??? About A/C

mblrds · June 12, 2004

Hi Ya'll,

Got a few questions concerning the a/c on my '00 GMC Sierra.

With the fan set on high and the recirculation/max button selected, the air leaving the center vent only reads around 58 to 60 degrees.

Shouldn't it be down to about 40?

If so then what do I need to do to get it down that low. It does seem that the a/c doesn't chill the air as well as when new.

How 'bout recharging the system using the DIY kits. I always heard that you want to watch out and not over-charge the system. How can you tell if the system is overcharged or not? What damage could result in a overcharged system? What are the symptoms of a overcharged or a undercharged system?

Inquiring minds want to know,

M.

asepgrad98 · June 12, 2004

turn down the fan setting to low and rev up rpm's to about 2k. It should go down to 40 or so. you should also check to see if the clutch is cycling on/off when AC is on. It should stay on steady. If if cycles you need to add R134a untill low side psi is about 35psi.

mblrds · June 12, 2004

What would I use to check the pressure? Obviously a tire gauge ain't gonna cut it.

asepgrad98 · June 13, 2004

a set of gauges at the parts store is about 80$. Shouldnt be needed in this case as i suspect your duct temp would be acceptable if you turned down the fan and reved up the engine.

Import Hater · June 13, 2004

Yup. The A/C performance at a standstill idle could be better. By taching the motor up or 2k or so, you're getting the fan to spin faster. Which in turn brings more airflow through the condensor, something that needs airflow in order for the A/C to work well. This is why you hear the fan come on with the A/C on a car that has electric fans, even if the coolant temp is cold.

zorrro · June 14, 2004

Yes, you should have cooler air at idle and you need to make sure the compressor clutch is steadily engaged.

If it is cycling these are the 3 things an unhampered with A/C can cycle on:

1. The low-pressure switch: Will do this when Freon is low or there is a blockage in the refrigerant line (Metering device or receiver drier).

2. High-pressure switch: This one disengages the clutch if the condenser pressure becomes too great. Too much refrigerant, a plugged up condenser coil (bugs and dirt etc.) or a fan that is failing to move air over it, can make it too great. Do not add refrigerant if this is the case.

3. There is a thermostat imbedded in the evaporator coil (the cold one). If the evaporator is getting too cold it will cycle the clutch to prevent it from icing over. It will get too cold if airflow is crummy over that coil. This will happen if it is plugged with dirt etc or the cab air filter is dirty. Do not add refrigerant.

Check these things before you add Freon. Too much is a bad thing. Remember if it is low you have a leak and it will be low again someday. I have found that newer autos and their fittings are pretty good and do not leak much. If you have a sight glass in the liquid line (small one connecting the evaporator and condenser) see if it is full, empty or bubbly. It should be clear on a moderately warm day after the a/c has been on 15mins or so.

If the clutch is engaged constantly make sure the large line going into the compressor feels cold. Some areas of the country it’s cold enough to condense water from the air. If you find this is the case, and your low side pressure is near 35psi I would suspect that heated air is being blended with your cool air due to a heater or air diverter malfunction. Make sure air is being routed over your cooling coil and not the heater core or if it is being routed over the core, the core is not warm.

I would buy myself the auto part store R-134a gauge set for $80.00 they are good enough and will let you get a good understanding of how your system functions both normally and when a problem arises. If you need to purchase refrigerant (some parts stores will sell it even they are not supposed too) there are a number of online testing agencies that for $20.00 and an open book test will get you certified....Hope this helps, Zorrro..............

snoman · June 14, 2004

Hi Ya'll,
Got a few questions concerning the a/c on my '00 GMC Sierra.

With the fan set on high and the recirculation/max button selected, the air leaving the center vent only reads around 58 to 60 degrees.

Shouldn't it be down to about 40?

If so then what do I need to do to get it down that low. It does seem that the a/c doesn't chill the air as well as when new.

How 'bout recharging the system using the DIY kits. I always heard that you want to watch out and not over-charge the system. How can you tell if the system is overcharged or not? What damage could result in a overcharged system? What are the symptoms of a overcharged or a undercharged system?

Inquiring minds want to know,

M.

THe tempature will vary at a idle and with ambient air temp and humidity. This sound more like low freon than anything as most sytems tend to leak over a period of time. If you get a recharge "kit" that comes with a low side pressure gage you can better confirm your problem (complete gages would be better yet) You should have around 50PSI at a warm idle on a warm day on low side with proper cooling and blower on low if you are reading much below this it is likely low. (It is possible for it to read less and be good it wouls have to be cooling better than it is too as well) Overcharging is not good but on a system that size it takes more than a few extra ounces to cause a problem. Add about 1/2 can to system (adaptor will only connect to low side) and recheck but my guess is that it will take a least a full can. My wife Cherokee acted simular to your this summer and it took about 2/3 's a can to get it correct again and it is a smaller system and hold less freon to begin with. (I do have a full gage set and I have worked with A/C's before)

silverado03 · June 14, 2004

my a/c has been acting up so I got around to checking it and the switch on the side of the accumalator is loose or messing up b/c when I wiggle the switch it starts flowing really cold air, which it was flowing warm air before wiggling it

zorrro · June 16, 2004

I've got some more info for servicing your A/C unit. I was mucking around with mine today as once or twice lately it just flat out stopped cooling. From perfect to none and then works great the next time I use it.

It was working well today and this is how I found it to run when working well:

At a warm idle about 75deg outside (average25-30% RH) and 90+ in the engine compartment and 80deg inside the cab. With high fan on I measured 40psig on the low and 180-200psig on the high. After the cab had cooled down some I revved the truck to around 2K the pressures were 30/210psig and the 30 was starting slowly climb a little. I was getting 50deg air out the vents in the cab and it was now very cool in the truck. I put the fan to low speed and got 47deg air. At this fan speed the clutch cycled on the low-pressure switch located on the accumulator. It would cycle off at 28psig and back on again at 48psig (This doubles as frost protection). I felt that this was OK and working fine for the conditions. The inside of the truck was frigid by now. The suction line to the compressor was sweating and very cold.

Your pressures will be higher if the humidity is higher or the temperatures are higher.

Then my clutch disengaged and stayed disengaged (I was hoping it would do this). Like Silverado03 my low pressure switch needed wiggling. I could bump the switch and wires and cycle the clutch as many times as I liked. I unplugged the harness and bet the tongs on the plug to contact the switch better. I could not repeat the problem after doing this. Time will tell..............

Oh, yeah.

This was on my '01 HD with the 8.1. The system has a low pressure switch on the accumulator, a high pressure switch in the dicharge line (hiding below the air box)no reciever drier, no expansion valve (fixed orfice instead) and no sight glass.