BMW 540i 1995 - Place key in ignition and often dash shows 'S4' instead of 'AP' Engine will turn properly but not fire. Turn key off/on a few times and dash will eventaully show 'AP' at which time it will start and keep running**.
I have found that if I get the S4 and only try to start for 2-3 seconds and then turn off/on I will almost always get the 'AP'
**Lately, I have had two problems where the car will cut out when running and dash shows S4. Turn key off/on and it will start. I have stopped driving. Local BMW says may cost thousands to diagnose.
Is there some sensor that detects transmission status?
Thanks
Glyn

Optional Information:
1995 BMW 540i 8 cyl

Already Tried:
New switch Auto/Manual