SRS/Horn Clock Spring - Introduction
TO: OUR MERCEDES-BENZ PASSENGER CAR DEALERSDate: April 1993
Ref. No. MBNA 46/2
Subject:
MODELS 129,140
INTRODUCTION OF SRS/HORN CLOCK SPRING CONTACT
A. DESIGN AND FUNCTION
B. REMOVAL AND INSTALLATION OF CLOCK SPRING CONTACT ASSEMBLY
C. NOTES REGARDING TYPICAL STEERING RELATED REPAIR JOBS
Starting with February 1993 production, models 129 and 140 are equipped with a clock spring contact instead of the previously used slip rings.
Effective Serial Number
Model As of Chassis End Number
129 1F-079966
140 1A-123446
A. DESIGN AND FUNCTION
The clock spring contact gives a continuous electrical connection from the steering column to the steering wheel.
Design
The clock spring contact consists of:
^ Housing lower section (32g, Figure 1)
^ Housing upper section (32f)
^ 4 conductor clock spring cable (32e), approx. 3.5 m long, that is wound into a spiral.
Function
The housing sections (32f and 32g, Figure 1) can turn in relation to each other. The housing lower half (32g) is secured to the steering column bracket by screws (22h, Figure 1). These screws also serve as a rotation stop when the clock spring contact assembly is not installed. The housing upper section (32f, Figure 1) turns with the steering wheel. When turning the steering wheel, the clock spring contact (32e, Figure 1) is unwound when turning to the right and wound when turning to the left.
The maximum amount of rotation from the center position of the steering wheel is approx. 2 turns of the steering wheel.
The symbols on the clock spring contact represent the following installation notes:
a CAUTION, wheels must he in straight ahead position.
b Vehicle straight ahead symbol.
C Do not turn front wheels symbol.
d With the front wheels turned Straight ahead, the arrows must align with the marks on the steering shaft.
The following components were modified:
^ Steering column and components lengthened by 7 mm.
^ Steering wheel.
^ Wring harness.
^ Additional horn relay module.
^ Turn signal switch and cruise control lever.
^ SRS control module.
^ Steering angle sensor
As a result, retrofitting vehicles built prior to February 1993 is not possible.
The steering angle sensor for ADS is clipped on to the underside of the clock spring contact assembly. It has been changed from magnetic to optical sensing.
IMPORTANT NOTE:
Due to the higher resistance of the clock spring contact, the SRS control modules also had to be changed. See the Parts Microfiche for the correct part number. Installation of the new control module in earlier production vehicles equipped with slip ring contacts is not feasible, since a Diagnostic Trouble Code will be set and the SRS malfunction indicator lamp will light. The same will occur if an earlier style module is installed on a later vehicle equipped with the clock spring contact.
The fanfare horns are actuated by the fanfare horns relay module (K39) located in the fuse and relay box (Figures 3 and 4).
The relay module is located in position "E".
Layout of relay module positions in model 129
Layout of relay module positions in model 140
B. REMOVAL AND INSTALLATION OF CLOCK SPRING CONTACT ASSEMBLY
The clock spring contact can be replaced using the following procedure:
1. Engage steering wheel lock with wheels in straight ahead position.
2. Disconnect driver airbag squib connector (X28/4, Figure 5) by pulling upper section off and pressing out of holder. Disengage wiring.
3. Press signal system connector (X87/1, Figure 5) out of holder and separate connector. Disengage wiring.
4. Loosen mounting screws (32h, Figure 6) only enough to allow removal of the clock spring contact (A45, Figure 7). The mounting screws (32h, Figure 6) also serve to prevent excessive rotation and as an installation aid.
5. Remove clock spring contact (A45, Figure 7) from steering column mounting bracket. Pull wiring Out of steering column covering.
6. Installation is in the reverse order of removal. During installation, use care to fully seat the clock spring contact (A45, Figure 8) and make sure that the area with the widest tooth spacing (arrow) on the clock spring contact is aligned with the locating tab on the steering shaft (arrow).
Note:
On vehicles equipped with ADS, the steering angle sensor is clipped to the underside of the clock spring contact (A45) and can only be replaced once the clock spring contact is removed from the vehicle.
Disassembling the clock spring contact
The upper and lower parts of the clock spring contact are snapped together. The two parts will only come apart if excessive force is applied or as a result of improper assembly. If this is the case, carefully wrap the clock spring and assemble the two sections without pinching the clock spring, then recenter the assembly.
CAUTION!
^ Do not grease clock spring contact.
^ Replace clock spring contact if kinked.
Clock spring contact center position
The correct centering of the clock spring contact is crucial for reliable operation of the SRS system and horn. The center position is maintained by the mounting screws before the clock spring contact is installed.
If the clock spring contact was turned out of the center position because:
^ the steering column joint was removed, or
^ the mounting screws were removed, the center position can be reestablished as follows:
1. Screw mounting screws completely into the clock spring contact assembly.
2. Turn clock spring contact to the right until resistance is felt.
3. Turn clock spring contact approx. 2-2.5 turns to the left until a position is reached where the mounting screws can be backed out so that the screw heads protrude through the holes to maintain the center position.
C. NOTES REGARDING TYPICAL STEERING RELATED REPAIR JOBS
Steering gear, removal/installation
CAUTION!
Before sliding off the steering shaft coupling, allow the steering wheel lock to engage. After disconnecting the steering coupling, a set installation position can no longer be assured and the clock spring contact can be destroyed by excessive turning.
Steering column removal/installation
With front wheels in straight ahead position, remove clock spring contact (A45, Figure 11) with plug connectors (38a, 38b, 38c).
Steering column disassembly
Push steering shaft (22, Figure 12) out slightly and loosen support ring (20) from snap ring (21). Remove snap ring (21) and support ring (20). On vehicles with clock spring contacts, an additional snap ring (21) is used.