1800ES wiring
 

Am I understanding the (72) 1800ES wiring diagram correctly in concluding that the reversing (back up) lights relay in manual cars was repurposed as a seat belt warning relay in BW35 automatics?

If so, the seatbelt warning light on manual cars operates without a relay, but is apparently wired through a gearbox "contact". What would that be? On the M41 which I am installing there is the fourth gear switch and solenoid for the overdrive, and the switch for the reversing light, but nothing else apparently electrical. Am I missing something?

Not exactly. Its repurposed as the starter interlock relay (#87 in the attached BW35 wiring diagram). Perhaps they could get away with that because item 64 on the BW 35 wiring diagram which controls the relay and the back up lights might be a real switch as opposed to that sorry excuse for back up control on the early M41

On an early M41 the electrical bits are limited to the 4th gear interlock switch, the solenoid and the reverse light contact (I hesitate to grace that thing with the the high technology term ' switch'). The later M41 had a different switch arrangement which included a cover mounted switch for I believe the control of the back up light and the seat belt buzzer.

https://vp-autoparts.com/en/artiklar...s_140_164.html
https://vp-autoparts.com/en/artiklar...act-3-pin.html

That's really helpful thankyou. I couldn't work out why the seat belt warning buzzer would be wired through the reversing light switch but I assume it is so that it does not sound if you release the seat belt to aid reversing?

Its the starter interlock part that has me confused. The starter interlock relay (87) can only close when the contacts on switch 64 are closed. As shown, the two sets of contacts in switch 64 are linked and operate together. Presumably, because it functions as a reverse light switch, both pairs of contacts are only closed in reverse which suggests that you can only start the engine when in reverse. That would be strange. There has to be something in the switch arrangement that is not apparent in the diagram; but, that is the 'way she is wrote'.

That is indeed how she is wrote but, as you suspect, not quite how it works (because it wouldn't). On the BW35 that switch 64 has 4 pins (poles?). 2 and 4 close when the box is in reverse. 1 and 3 close when the box is in Park or Neutral.

That makes sense for the starter, but less so for the seat belts. The way it is shown, the seatbelt warnings would only show in Park or Neutral?