See Figures 1, 2, 3 and 4
Beginning in August of 1973 (1974 models), all Mercedes-Benz cars conformed to the regulation requiring a starter interlock system that prevented starting the engine if the seat belts were not buckled. To eliminate the possibility of defeating the system by permanently buckling the seatbelts, the system required that buckling the belts and starting the car take place in a preset sequence. Each front seat contains a contact switch that closes when the seat is occupied. The buckle on the front seat belts also contains a switch that closes if the belt is unbuckled. 1974 cars can be started by reaching in through the open window and starting the car with the key.
1975 models are equipped with the same basic system, but with an additional override switch, located in the engine compartment. In case the engine cannot be started due to a malfunction in the seat belt warning system, the starter interlock can be bypassed for ONE starting attempt, by pushing on the button on the switch with the ignition ON and the transmission in N or P. As soon as the transmission is shifted out of N or P, or the ignition is turned OFF, the relay in the over-ride switch is opened. To repeat the process, the switch must be depressed again.
DISABLING THE INTERLOCK SYSTEM
As a result of Federal legislation, the starter interlock system used on 1974-75 cars was replaced with a light and buzzer reminder system. The new law, which took effect 12/26/74, permitted the disconnection of the starter interlock system (but not the warning light) and Mercedes-Benz does not advocate that this be done.
To bypass the interlock feature on all models so equipped (with or without an over-ride switch), replace the seatbelt logic relay (Part No. 000 545 69 32 or 000 545 68 32) with a new relay (Part No. 001 545 00 32). This will disable the interlock feature, but still allow the warning buzzer and buckle-up sequence to remain in effect.
Under no circumstances should the interlock system be bypassed by bridging either the over-ride switch relay or the logic relay, since this could allow the car to be started in gear.