Overvoltage Relay/Harness - Update
REF. NO. 54/45TO: OUR MERCEDES-BENZ PASSENGER CAR DEALERS
DATE: November 1987
SUBJECT:
A. OVERVOLTAGE PROTECTION RELAY
B. ENGINE WIRING HARNESS
MODELS 107, 124, 126, 201
A. Overvoltage Protection Relay Models 107, 124, 126, 201
On the above mentioned vehicles a 4-pole, 5-pole or 7-pole overvoltage protection relay may be installed depending on vehicle model (Figures 1-3).
Note:
The 4-pole relay is no longer available, and when necessary, is to be replaced with the 5-pole relay.
The overvoltage protection relay protects the control unit(s) of various electronic systems (e.g. CIS-E, ABS, etc.).
Note:
A 9-pole relay is in use on European version vehicles equipped with 4MATIC, ASD or ASR. The 9-pole relay will be available on U.S. vehicles with the introduction of one of these systems.
On 4 and 5-pole relays battery voltage is continuously supplied via circuit 30. On the 7 and 9-pole relays, the voltage is additionally supplied to circuit 30a via the 10A relay fuse.
In steering lock position 2, voltage is applied to circuit 15 and the relay closes the circuit. Voltage is supplied to circuit 87 (5-pole relay) or circuits 87E and 87L (7 and 9-pole relays).
When the main power supply exceeds 22 volts, the Z-diode (zener diode) directs the voltage to ground, causing the fuse to blow.
In case of a voltage overload occuring through the overvoltage relay, the electrical circuit is interrupted by the 10A relay fuse.
Note:
The 9-pole relay is separately fused on circuit 87E and 87L, whereby in case of an open circuit (blown fuse) on 87E, circuit 87L will also switch off (see Figure 4).
Testing the Z-diode in the Relay
a. 4- and 5-pole relay
Perform the test with the ABS test adapter 126 589 09 21 00 and the intermediate plug 126 589 15 63 according to test step 12.
b. 7- and 9- Relay
Perform test is describes using a multimeter.
Special Tool Information
Conventional Tools
Parts Information
B. Engine Wiring Harness
Models 107, 124, 126, 201
Engine wiring harnesses will now only be available with a 7-pole connection for the overvoltage protection relay.
When installing a new engine harness with a 7-pole connection into a vehicle with either a 4- or 5-pole connection, it is necessary to also replace the overvoltage protection relay and the bracket for the control units.
Note:
On models with ABS, connect the voltage supply line for the ABS control unit to terminal 7 of the separate 3-pole connector of the overvoltage protection relay connector (Figure 6).
The cable connector of the removed bracket for the control units is to be replaced with a new cable connector and secured with cable straps to other cables in the component compartment behind the battery (Figure 7)
Parts Information