'86 100 Auto problemlo

[Nigel Houghton-Allen]

Hi all
my 86 100 avant is no longer avanting down the highway.
Symptoms as follows; Park, reverse and N all seem to work. No drive to the
wheels  in D -2 -1. There is no 'clunk' as I shift from N to D. The engine
revs freely in D-2-1. The transmission was working very well before, no
noises, clunks etc. I took off the T-Bar shifter housing and could see the
shifter and cable moving all the way from that end. [Naturally it was dark
and raining when this happened!!]
The only thing I did was slip the shifter from D to N at the lights asI have
just got the car and had adjusted the idle down, but when the a/c comes on
the revs are too low and the car vibrates. I was just turning the a/c off
but this once I slipped it into N to see if the car idled ok at those revs.
Idled ok but then no drive!
I would have thought some solenoid or electronic adjustment would increase
the engine revs when the a/c comes on. Is this true? If so where is it?
I have only driven the car 100km since I got it, so its a nice surprise
after getting it registered yesterday after doing lots of fixing. Any way
any help or suggestions will be appreciated before I find a local auto
mechanic [Albert Park - Melbourne,Australia! -about 1.0km from the F1 GP
Track]
many thanks
Nigel