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.

[davep]

Just saw this thread. I worked on this about 3 years ago. There appears to be about 4 circuits: 2 x 74, 1x 75 1 x 76
1974 there is a ground terminal (1/4" spade) riveted to the floor pan, and is either used or not. This uses the two relay system.
1975 uses just the dual relay
1976 uses just a single relay

The two minute restart delay comes from the oil pressure switch, and the time that it takes for the oil pressure to drop, the switch closes, and the idiot light comes on. SO, if you want to thwart the interlock system, then just pull the oil pressure switch wires from the relay and join them together so that the idiot light works as usual. Then the logic relay is tricked into thinking the oil pressure is always high.

I have some better drawings if you need them.

[Superhawk996]

The two minute restart delay comes from the oil pressure switch, and the time that it takes for the oil pressure to drop, the switch closes, and the idiot light comes on.

I have some better drawings if you need them.

@davep glad to hear that about the oil pressure input onto pin “B” - that was the only way I can see the time dependency being met based on the bench testing.

I was thinking that but didn’t quite want to say that out loud just yet. 2 minutes +/- 1 minutes is pretty ambitious for oil pressure bleed down. May just have been my cars. While I can see 1 minute being pretty reasonable I’m not sure any of my cars held oil pressure switch off for 3 minutes after a shutdown when hot.

By all means if you have info to add about year to year dependency - post it up. Since I have never owned anything other than 73’s I’m at a disadvantage on this little journey.