As previously required by law, all 1974 and some 1975 Chevrolet passenger cars were designed so that they could not be started until the front seat occupants were seated with their seat belts fastened. If the proper sequence is not followed (for example, the occupants fasten the seat belts and then sit on them), the engine cannot be started.
If, after the car is started, the seat belts are unfastened, a warning buzzer and light will be activated in a similar manner to that described previously for 1972-73 models.
The shoulder harness and lap belt are permanently fastened together, so that they both must be worn. The shoulder harness uses an inertia-lock reel to allow freedom of movement under normal driving conditions.
This type of reel locks up when the car decelerates rapidly, as during a crash.
The lap belts use the same ratchet-type retractors that the 1972-73 models use.
The switches for the interlock system have been removed from the lap belt retractors and placed in the belt buckles. The seat sensors remain the same as those used in 1972-73.
For ease of service, the car may be started from outside, by reaching in and turning the key, but without depressing the seat sensors.
In case of system failure, an override switch is located under the hood. This is a "one start" switch and it must be reset each time it is used.
On Chevrolet products, this system consists of seat belt retractor switches, pressure sensitive front seat switches, a parking brake switch (MT), or a transmission switch (AT), a warning light, and a buzzer.
The seat belt warning system is wired through the 20 amp "Gauges" fuse.
The warning light is located in the instrument cluster; and the buzzer, which is shared with the ignition key warning system, is taped to the instrument cluster wiring harness.
The warning system is activated when the ignition switch is ON, the front seats are occupied, and the seat belts are left in their retractors. Only when the front seat belts are extended and properly fastened will the warning light and buzzer stop.
Two different types of switches are used to control the operation of the system, depending upon the type of transmission used:
On manual transmission-equipped cars, the parking brake warning light switch is used to activate the seat belt warning circuit through a relay as soon as the parking brake is released. A diode is used in the circuit to prevent feedback to the brake warning system.
On automatic transmission equipped models, the seat belt warning system is activated by the neutral safety/back-up lamp switch, when the car is placed into any forward gear and the seat belts are not used.
DISABLING THE SEATBELT INTERLOCK SYSTEM
Since the requirement for the interlock system was dropped during the 1975 model year, those systems installed on cars built earlier may now be legally disabled. The seat belt warning light is still required.
- Disconnect the negative battery cable.
- Locate the interlock harness connector under the left side of the instrument panel on or near the fuse block. It has orange, yellow, and green leads.
- Cut and tape the ends of the green wire on the body side of the connector.
- Remove the buzzer from the fuse block or connector.