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]

So this is where things start to go astray.

I cannot duplicate the “logic” operation of the training information.

I have two different interlocks that appear to be functional. Neither seems to require that the seat cushion be loaded to enable the starter. I’ll detail this more later in the thread as it progresses.

Second, I have been unable to verify that there is a time aspect as referenced for a restart within 2 minutes +/- 1 minute.

There are 3 electrolytic capacitors in the circuit and it is possible that they are using a RC time constant to affect the circuit. I just haven’t been able to duplicate this and this is why I intend to put the circuit into LTSpice so I can better understand why this isn’t working as it is described by that training document. I am currently working on verify the capacitors. They are 50 years old and although they measure at their rated capacitance, they seem to leak a little more current than they should.

[5/16/25 update - I’ve replaced the 3 electrolytic’s in the best operational relay - still no change in being able to get a seat cushion dependency or a time dependency]

The one thing I haven’t been able to find is a 74’ -76’ owners manual to see how the operation is described there. If anyone cares to post that it would be appreciated.

From here on out I’ll be posting the actual results I’ve observed bench testing the two “functional” relays I have.