Teasing you with the TCU

What to do when the transmission control unit goes faulty?

Published:  05 April, 2017

By ACtronics

"When you say Mercedes-Benz, most people think German reliability and luxury" says Kevin Barnbrook, marketing manager at vehicle electronics remanufacturing and repair specialists ACtronics. "It's the same regardless of whether it's the bigger C or E Class or the smaller models in their range such as the A Class and B Class.

"The smaller models come with the same plush extras as the bigger models have, but they also come with automatic transmission to make sure that the car drives as smoothly as its larger counterparts.

The management of this CVT with torque converter is conducted by mechatronics. This is a mixture of mechanics and electronics. To the uninitiated it might appear to be just a warren of channels and valves, but these control various other components. We said you wouldn't like it. Central to this is the transmission control unit (TCU).

"This TCU is basically what controls the electronic automatic transmission or gearing in all modern automatic Mercedes-Benz A and B Class vehicles since the change over from hydro mechanical controls," explains Kevin. "What the TCU does is uses sensors and data provided by the ECU to calculate when to change gears. This provides optimum performance and much better fuel quality for the vehicle. It's essentially the brain of the automatic gearbox."

"When the TCU goes faulty, there are a whole host of codes that relate to faults within the TCU. The most common ones we see are P0720, P0722, P0793 and P0794, which relate to either the output speed sensor or shaft speed sensor."

There's nothing more unsettling than fault codes that prompt more questions than answers - and this is clearly something you don't want to try fixing yourself.

"These can sometimes be a tricky repair," says Kevin, due to the small nature of the circuits, processors and soldering needed to replace and remanufacture."

Inside the TCU

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