Modifications To Automatic Locking Differential (35/74)
Publication date: 1992-11-01Reference number: 35/74
MODIFICATIONS TO AUTOMATIC LOCKING DIFFERENTIAL
TECHNICAL SERVICE BULLETIN
MODELS 124.128, 129.061, 140.134, 201.028/029 - MODIFICATIONS TO AUTOMATIC LOCKING DIFFERENTIAL (ASD)
Model(s): 1993 Mercedes-Benz 190 E, 300 E, 300 SD, 300 SL
Group: 35 - Rear Axle, Automatic Locking Differential (ASD), Electronic Traction Support (ETS)
Bulletin No.: 35/74
Date: November 1992
SERVICE INFORMATION
As of June 1992, a modified automatic locking differential containing a hydraulic unit without pressure reservoir has been installed. As a result, the following components have been changed:
- HYDRAULIC UNIT
- ASD CONTROL MODULE
- RING CYLINDER - REAR AXLE CENTER PIECE
A: HYDRAULIC UNIT
The ASD hydraulic unit no longer contains a pressure reservoir. See Fig 1 . When ASD is activated, the oil pressure from the pressure oil pump is directed via the hydraulic unit to the ring cylinders in the rear axle center piece. The system pressure ranges from 50-62 bar depending on engine speed and delivery of the hydraulic pump.
REPAIR NOTES
- The hydraulic unit with pressure reservoir is replaced by the new hydraulic unit without reservoir. Only hydraulic units without pressure reservoir will be supplied as replacement parts.
- In the case of the new hydraulic unit, an expansion line in the HS and T lines is no longer installed. In cases of hydraulic unit replacement, the existing expansion lines may remain in place.
- The connections and lines in addition to the body mounting points are the same as for the hydraulic element with pressure reservoir. During replacement, note that the lines are connected correctly. See Fig 2 .
- The ASD solenoid valve (Y38) of the hydraulic unit cannot be exchanged separately and is no longer available as a spare part. See Fig 2 .
- The system pressure can be measured at the HS line on the rear axle center piece. Use pressure gauge 126 589 14 21 00 for pressure testing. See Fig 3 . Test values are 50-62 bar at an engine speed of 650-750 rpm. Before testing, on models 124, 129 and 201, the ASD control module must be removed and sockets 8 and 10 bridged in the control module plug. On model 140, remove the ASD control module, connect the socket box and bridge sockets 2 and 27 at the socket box.
B: ASD CONTROL MODULE
The control module (third generation) recognizes and stores several diagnostic trouble codes (DTC's). In case of a fault, the ASD is continuously shut off, or shut off only until the next engine start. In cases of faults indicated by diagnostic trouble codes 3-8, the ASD system is only then continuously shut off if the fault occurs at least five times in a row. In case of a control module fault (diagnostic trouble code 2), the ASD is continuously shut off. This diagnostic trouble code cannot be erased and the control module must be replaced. The diagnostic trouble code memory has been expanded to include diagnostic trouble code 9, allowing the recognition of incorrect tooth count readings at the rotors. If a malfunction with trouble code 9 occurs, the ASD is immediately shut off. This trouble code may also be erased; however, the control module must be reinitialized afterwards (refer to repair step 3). The control module software is set up for front wheel hubs with 96 or 48 teeth. For this reason, the number of various control modules is kept to a minimum.
PROCEDURE
- Diagnostic trouble code readout and erasing can be performed using impulse counter scan tool, special tool no. 124 589 19 21 00. Trouble code readout via the ASD malfunction indicator lamp is no longer possible. Stored trouble codes are read out in order from 2 to 9. The trouble codes must be read out and system trouble codes eliminated until code 1 once again appears. The trouble codes may be erased only after having been read at least once. When erasing the stored trouble code, only the trouble code displayed is erased. All codes must be erased individually.
- Pins 7 and 9 have been removed from this third-generation control module; however, pin 6 is new. See Fig 4 . Pin 6 leads to the diagnostic connector (X11/4), allowing retrieval of stored codes, if any. When installing the new control module in place of the previous module, pin 2 still functions as a diagnostic trouble code output. In addition, a plastic plug in socket 6 of the control module connector must be removed. See arrow, Fig. Fig 5 .
- During initial operation of a new control module, the ASD malfunction indicator lamp blinks and goes out first when the rotors on the front and rear axles are synchronized (initializing). A synchronization is accomplished if the vehicle speed is held for a short time above 30 km/h (19 mph) without braking. In models with the more powerful engines, the synchronization occurs over a longer driving distance. During synchronization, under acceleration, the function indicator lamp also comes on.
C: RING CYLINDER - REAR AXLE CENTER PIECE
The material for the O-rings (70i and 70h) on the ring piston (70b) has been changed. During repair, only these new O-rings may be installed.
PARTS INFORMATION
See respective parts microfiche for spare parts information.