IIRC, on the '87, the low speed rad fan should come on as soon as you turn the A/C on.  The A/C high pressure switch kicks on the second speed rad fan.  It doesn't shut the compressor off though.  If the rad fan switch closes the second (high) temp contacts, it kicks on the high speed fan, which disengages the compressor clutch (not so much to drop the load on the engine, but to drop the heat load on the radiator).  Sounds like your pressure switch, or wiring, may be bad, and your low speed fan isn't enough to keep engine temps down, and what you heard was the high speed fan come on.  The "running after" or "dieseling" is typically high engine temps (hence the high speed fan).

With R-134A there really is no way to just add refrigerant and really get it right. Very easy to overcharge which will cause increasing problems as the ambient temps get higher.  You can get close, but often that's not good enough.

I'd turn the A/C on, with a cool engine, and immediately jumper the high refrigerant pressure switch and see if the mid speed fan kicks in.  If not, then you'll need to start with that fix.  I actually installed a rocker switch under my dash to kick the second stage fan on when I'm in traffic.

If the high pressure switch works OK, then likely the second stage fan is not sufficient to cool both the refrigerant in the condenser and the engine coolant in the radiator, so it just climbs up until the high temp rad switch closes.  If you're issues were observed at idle, that's very likely the case.

Keith Hughes
'86 Westy Tiico (Marvin)
 
 Date:� � Sat, 12 Jul 2014 20:05:01 -0700
 From:� � Mister Tom <TomsGroups@SALICOS.COM>
 Subject: 87 Westy, A/C Question
 
 In my '87 Westy, I put a couple cans of R-134 in a couple of
 weeks ago. The
 van mostly sat for three years after being converted from
 R-12 to 134.
  
 The air now blows cold, OK.
  
 Today temps are in the low 90s and I fired it up to see if
 it was still
 blowing cold.
 
 After a few minutes, what I think is the mid-speed radiator
 fan came on and
 stayed on.
 
 When I shut the van off, the van ran for just a few seconds
 and went off.
  
 Does this sound like the high pressure switch, or an outside
 air temp
 problem? I've heard that the A/C systems don't like to work
 when it's really
 hot out.
  
 Back when I added the refrigerant, temps were in the 80s.
 
 I ran the van for a LONG time and never heard the fan come
 on.
  
 I do remember the mid-speed fan coming on in the past, in
 the R-12 days,
 when the temp was really hot and we were traveling at 70 mph
 on a hot night.
  
 Thanks for any help.