Expansion valve vs orifice tube (was: compressor nightmare)

[mark grady]

| Greg,
|
| I can confirm there is no orifice tube if your dash AC is still stock. It
| may not be, for one never knows what has happened over the past
| twenty five
| years.
|
| These units came from factory with a thermostatic expansion valve
| instead of
| orifice tube. It is located on the evaporator and visible when
| you pull the
| front cover off the big black AC / heater box.

So much learning that my head is hurting!

Don, I didn't realize this. My '77K has a fitting on the outside of the
heater A/C box, I always assumed there was an orifice tube at that point,
right where the line enters the air box.

Since it uses a cycling clutch with high and low pressure switches, it
looked, acted like and I assumed was a CCOT (GM orifice tube) system. The
line from the receiver connects at this point to the evaporator, and it
didn't occur to me that there was yet another fitting inside the air box to
an expansion valve.

That's one area I haven't checked for leaks, but I may have a slightly loose
fitting. (I've been trying to tighten The General up for about 4 years now.
I don't really have a complaint about dash cooling, but after hearing Dick
Kennedy's re-hab job and his ready-to-frost expectations, perhaps it's time
for the air box to come apart for a little look see and a re-do.

I assume you've got a coach of the same vintage, so I've learned another new
thing today. (Number 1 was that 30 amp campground circuits are 110 volts.)

Mark

 

[donald w. miller]

> So much learning that my head is hurting!
>
> Don, I didn't realize this. My '77K has a fitting on the outside of the
> heater A/C box, I always assumed there was an orifice tube at that point,
> right where the line enters the air box.
>
> Since it uses a cycling clutch with high and low pressure switches, it
> looked, acted like and I assumed was a CCOT (GM orifice tube) system. The
> line from the receiver connects at this point to the evaporator, and it
> didn't occur to me that there was yet another fitting inside the air box
to
> an expansion valve.
>
> That's one area I haven't checked for leaks, but I may have a slightly
loose
> fitting. (I've been trying to tighten The General up for about 4 years
now.
> I don't really have a complaint about dash cooling, but after hearing Dick
> Kennedy's re-hab job and his ready-to-frost expectations, perhaps it's
time
> for the air box to come apart for a little look see and a re-do.
>
> I assume you've got a coach of the same vintage, so I've learned another
new
> thing today. (Number 1 was that 30 amp campground circuits are 110 volts.)
>
> Mark




Hi Mark,

What I posted was based solely upon my Parts book, published in 1979, which
says all units have thermostatic expansion valves.

I know for certain my 1975 Glennbrook has no orifice tube. Other coaches
could be different for AC shops will sometimes replace the GM expansion
valves with orifice tubes. It would be interesting to find out why they do
this because AC reference books seem to consider the TXV valve system
superior.

I have seen misunderstandings before when someone assumed all coaches are
identical to theirs, so I tried to cover my ass in my original post when I
said:

> I can confirm there is no orifice tube if your dash AC is still stock. It
> may not be, for one never knows what has happened over the past
> twenty five
> years.

I remember some 1979 GM vehicles with A6 compressor and the orifice tube
systems you mention, but don't know if that was the first year for them.
I think those systems had a muffler in the compressor output line and a
larger aluminum receiver (or whatever it was) located outside the evaporator
box ?

Another interesting component inside our GMCMH big black box is a screw
driver adjustable thermostat which controls the compressor cycling by
sensing evaporator output air temperature.

The book doesn't say, but it would seem optimum to adjust this thermostat
so the evaporator fins are just above 32 degrees.

If this is possible, the evaporator won't freeze up but will be operating
near it's maximum capacity.

I wonder if anyone has played with this thermostat ?

Don