AC Problem - lines froze

Situation where if AC on low fan, both the high pressure and low
pressure lines froze - frost on both lines. Pretty soon, blowing warm
air only. Fan on high, not a problem.

Also compressor seemed to fail to cycle, running all the time, and gas
mileage dropped badly.

How do Volvo's control the clutch and cycling of the compressor?

Thanks

 

my guess is that you are low on coolant .... R12 in older systems, R134a in
newer ones...

 

We really need to know model/year on this, and if the year is earlier than
about 93 whether the system has been converted to R134a. Something that
strikes me is "frost on the high pressure line." Is this the line from the
condensor, past the valve, where it enters the evaporator? (If not, I'm
lost.)

Mike

 

High pressure line = liquid line = the line from the compressor to the
orifice tube or expansion valve.

 

That's what doesn't figure. That part should be hot under all conditions
(except compressor off). The section from the valve to the evaporator can be
cooled to frost by flash gas if the system is running full bore. To that
extent, I think the OP was doing the best by focusing on why the compressor
was always engaged.

Mike

 

If there is frost on the liquid line and the system has the proper
charge then there is a restriction in the line just prior to where the
frost starts that acts like an expansion valve. If it is a model that
has the orfice tube under the hood it's possible for the high side line
to frost when the system charge is low.

Bob

 

Er, I forgot to mention the condenser in there, the line goes from the
compressor to the condenser, and then to the expansion device. The line
between the condenser and expansion device shouldn't be hot, but it should
be a bit warmer than ambient depending on condenser efficiency. If that line
is iced up it can be one of two things, a kink or blockage causing a
pressure drop and some of the refrigerant to boil off in the line rather
than in the evaporator, or the evaporator can be so iced up that the liquid
line ices up nearby just from conduction.

 

1998 S70 - so it's the new stuff
plenty of freon though ~40# per mechanic

actually, the high and low pressure lines were freezing as they exited
and entered the firewall.

think it may be fixed now, replacing the high pressure switch and
expansion valve seems to have done the trick

 

40 pounds?! That can't be right, the system only holds about 2.5 pounds. If
that's the pressure he's referring to it sounds far too low for the high
side and too high for the low side.

If the expansion valve is stuck open it can indeed cause the evaporator to
completely freeze up which will also ice the lines going through the
firewall. The valve's purpose is to regulate the flow of liquid refrigerant
into the evaporator by monitoring the temperature of the suction line, you
want just enough so that it boils almost completely away using the entire
surface area of the evaporator but still leaves just enough liquid
refrigerant to cool the compressor.

 

40 psi? That's too high, IIRC about 20-25 psi is a normal reading on
the low pressure side (where you add freon) while the system is running.

 

2.5*16=40 oz.

Bob

 

Am I the only one kicking himself for missing that?

Mike

 

Well, my '98 S70 only takes 750 grams = 1.65 lbs = 26.4 oz. So 40 oz.
would be way too much. Also, they put the expansion orifice (not valve)
in the high pressure (or liquid) line more than a foot from the firewall
to reduce the hissing noise inside the cabin. So both lines can be
covered in frost from this point to the firewall.

--
Mike F.
Thornhill (near Toronto), Ont.

Replace tt with t (twice!) and remove parentheses to email me directly.
(But I check the newsgroup more often than this email address.)

 

Mike, I have correct pressure in system and it blows cold for awhile then shuts off. Frost forms on the low pressure line (smaller diameter line) where it enters the firewall. Does this indicate it needs a new orifice tube? or??? I'm in Caledon ON and could call you if I had your number. It's a 1999 V70 n/a in case that matters. Thanks! Bob

 

OOOPS That should read 'high pressure line' (smaller diameter). It frosts up between the firewall and where the orifice tube is in the line.