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]

This video shows behavior of the relay under a couple conditions

Starts with power being switched on as if someone is sitting in driver seat - unbelted

Seat belt is then Fastened (removal of switched ground from pin GF)

Then Key turned to start (yellow +12v applied to pin C) which is shown by the lamp between pin C and pin 50 lighting - as if the starter was now being allowed to crank the engine.

So that is the basic start sequence

After that the video proceeds with unfastening the seat belt (switched ground added back to pin GF)

After that we see that adding a switched ground to pin A - simulates applying the park brake. Ground on pin A = silence the lamp and buzzer

Then the switched ground is moved over to pin B. Pin B represents having oil pressure . As DaveP correctly stated earlier if the relay thinks the engine is running - or still has oil pressure while it bleeds down - the lights and buzzer are disabled and the vehicle should be able to be restarted. NOTE: this restart attempt is NOT shown in the video - doesn’t always work - I’ll come back to the later regarding anomalies I’ve seen when testing this relay

After moving the switched ground back to pin A (park brake applied) and the seat belt unfastened - 12v is again applied to pin C, we see the FSB lamp lights and the buzzer comes on but the starter interlock isn’t enabled - basically reminding the occupant to fasten the seat belt before the engine can be started

The seat belt is then fastened (switched ground removed from pin GF) and 12v is applied to pin C and we can see the vehicle would be allowed to start


https://www.youtube.com/watch?v=qo55bVLfsYM?si=ZCez5BhdQteaLknO