Seat Belt Interlock Relay, Late 74-76’ - how does it work? Options

[Superhawk996]

For those of you that despise the seatbelt / starter interlock and the horrific screeching noise the seat belt buzzer makes - let me say I understand completely. As I’ve been playing with this on the bench my wife now hates the noise just as much as I do.

This thread is going to explain how it works and how to bench test it over the coming days.




Credit is owed to @JeffBowlsby , @Bdstone914 , and @Montreal914 for providing me with donor parts that will make this thread possible.

Eventually I will post the schematic of this ridiculous conglomeration of components. I hope to eventually put the circuit in LTSpice to help verify its theoretical operation. I make no warranty as to the accuracy of the schematic - this is just me satisfying my own curiosity.



If you have no interest in my personality defects that would lead me to take on this little project; ignore this thread.

If you have a late car with an in-op system and eventually want to get it working - this thread is for you.

Toward the end I’ll share some ideas on how to get rid of or to change that horrific buzzer noise.

[Superhawk996]

The Fasten Seat Belt light is powered by 12v supplied to one side of the bulb

The light is turned on/off via switched ground coming from the interlock relay on pin “L”,



In the photo above you’ll notice that with no inputs from the seat cushions and no input from the seat belt latch switches - the Fasten Seat Belt (let’s abbreviate as FSB) light is not lit. The buzzer is not active.

This setup represents:
Key in ignition - 12v on pin 85
Ignition on - 12v on pin 15 and 30
Doors closed - no ground connection on pin 86