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]

OK. Been sidelined for a little while but time to get back to this insanity.

Thanks again to @JeffBowlsby @BdStone914 @Montreal914 for the parts to play with -- this would not be possible without you guys (IMG:style_emoticons/default/smilie_pokal.gif)

I've been working on reverse engineering the circuit and getting it put into LTSpice. Mostly for my own entertainment and education.

This little relay does some odd things which I mentioned earlier. LTSpice is confirming that some of the behaviors I'm seeing appear to be inherent to the circuit design and how it powers on. I haven't gone though them all but here's an example.

When the circuit first powers on (key on but not trying to Start):

If either of the seat cushions are occupied and a seat belt is fastened, the Fasten Seat Belt Lamp and the buzzer come on for about 1/4 - 1/2 second then shut off as power is applied. The vehicle start interlock relay engages and the vehicle would be allowed to start if the key is moved to start position.

I have two relays that do this but the time they are on varies between them. I suspect this is related to the fact that one relay, I've replaced the elecrolytic capacitors with new ones while the other one still has 50 year old capacitors in it.

The relay with the fresh capacitors stays on longer (almost 1/2 second) while the old unit is very short (like 1/4 second) but is noticeable.

I know this is a complete boondoggle given most of these systems have been either bypassed or completely disabled but I'm wondering if anyone else has a functional late (74' - 76') seat belt interlock system and wondering if others have seen this behavior from the lamp and buzzer.