Airbag - Production Modification SRS Control Unit
REF. NO. 91/43TO: OUR MERCEDES-BENZ PASSENGER CAR DEALERS
DATE: April 1990
SUBJECT:
A. MODIFIED SRS CONTROL UNIT
B. SRS CONTROL UNIT PARTS PROCUREMENT
C. REVISED SRS TEST CARD
MODELS 107, 124, 126, 129, AND 201
A. Modified Control Unit
As of July 1989, only SRS control units with a reed contact switch, instead of the previous mercury switch, are installed. (The mercury switch or reed switch functions as a mechanical "safing sensor" and must close before the electronic sensing system can trigger a deployment).
The modified control units are recognizable by the raised oval on the control unit housing (1, Figure 1).
Important Note:
Defective control units with the raised oval which are out-of-warranty may be discarded without further precautions.
Defective control units without the raised oval (mercury switch) must be returned to MBNA, Warranty Inspection regardless of whether or not warranty is still in effect. Federal law prohibits the disposal of any mercury product (except as a toxic waste), even in such small quantities as contained in the control unit's mercury switch. MBNA will assume the responsibility of toxic waste disposal after the mercury has been recovered from the control units. In-warranty units will be requested for return by MBNA via the Parts Return Request form. Dealers should collect out-of-warranty warranty units and periodically ship them to MBNA, Warranty Receiving Department.
Control units with the raised oval which are under warranty shall be returned in the usual manner.
B. SRS Control Unit Parts Procurement
Parts Information
As of July 1989, after existing supplies of the previous version control unit have been exhausted, only the modified version is available as shown.
Note:
When using the modified version SRS unit as a replacement for previous version control units up to 9/87 (production code 749), The energy accumulator and voltage converter installed in the vehicle are no longer required as these are integrated into the modified version control unit's design.
However, the energy accumulator and voltage converter will continue to be available as separate units should their replacement become necessary (while retaining the previous version control unit)
Note:
When replacing a SRS control unit manufactured prior to 9/87 (production code 749) with the modified version, the existing voltage converter as well as the energy accumulator must be removed from the vehicle and discarded (if installed).
The wire ends should be insulated with electrical tape and secured to the existing harness (2 & 3, Figure 2).
In addition to the installation of the modified control unit, part no. 004 820 11 10, either harness "A" or "B" (Figure 3) must be used depending on which control unit was installed in the vehicle (see "Cross Reference: Control unit/harness").
Cross Reference: Control Unit/Harness
A reminder that the modified version of the SRS control unit has been installed, and that a revised diagnostic test must be performed, is imprinted on the top of the test connector (as "Airbag 1/3, modified test"). The modified SRS control unit made a revision to the diagnostic procedure necessary. The revised diagnostic test card should be kept with the tester. All previous versions of the card are invalid and should be destroyed.
Installation of the modified SRS control unit, part no. 004 820 11 10, will cause the SRS warning lamp to go out 4 seconds after turning the key to steering lock position 1 or 2 (10 seconds with previous version control unit).
When disconnection the 10-pole plug connection (X29/9), pins 3, 5, 6, 7, 8 are short-circuited on the lower half of the plug.
When disconnecting the connection for the ETR squibs (R12/1, R12/2) and the airbag squib (R12/3), they are short-circuited.
In Case of Warranty:
C. Revised SRS Test Card
As a result of the modifications to the SRS covered in this Service Information, the SRS test card used with tester 126 589 10 21 00 has been revised.
The revised test card (MBNA/MBC 2277 [3/89]) is intended for use on model year 1984 through 1987 vehicles only. Diagnosis of model year 1988 and later vehicles should be made using an impulse counter and respective diagnostic literature. The card should be kept with the tester and the previous edition should be discarded.
Additional test cards may be ordered from Service and Parts Literature in the usual manner.