starter cranks but no ignition

[gordy.schesel]

Seeking news of anyone else experience this problem.
 
I had a recent no start problem on my '91 100 quattro.  A Friday
morning a week ago, it cranked fine but would not start.  I left
it parked in the driveway and used the 200 to drive to work.
Towed it to my mechanic on Monday night.  He was unable to get it
to start outside his garage so pushed it into a stall.  It then
started fine.  This has never happened in the almost 10 years I
have been driving it.  My mechanic was unable to locate a
problem.  Other symptoms: the horn would not work with the key in
the on position, the gas gauge barely left the E mark when the
tank was half full, the battery voltage on the gauge read 12 or
more volts; where I would call normal prior to a start with the
key in the on position.  That is all I noticed..
 
My question:  I know the problem is not the starter.  But with
232k miles on the ignition key lock, is there an electrical
contact that may not have worked inside the key lock?  I have the
Bentleys, so I can take a look at the wiring diagram for a link
to an ignition control unit? or something else otherwise?  I
admit the key lock has gotten to feel like it is wearing out.
 
Could this related to the auto-check system? It has become
problematic as of some months ago, giving me the audible and
visual brake light out indication, although there are no bulbs
out. The auto-check brake light out alarm used to come and go as
I was driving, but since this no-start problem came and went
away, the auto=check alarm has changed it's behavior...it now
comes on and beeps once immediately after the successful start
with the light staying on but no more beeping.  I don't think it
is related.
 
I think I may need to install a new ignition key lock.  The
auto-check alarm problem may be remedied by replacing the
instrument cluster and maybe the auto-check control unit also.
 
TIA for any BTDT.
 
Gordy in St Paul
91 100 quattro sedan, 232k miles
91 200 20v, 118k miles, avant