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]

Here's how it works (Big Picture Overview):

Power is supplied into the seat belt warning light via pin 15 coming from the ignition switch. When the ignition is ON, there is +12v supplied to the warning lamp circuit at all times.

The seat belts buckles receivers (both driver and passenger side) and the passenger seat cushion switch are switched ground circuits that ultimately provide a ground to the seat belt warning lamp to turn the Fasten Seat Belt light on depending on the switch conditions.

I will cover the details of how transistor circuit works later in this thread.


Here are the basics of how the seat belt and switch cushion switches work before moving on:

The driver and passenger seat belt switches are normally closed. i.e. when the seat belt isn’t fastened, the switch is closed and able to pass the ground signal.

The passenger seat cushion switch is normally open. i.e. unless someone is sitting in the passenger seat the seat cushion switch is unable to pass the ground signal though it.