recurrence (especially after replacing the computer) is to remove enough insulation to create a bare section on each wire and drop a little solder into each of them, then nicely heat-shrink each wire and dress it all up. It would be best to stagger the bare areas so that you do not end up with one large lump from all the heat shrinks. The solder will block the capillary action and you will not have to be concerned about the car coming back with the same problem.
One note: We may see this same problem occur with vehicles that use the ZF 6HP26 and the 6L80, as they have connector features similar to those of the 722.6. This is just an observation. We will have to wait and see.
Many thanks to Wayne Duvall from Crownsville Transmission for the TCM pictures used in this article.