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]

I know almost no one cares about the internal details but tough luck - I do so skip the nerd stuff if you don’t care.

I don’t know exactly what I was expecting to happen when the seat belt and cushions conditions were met but I was greeted with nothing. No activation of the starter solenoid relay . . . Nothing just silence. For some reason I was sort of expecting that relay to pull in on its own but it didn’t.

Wanting to know more about how this black box works - I started digging into and documenting the circuits within the black box.

What I found was that the starter interlock solenoid needs to have 12v on pin C in order for the relay to actuate. This is because Pin C is attached to the solenoid coil on one side. The other ground side is supplied inside the black box by a transistor (T9) that is turned on allowing a path to ground for the other side of the solenoid coil. Of course this transistor is only turned on when the seat belt has been fastened.

Here’s my crude notes for those that are really interested in the gory details.



Part of my goal here was to be able to troubleshoot and repair a defective interlock relay (more on that later).

In hindsight the fact that the solenoid needs 12v on pin C makes sense. It wouldn’t make sense to keep the solenoid relay within the interlock relay energized all the time. This way it only temporarily energizes when the key is actually in the Start position.

I’m also a little surprised by how little the relay is considering that the actual starter solenoid in the stater motor pulls about 25 amps through this interlock relay. Strangely - not one of the three interlock relays I have looks the least bit worn or burned though. It certainly worked when these cars were new.

I sort of suspect that many of these interlocks got removed over the years simply because they are a black box and it’s not intuitive to understand what’s going on in that little silver box. So if someone was having starting issues for other reasons (bad ground strap for example) it would be easy to see how pins C and pin 50 got a jumper - end of story. Don’t get me wrong - I don’t think this is a good design.

To me it’s just an interesting time capsule of 70s vintage electronics and VW / Porsche trying to meet the mandates imposed by new FMVSS regulations.