Early 72' & 73' Seat Belt Warning Lamp, How does it work? Options

[Superhawk996]

Ever wondered how the seat belt warning circuit works in early 72’ & 73’s?

Lets start with the description of operation in the owners manual:

“An audio-visual warning system will remind the driver and the passenger to put on their safety belts. The buzzer will sound and the fasten seat belt sign on the dashboard will light up when the ignition is on and the parking brake is released. The warning system will also be activated if the passenger is not wearing the seat belt.”

The entire “logic” to make all this happen is contained in the seat belt warning light. The whole thing is on a small PCB (see picture below). There really is no smarts or logic in the modern sense. It is just a very simple transistor based circuit that reads switch inputs.






Note: one of the original diodes in this picture has been swapped for a 1N4001.

The vehicle side electrical connector slides onto the four PCB traces to make the connection to the circuit board.

The schematic is contained in the Haynes Manual for the 73’ model year. In my copy of Haynes the 72’ schematic is there but isn’t printed as clearly not sure if this is the case with all printings. Item 66 is the buzzer. The buzzer (which is really just a solenoid) is self actuated meaning that when 12v and ground is supplied to it, the solenoid repeatedly makes & breaks the connection that energizes the solenoid, causing it to "buzz". The buzzer has 12V to it at all times and the ground path is supplied though the transistor on the PCB.




When the Fasten Seat Belt lamp is on, the seat belt buzzer will also be on. From here on out, I’m going to neglect to mention the buzzer. But remember, if the Fasten Seat Belt light is on, the buzzer would be on also and is sourcing current through the Fasten Seat Belt PCB circuit.

NOTE: Special thanks to Montreal914 for donating this part for me to document and modify

[Superhawk996]

Operation:

Driver Only – seat belt unbuckled. (Lets call this the default condition):

Ground is passed though the driver side seat belt latch switch because its normally closed. That ground signal goes to pin “G” on the seat belt warning light and causes the transistor circuit to light the Fasten Seat Belt lamp.

Once the driver buckles the seat belt, the normally closed switch is opened, the ground signal cannot pass, and the Fasten Seat Belt lamp is turned off.


Passenger Only – seat belt unbuckled:

Ground is passed though the passenger side seat belt latch switch because it’s normally closed. The signal then goes to the passenger side seat cushion switch.

If no one is sitting in the passenger seat, the ground signal cannot pass to the warning lamp because the seat cushion switch is normally open. The Fasten Seat Belts lamp remains off.

If someone is sitting in the passenger seat, the seat cushion switch is closed (by the passengers weight) and the ground signal from the unbuckled latch is able to pass on through to the warning lamp pin “G”, lighting the light regardless of whether the driver is buckled or unbuckled.

When the passenger buckles the seat belt, the normally open switch at the buckle receiver is opened and the Fasten Seat Belt lamp is turned off.