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]

Circuit Overview:

There is a block diagram schematic of the Fasten Seat Belt warning circuit in the Haynes and factory manual. If you understand electronics & schematics, there is enough information present to see how the circuit operates as outlined above.

The transistor is just acting as a electronic switch for the warning lamp bulb and the buzzer which are in parallel. The transistor allows current to flow though the Fasten Seat Belt lamp and the buzzer.

The big picture is that when 12v is on pin 15 and ground is applied to pin “G”, the transistor switches “ON” and lets current flow of the Fasten Seat Belt light & buzzer (via pin 86) to pass through the transistor.

If Ground is removed from pin “G” of the warning lamp by opening the seat belt switches or the passenger seat cushion switch, the transistor switches “OFF” and doesn’t allow current to flow through it.

I don’t suspect anyone cares about the math or the details behind how the transistor is biased. The short version is that the 3 resistors on the PCB set up a simple voltage divider to bias the transistor such that its just sitting there waiting to see a ground on pin “G” and when it does, the transistor is switched on and can flow current to the lamp and the buzzer.