Weird AC Behavior (sorta long reply)

[Mike Veglia]

Several things combined to solve "Weird AC Behavior" on my '87 5ktq a few
years back (never mind that I have no AC behavior now--pending finding time
to install the used compressor sitting in a box in my house).

What you're looking for are either vacuum leaks, or vacuum restrictions in
the climate control system. I had both. One good tip off is how well do your
diff locks work? If they are sluggish it may be a clue that the vacuum
source (check valve, reservoir, etc.) could be part of the problem.

Common problems include:

Heater valve vacuum leak (these should be replaced periodically anyway since
they are plastic and often overlooked until they break causing sudden, and
often catastrophic, coolant loss).

Bad check valve in the vacuum feed to the reservoir (mine broke actually at
the small tee off to the diff locks.) which is located below and in front of
the brake fluid and PS oil reservoirs--check the condition off all the
vacuum connections in this feed line area.

Climate Control Programmer Solenoids. There were many threads about these
back when I first joined the list. Supposedly, they leak. I recall some who
would go get used Programmers and cherrypick the good valves (tested by
removing from the board, applying a test voltage--which escapes me now--and
checking with a mighty-vac). What the Programmer is notorious for (this was
the case on my car) are cracked solder connections. The programmer is
located behind and under the glove box--sorta. Look for the box with all the
colored vacuum lines coming out one end and a Bowden Cable off the side. Be
careful not to break the Bowden Cable when removing this box. The
intermittent nature of the failures in question suggest cracked solder
connections in the Programmer to me. Easy fix. Remove and open the box.
Critical visual inspection will turn up circular hairline cracks in the
colder--mostly at the solenoid pins and the drive transistors. If in doubt,
just resolder the whole board--it's not that big. Use a good soldering iron
and electronic (rosin core) solder. A dab of liquid solder flux (again,
electronic grade--rosin) on the more difficult connections (those where the
solder has heat cycled so many times it has crystallized and doesn't melt
well) helps a lot too.

Recirculation Flap "motor" mount broken. Very common problem. Result is the
vacuum line to the "motor" gets either pulled off, pinched, or cut. You'll
need to get the "repair kit" and do the contortions to fix this. This would
likely be an all or nothing problem--not intermittent.

Pinched vacuum lines. I found a few of these when troubleshooting my
system--all the result of the so-called "professional mechanics" who worked
on the car for the PO of the car (she didn't do any of her own work). The
worst offender was the vacuum line that fed the vent flaps "motor" which was
run through a flap pivot in such a way that when the pivot turned, it
pinched off the hose. This was one cause of intermittent dash vents on my
car.

To summarize:

Resolder Programmer.

Check Recirculation Doors.

Mighty-Vac all the vacuum lines.

Once you fix it, the reward is nice (hopefully) cold air from the dash
vents, instead of the defroster ;-)

Mike Veglia
Motor Sport Visions Photography
http://www.motorsportvisions.com