VW error codes stored in the transmission control unit:
- “P1735 (10666) Position sensor clutch 1, electrical fault”
and/or
- “P1736 (10668) Position sensor clutch 2, electrical fault”
indicate an electrical fault in the pressure sensors in the transmission mechatronics.
The VAG DQ381/DQ500 dual-clutch transmission has two pressure-controlled clutches that perform the gear changes in the sub-transmissions. Pressure sensors are used to measure the pressure required for this in the hydraulic circuits.
System description:
The abbreviation DCT stands for dual-clutch transmission.
This automated manual transmission automatically changes gears and disengages and engages the clutches.
The special feature of this transmission is that the manual transmission is divided into two “sub-transmissions” (TG1 and TG2) and two clutches (K1 and K2).
To change gears, the active sub-transmission is disengaged with the corresponding clutch and the second sub-transmission is engaged with the preselected gear and the corresponding clutch. A gear change is equivalent to a change of sub-transmission with each shift.
All actions, such as shifting and coupling, are performed hydraulically. The pressures of both sub-transmission hydraulic systems are monitored by one pressure sensor per system.
The pressures thus represent one of the basic functional pieces of information for the sub-transmissions.
Fault description:
The error code does not refer to a clutch position sensor actually installed in the transmission. A pressure-displacement calculation model is used to determine the clutch position.
This is supported, among other things, by the measured pressure in the hydraulic systems of the TG1 and TG2 sub-transmissions and thus the K1 and K2 clutches.
The calculated clutch position is therefore based on pressure measured incorrectly by the pressure sensors integrated in the control unit circuit board. The control unit compensates for the deviating pressures by means of clutch adaptations.
During the phase of sensor drift, the clutches are subject to increased wear and the transmission changes its shifting behavior, possibly leading to emergency operation.
Possible causes:
- Electrical defect in the pressure sensor for sub-transmission 1
- Electrical defect in the pressure sensor for sub-transmission 2
- Metal-containing dirt particles have entered the pressure measuring chamber of the sensor
Consequences of the fault: The transmission only shifts up and down with one sub-transmission in gears 1-3-5-7 or 2-4-6-R with very delayed shifting. Due to the large gear jumps, the gears are held for a long time and the subsequent changes are noticeably hard. As a result, the gear indicator in the instrument cluster flashes until the aforementioned faults are rectified. The transmission is in emergency mode.
Diagnosis:
For diagnosis, check the parameters/actual values for transmission oil pressure when the engine is not running and the transmission is in “P” mode. Under these conditions, the values should show 0.00 bar on both sensors.
The pressure displayed by the sensor in hydraulic circuit 1 of clutch 1 is correct. The pressure displayed by the sensor in hydraulic circuit 2 of clutch 2 deviates significantly from the offset pressure (0 bar).
