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Mercedes-Benz Temic VGS (722.8)

Mercedes-Benz would not be Mercedes-Benz if the brand would economize on luxury. Therefore even the smallest models are equipped with a highly comfortable automatic transmission. Both the Mercedes-Benz A-Class (W169) and the B-Class (W245) are provided with a 722.8: a Continuously Variable Transmission (CVT) with torque converter. This combination was deliberately chosen, since a CVT usually requires relatively little space and functions quite comfortably. Therefore, ideal for the smaller Mercedes-Benz. The addition of the torque converter is, in a technical sense, not necessary, because it could have easily been done with a (somewhat cheaper) automatic plate clutch. However, a torque converter generates a total ‘‘Mercedes feeling’’. This is because a torque converter slips: as it also does in the more expensive Mercedes-Benz models with automatic transmission. In addition, a torque converter multiplies the engine torque until there is no slippage anymore. A characteristic that empowers engines while driving off, especially those with a low torque. This combination enables a luxurious driving experience, which is exactly what you would expect from a Mercedes-Benz.

 

AC1

 

What goes defective in many cases?

The management of the CVT with a torque converter is conducted by mechatronics: a component that consists of mechanics and electronics. By mechanics, we mean the labyrinth of channels and valves that control various components within the CVT through these channels. The electronic component is the TCU of the Mercedes-Benz Temic VGS (or Autronic: how Mercedes-Benz calls it). Defects may arise in these electronics, resulting in error codes. The rather large list below shows which error codes relate to a defective Mercedes-Benz Autronic:

 

AC2

 

0657 The voltage supply of the control solenoid valves in the CVT is faulty
0705 Transmission Range Sensor Circuit Malfunction
0706 The signals of the range selector are implausible in comparison to the signals from the electronic selector lever module
0717 The RPM signal from component Y3/9b5 (CVT output rpm sensor) is not available
0720 Speed Gradient nPrimGrd > nPrimGrdMax
0722 The RPM signal from component Y3/9b5 (CVT output rpm sensor) is not available
0723 The RPM signal from component Y3/9b5 (CVT output rpm sensor) is implausible
0730 The gear ratio in the CVT is not permissible
0739 Secondary RPM signal of component Y3/9B5 is not available
0746 An open circuit has occurred in the control loop for componentY3/9y1 (CVT) primary control solenoid valve
0766 An open circuit has occurred in the control loop for componentY3/9y4 (CVT) torque converter lockup clutch control solenoid valve
0776 An open circuit has occurred in the control loop for componentY3/9y2 (CVT) secondary control solenoid valve
0778 a short circuit has occurred in the control loop for component y3/9y2 (CVT) continuously variable automatic transmission secondary control solenoid valve.
0793 Intermediate Shaft Speed Sensor Circuit No Signal
0794 Intermediate Input Shaft Speed Sensor Signal
0796 An open circuit has occurred in the control loop for componentY3/9y3 (CVT) clutch control solenoid valve
0766 An open circuit has occurred in the control loop for componentY3/9y4 (CVT) torque converter lockup clutch control solenoid valve
0842 The output voltage of component pressure sensor is faulty. (Short circuit to ground)
1634 Component Y3/9n1 (CVT) is defective or the voltage supply is faulty (undervoltage)
1744 Torque Converter Clutch System Performance
2722 Impermissible closing of hydraulic brake ‘Reverse gear’ has occurred in the continuously variable automatic transmission (CVT) when selector lever ‘N’ was engaged.
2723 Impermissible opening of hydraulic brake “Reverse gear” has occurred in the CVT
2731 Impermissible closing of hydraulic clutch “forward gear” has occurred in the CVT when selector lever position “N” was engaged
2732 Impermissible opening of hydraulic clutch “forward gear” has occurred in the CVT

We will explain more about the functioning of the Autronic later, but we can already recommend to think about defective mechatronics once one of the error codes above appear, even though these error codes might also refer to a mechanical defect.

There are also error codes that refer to a mechanical defect in the CVT itself, rather than a defective mechatronic. Due to wear, slack might occur with all the associated consequences. The error codes we see here have often to do with the functioning of the pushbelt or an incorrectly measured oil pressure:

0841 While driving, the actual hydraulic pressure is implausible in comparison with the specified hydraulic pressure in the CVT
0894 The steel thrust belt in the CVT is slipping
0896 Impermissible adjustment of the step-down ratio in the CVT

AC3

 

The 722.8 in detail

A description of how the Temic VGS functions would be incomplete without involving the CVT with a torque converter. Before you can understand what a TCU like the Temic VGS controls, it is important to understand how the CVT roughly works.

Therefore, we’ll first try to briefly explain the principle of the CVT:
A gearbox with a continuously variable transmission basically consists of two pulleys and a pushbelt. The gear ratio from the engine to the drive shafts is steplessly adjustable, due to adjustable radiuses of both pulleys. This adjustment functions hydraulically. By providing or reducing the oil for the pulley, the sloping sides of the pulleys move towards or from each other.The closer these sides get towards each other, the more the pushbelt of the pulley is pushed of its central ‘‘position’’ in an outward direction: leading to a higher radius of the pushbelt around the pulley. By varying the distances between these sides, the gear ratio can be changed.

 

AC5

 

Solenoid valves and speed sensors are required in order to manage the extent in which the pulleys adjust themselves. The velocities of both the driving pulley (motor side) and the driven pulley (differential side) are measured. It is up to the TCU, the Temic VGS, to determine the amount of oil that the valves transfer to the pulleys, in order to adjust the gear ratio. However, the Temic VGS manages four valves, not two. What are the other two for?

Except for controlling the gear ratio, the Temic VGS is also responsible for operating the torque converter: this is where the other two valves are used for. A negative characteristic of a torque converter is that the circulated liquid in its housing is never able to ensure that 100% of the engine torque is converted into the rotating output shaft. Unfortunately, there is always some loss. In order to eliminate this loss, a lock-up was placed in the torque converter itself: turning it into a solid connection between the engine and the CVT, just like a conventional clutch plate does in manual transmission. This prevents any slippage.

 

AC6

 

An image of the Temic VGS, assembled with valves on the labyrinth. Purple indicates the position of the valves.

Now that it is clear that the Temic VGS is responsible for controlling all these components, it might be easier to imagine that various complaints may arise once one or more valves are not controlled like they should. The TCU is literally the key element of the system. However, except for errors regarding sending commands to the valves, other problems might arise when the TCU does not receive the correct input. The Mercedes-Benz Temic VGS receives its signals through 3 speed sensors (that are attached to the TCU itself) and a CAN bus connection. An interruption or deviation in these signals can have major consequences.

Finally, another tip: a test you can perform yourself as a technician is measuring the voltage going through the plug. Therefore the pin assignment of this plug is showed below:

 

AC7

 

Pin 1 CAN-L
Pin 2 CAN-H
Pin 3 Mass
Pin 4 12V

 

SOURCE: ACTRONICS