A/C Problem 99 Suburban won't cycle off

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vince - 23 Apr 2006 21:04 GMT

I have a good deal of Automotive knowledge, though not a professional
tech, and am a bit stumped by this one. I live in Phoenix, for
starters. The first time I fired up the A/C on my dual-air Suburban
for the year, I discovered that the clutch was never cycling off. Even
though the symptoms aren't correct for this accessory, I replaced the
clutch cycling switch as it was easy and inexpensive, to no avail. The
clutch disengages properly when the A/C is turned off, and when the
clutch cycling switch is unplugged, so I assume that the relay is
working properly. I have dealt with multiple situations where an A/C
clutch wouldn't cycle "ON", but this is the first one where it wouldn't
cycle "OFF". The air-gap is correcton the clutch... could the system
be undercharged (my experience says that this usually results in a
clutch that cycles on and off quickly)? Is there another switch in the
system (there is a switch on the High Side of the Compressor's
manifold, perhaps a High Pressure Switch) that could be causing this?
Please help, as my wife is about to KILL ME, and I don't trust anyone
else to work on my rig...

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DFBonnett - 23 Apr 2006 21:37 GMT

You might take a look at the inside temperature sensor if the 99 Blazer
is set up to maintain a certain temperature.

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DFBonnett

http://www.automotiveforums.com

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fiveiron@webtv.net - 25 Apr 2006 05:20 GMT

may be that the thermostat (expansion valve) is shot, and continues to
call.

why not get a cheap pencil-type gauge and check the low-side - it'll
give you a reading.

with a situation like that - charging indiscriminately is a no no.

mho
ve

>double dog dare you, sound familiar?

>to reduce your driving by - 10%.

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Kevin - 23 Apr 2006 21:46 GMT

You didn't say weather or not you have a problem with insufficient cooling
or not. I assume you do and that is why you are worried about the compressor
cycling. If it the system is providing sufficient cooling in the cab then
you probably don't have anything to worry about

So, the compressor should only cycle off when the low pressure clutch
cycling switch senses the evaporator pressure is below a preset pressure,
usually around 26 psi. There could be several reasons why the evaporator
pressure would not reach that point. Most of the reasons are normal. A high
heat load on the evaporator will keep it's pressure up. That could be due to
high in cab temperatures or hot outside air getting into the evaporator
case. There could be other reasons but I doubt that is what's happening.

Maybe your compressor just doesn't have enough suction to pull the low side
down at idle. Try speeding up the engine a little for about 3 minutes. If it
still doesn't cycle then you may not be getting enough air flow through the
condenser. The fan clutch may be going out and the condenser pressure is
staying so high the low side pressure is also affected. To check the fan
clutch for proper operation warm the engine up to normal operating
temperature and have an assistant turn off the ignition while you watch the
radiator fan. If it continues to spin for a couple of seconds after the
engine quits turning then, it needs to be replaced..
For proper diagnosis you would really need to hook up a manifold gage set
and monitor the high and low system pressures. An experienced tech should
be able to tell from the pressure readings weather the system is working
properly or not.

Signature

Kevin Mouton
Automotive Technology Instructor
"If women don't find you handsome, they should at least find you handy"
Red Green
.

> I have a good deal of Automotive knowledge, though not a professional
> tech, and am a bit stumped by this one. I live in Phoenix, for
[quoted text clipped - 13 lines]
> Please help, as my wife is about to KILL ME, and I don't trust anyone
> else to work on my rig...

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vince - 23 Apr 2006 22:02 GMT

Kevin,
Thanx for the response...I know that the accepted way to determine
proper level of charge today is to use an infrared thermometer to
measure the difference in temp between the inlet and outlet of the
evaporator....the inlet temp appears to be correct, but the outlet is
much too high...does this mean that I have an undercharged condition?
I am not sure if this temp difference is the same for a Suburban...

Vince

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Kevin - 24 Apr 2006 01:13 GMT

> Kevin,
> Thanx for the response...I know that the accepted way to determine
[quoted text clipped - 5 lines]
>
> Vince

Well, using the latent heat method (sometimes referred to as the super heat)
to judge refrigerant charge works well on home A/C systems, but automotive
systems are different. You do a much better job of analyzing the system
based on pressure readings rather than evap temperature.. Home units do not
have to operate in such a wide range of operating and load conditions and
do not have the same sort of control systems. Checking the inlet and outlet
temp of the evap is only one small piece of the whole story.
Also the latent heat difference for various refrigerants is different. Home
units use R22 while most automotive applications now use r134a. Both have
different boiling points and operate at different evap pressures with
different latent heat values.

An interesting article on refrigeration:
http://en.wikipedia.org/wiki/Vapor-compression_refrigeration

Besides what is in that article there are only about a thousand other things
you need to know to diagnose and repair most automotive A/C systems. That's
really not a lot compared to the many thousands of things you need to know
to diagnose and service some other automotive systems such as the electronic
engine controls, but it is a good place to start.

Signature

Kevin Mouton
Automotive Technology Instructor
"If women don't find you handsome, they should at least find you handy"
Red Green

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davehoos - 24 Apr 2006 08:00 GMT

the symptom sounds like its slightly low on gas.dont take much.
if it hasnt been used sometimes you can loose the good bit of the gas
mix and be left with the not so good and oil.this is why you stay away
from blended gasses use R134A and leave the a/c on all year.

Im guessing that you checked the basics like the blend doors and recirc
door,leaves in the evaporator...with these not working/adjusted
radiator restricted/ cooling fan not working 100%.very basic stuff.belt
tensioner-harmonic balancer.

if it has a simple system the temp idea works,you give it a rev and the
suction pipe pulses with cold gas and you can look at the frosting on
the evap.,with new systems we have to measure the suction hose as below
freezing after 1/2 hour.as the gauge pressures are needed but not as
staight forward as the older cars.

what i was wondering if its an orifice system the orifice is out the
front and all the 2 pipes at the evap will be cold the hot pipe stops
at the orifice.change of state gas to liquid.Why i added is that i
wanted to know if this model has the control valve compressor.the later
dephi compressors fitted to GM cars idle back when the low side reaches
40 psi so they run all the time but dont destroy the clutch.they use
the cycling switch to prevent freezing.in low humidity they run all
day.

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vince - 25 Apr 2006 03:40 GMT

Thanx for the reply....it's GM DA6 comp with orifice tube in liquid
line, cold after orifice tube. Checked evap doors,use only Dupont Suva
refrigerant as I have had trouble in the past with the "offshore"
stuff.

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vince - 25 Apr 2006 03:41 GMT

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