I had same problem with Stannah Stair lifts, I did in the end find a wiring diagram of sorts, but it's one of those where there is a block which it does not show what is inside the block. As it if a stair lift comes under same rules as standard lifts I don't know? Unlike a standard lift an able bodied person can simply stand up and walk away from it, it it fails one is not trapped in a box.
However when for the use of the disabled it is very different, and my mother with only one leg would be trapped if the stair lift failed, it would require call to fire department to rescue her.
I have found the Stannah we have installed has safety features which make it less safe, it will not allow one to descend until the leg at bottom is extended, so the leg has to be extended from a position where it is out of sight. When the battery voltage drops it stops, I can see how it would need in the end to stop, but would have expected a buzzer or other indicator first to warn battery becoming discharged, before it finally stops. There are no safety beams or switches on the leg which needs extending before it will travel, although loads of safety switches on the lift its self.
So in real terms they are not really suitable when the user relies on them, they are only to make life easy. Yet fitted by council for my mother with one leg. I find it is great for sending up washing and things, but not really enough room to transport me, my legs are too long.
It has failed, for two unconnected reasons. First was simply a faulty battery, charge voltage is 29 volt which to my mind is too high, so if used every day it's OK, but once a week and batteries don't last long, and then one has the job of manually winding it back up stairs which takes ages, due to extending leg it can't be left down stairs. Second fault was a multi-pin connector coming apart under the seat. It only needed plugging back in, but it took ages to find. For that fault I was trying to use the circuit diagram to trace the fault, but at the end of the day it was my son who found the fault by simply ripping off anything which hid wiring harnesses.
Being fair to Stannah the lift is around 15 years old, but hardly used. It is so slow you would only use it if forced, except as a dumb waiter. When really lazy I send up breakfast for wife on it, saves spilling coffee on the stairs.
What seems odd is the way everything is RF linked, the leg being extended is RF linked, the wall mounted call buttons RF linked, they need a PP3 battery to work, the whole chair is battery operated, the 230 volt supply only re-charges the battery at top or middle of stairs, well there is a charge point at bottom, but that would block access to front door.
There is a line of LED's to show what it is doing, but you can only see them with safety covers off. It's like one of those projects you do in University, they show what could be done, but no one has done a proof testing on it, and it needs more work before it would be commercially viable. I keep it running as it suits me, but if I was to rent out the house, it would need ripping out. For one thing can't get a double bed upstairs.
I wish you luck, I hope if you understand some of the general failings with stair lifts, it may point you in right direction?