Weird AC Behavior

[Wallace White]

I'm a little late to this thread... but I think there's an error in the
following:

 > Also note that the heater valve is OPEN on all settings except OFF
and > Auto with the low fan speed.

The heater valve (and the recirc flap--same solenoid) is closed only in
Auto when the system is trying its hardest to cool the car (i.e. large
positive error between target and actual temp). That means high fan
speed instead of low.

 > On the Auto setting the dash vents are meant to be closed on the high
 > fan setting but full-open on the low fan setting.

This also seems reversed... but even then I find it surprising. The dash
vents can (and logically should) be used in Auto or Econ whenever the
target temp is below the actual. My car _usually_ behaves this way.

TMK, most failures in this system stem from vacuum leaks (e.g.
disconnected hose at the recirc flap piston). The actuators are set up
such that defrost is the default position of the flaps, if no vacuum is
applied. To get center vents and recirc/heater valve off, all four
solenoids must be open and there must be no vacuum leaks and enough
vacuum to move all pistons.

So knowing that, let's look at Doyt's experience again:

> This is how my system is behaving:  On a two or three hour trip, it will
> usually flip over from blowing through the defrost vents and the foot well
> vents, and suddenly blow full force through the dash vents. And it usually
> stays in that mode until I stop somewhere and turn off the engine. Then,
> after maybe a 30 minute interval while I eat lunch or something, it comes
> back on through the defrost again when I start the car.

I'd guess Doyt's recirc circuit leaks (common if the piston's bracket
has broken--then the piston just pulls off from its hose). Doyt starts
up and it's hot enough that the system tries to do center vents and
recirc (all four solenoids). The circuit leaks and he gets defrost vents
instead. Then later, the car cools enough that the system switches the
recirc valve off. Then the leak is cut off and the other flaps function
again, so the flow is directed to the center vents.

The other category of failures is solenoid related. I can't remember
what happens to them exactly... Mike Arman?

My system still has some problems too, where on hot days I get defrost
vents. Sometimes it helps to "reboot": press Off, wait a few
paralyzingly hot seconds, and press Auto again. I find this works better
than just changing to Econ mode, though I don't know why... Time to try
out those spare programmers I picked up at the boneyard...

- Wallace
   '87 5kcstq 189k