Definition
If error codes relating to the speed sensors or the transmission ratio are stored in Mercedes-Benz vehicles with front CVT automatic transmission, a hardware defect in the control unit is to be assumed.
The following error codes indicate the defect:
• 0717 (P1815) Speed signal from component Y3/9b3 (primary speed sensor CVT) not available
• 0722 (P1826) Speed signal from component Y3/9b5 (output speed sensor CVT) not available
• 0793 (P1939) Speed signal from component Y3/9b4 (secondary speed sensor CVT) not available
The error code “0896 (P2198) Transmission ratio – transmission ratio incorrect” may be set in conjunction with one of the above-mentioned error codes.
Proceed to Repair!
System description
Starting with the 2005 model year, Mercedes equipped the A-Class (type 169) and B-Class (type 245) models with an optional continuously variable automatic transmission (CVT transmission), which Mercedes refers to as the VGS-FCVT transmission or transmission type 722.8, or Autotronic.
In the CVT transmission, the gear ratios are continuously adjusted to the current operating conditions. The continuously variable transmission is made possible by the primary and secondary disc sets. The engine torque is transmitted from the primary to the secondary disc set by the push rod belt, with the primary and secondary disc sets being wrapped around the push rod belt.
CVT solenoid valves are controlled by the CVT control unit to adjust the disc sets. The oil pressure required for the variator control and the contact pressure of the push band are regulated by pressure slides. Control is dependent on the incoming speed signals from the sensors, the applied oil pressure, and the oil temperature, which are installed in the control unit located inside the transmission. The signals from the speed sensors are recorded together with the wheel and engine speeds and other data in the CVT control unit as input signals for the control system.
The FCVT mechatronics is bolted to the housing below the wheel set in the transmission. The component consists of the hydraulic shift valve box and the CVT control unit with sensor carrier attached to it. The control solenoid valves are mounted in this and electrically connected by means of clamping contacts.
Fault description
The CVT control unit calculates the control variables for the variator positions from the input data, which are implemented by the control valves using oil pressure. At the same time, a redundant comparison model is calculated using CAN data from the engine speed, load, torque, wheel speed, and speed. If the results deviate too much, the system starts an analysis program, which sets the error codes. The internal speed sensors often fail, initially sporadically and later permanently.
Possible emergency strategy for the transmission control:
- The transmission is operating in emergency mode.
- The engine control lamp is illuminated.
- Driving is only possible with limited speed and power.
- The speed is limited to Vmax = 50 km/h (max. speed 2500 rpm).
Diagnosis
In addition to the selection of the errors mentioned, the speed is no longer visible in the parameters for the faulty sensors. Depending on the diagnostic device, the displayed value may be output as implausible with 0 min-1 or —- .
The sensors do not necessarily all have to fail at the same time.
Diagnostic screen showing the most important parameters. The primary disc set speed should correspond approximately to the engine speed. As this information is missing, the speed difference of the converter cannot be calculated. This means that the value is invalid. Both confirm that the primary disc set speed sensor is defective.
If the error code „0896 (P2198) – Gear ratio – transmission ratio incorrect“ is stored in conjunction with error codes relating to the transmission oil pressure, this may be caused by a worn oil pump or cracks in the valve box.
Ignoring oil change intervals and filter changes (e.g., after 60,000 km) can also lead to this symptom due to deteriorated oil quality.
Mostly in combination with the error codes:
- “P 2732 – improper opening of the forward clutch”
- “P 2722 improper closing of the brake for reverse gear”
- “P 2723 – improper opening of the brake for reverse gear”
- “0730 – improper gear ratio”.
