A/c recharge, has this happened to you before

[dodgethis]

I had a complete dry system, and had pulled a vacuum on the system for at least two hours. The manifold gauges held -30 inches/mg for at least 45 minutes with no problems. I proceeded to add 1 can of 134a, The compressor never cycled on. I then jumperd the terminals to cycle it, and it pulled in more refrigerant. The low side states around 32psi, and the compressor won't engage on it's own. I stopped adding freon because of this, and the pipe connecting to the compressor was hot as hell. I didn't want to damage anything. The compressor, and accumulator was given oil per stated in the OEM manual. Any thoughts?

[Imp558]

Did you test the low pressure switch? Or is it new enough to have a 0-5v sensor?

[dodgethis]

Didn't think about testing it, I'll try that when I get home.

[RobertISaar]

should have just the sensor... all w-bodies from 91+ used a sensor instead of the high switch/low switch setup. the sensor only monitors high-side pressure, going off of a 95 3100 calibration, high-side pressure needs to be 41.25PSI to allow compressor operation. if it drops below 37.5, compressor is disabled.

 

with the compressor off, high and low side pressures will be equal. sounds like it is doing what it should if you're at 32PSI with the compressor off.

 

when the compressor engages, low-side pressure will be targeted at 40PSI or so if I'm remembering how the variable stroke stuff works on the V5 compressor. high-side will vary depending on a lot of factors.

 

do you have a high-side guage? if not, I'm not saying it's the greatest idea, but the pressure sensor data may be available over the datastream and you'll be able to view it that way.

[l67ss]

There is an actual measurement of how much freon to use

[Imp558]

Potentially scour the internet for a table on that sensor and stick a sewing pin into the back alongside the red/black wire and read it's voltage

[RobertISaar]

a/c pressure sensor is scaled as such:

 

X * 1.875000 + -18.750000 = PSI

 

X being A/D counts, but you can substitute A/D counts for roughly .02 volts.

 

ex: 1 volt =

 

(1/.02) * 1.875 + -18.75 = 75PSI.

[Imp558]

a/c pressure sensor is scaled as such:

 

X * 1.875000 + -18.750000 = PSI

 

X being A/D counts, but you can substitute A/D counts for roughly .02 volts.

 

ex: 1 volt =

 

(1/.02) * 1.875 + -18.75 = 75PSI.

Cleared that right up

[RobertISaar]

[dodgethis]

Thank you all for the suggestions. I'll give them shot once I get back home. I do recollect that the A/C system Capacity is 1.88lb of refrigerate. So if I convert that to onces as I have 12 oz R134A cans, it will take roughly 2 and half cans. I think it only has about a little over 12oz. I do have a manifold guage, and will see what the high side pressure is. I'll report back soon.

 

1.88lbs = 30.08oz

[dodgethis]

Where is this A/c pressure switch?

[Imp558]

Sensor, it's in the high side a/c line.

[RobertISaar]

Thank you all for the suggestions. I'll give them shot once I get back home. I do recollect that the A/C system Capacity is 1.88lb of refrigerate. So if I convert that to onces as I have 12 oz R134A cans, it will take roughly 2 and half cans. I think it only has about a little over 12oz. I do have a manifold guage, and will see what the high side pressure is. I'll report back soon.

 

1.88lbs = 30.08oz

 

I don't suspect the a/c will remain engaged on its own until you have at least half of the 2nd can in.

 

Sensor, it's in the high side a/c line.

 

yar, airbox area.

[dodgethis]

Well, the compressor cycles on its own. I haven't added a half a can of 12 oz yet. The low pressure is reading right at 65 psi, and the high pressure is around 150psi. The outside temperature is 65 degrees. Does the pressures seem off?

[RobertISaar]

without the full charge in it, things aren't going to read quite as they should... your low-side pressure does seem a bit high to me.

 

GM says with the system full and the compressor off, both sides should read around 50PSI. slightly higher is okay(though too high is an obvious overcharge), lower indicates a leak or it not being fully charged. I imagine ambient temps will have some effect on this reading though, with low temps probably dropping the system pressure low enough to not allow the compressor to engage(which may be intentional).

 

this performance and temp-pressure chart may be helpful.

 

http://imgur.com/a/s0OIs

 

if I'm reading the temp-pressure chart correctly, assuming it isn't meant entirely for shop usage, ~65PSI at ~65*F on the low-side with a high-side pressure around 150PSI, high-side temp should be somewhere around ~112*F?

[Imp558]

A little high, it's wether it's a problem that's in question, https://www.google.com/search?site=&source=hp&ei=aV0QVsuQJMOs-AH-8oTgDA&q=ac+pressure+chart&oq=ac+press&gs_l=mobile-gws-hp.1.1.0i131j0j0i20j0j5.2904.7919.0.10194.10.9.1.2.2.0.465.3197.3-3j5.8.0....0...1.1.64.mobile-gws-hp..1.9.2437.3.7_6VfzGcE2s

[Schurkey]

I had a complete dry system, and had pulled a vacuum on the system for at least two hours. The manifold gauges held -30 inches/mg for at least 45 minutes with no problems. I proceeded to add 1 can of 134a, The compressor never cycled on. I then jumperd the terminals to cycle it, and it pulled in more refrigerant. The low side states around 32psi, and the compressor won't engage on it's own. I stopped adding freon because of this, and the pipe connecting to the compressor was hot as hell. I didn't want to damage anything. The compressor, and accumulator was given oil per stated in the OEM manual. Any thoughts?

Was the system empty of oil, or is it now double-filled?  Too much oil will screw-up system performance.  How much oil did you drain out?  Were the compressor and accumulator all-new, or flushed clean of the old oil?

Well, the compressor cycles on its own. I haven't added a half a can of 12 oz yet. The low pressure is reading right at 65 psi, and the high pressure is around 150psi. The outside temperature is 65 degrees. Does the pressures seem off?

You already know the approximate system capacity.  You said you started with an empty (vacuumed) system.  Until you put in somewhere near the proper amount of refrigerant, the system CANNOT WORK PROPERLY.  You've said you have less than 1 1/2 cans in a system that holds 2 1/2 cans.  That system is undercharged.  What are you waiting for?  Your low-side is high, your high-side is low.

 

As said, there is a pressure/temperature relationship with refrigerant.  Given the pressure/temperature chart for whichever refrigerant you're using, you know the temperature of the refrigerant if you know the pressure, and you'll know the pressure if you know the temperature.  Ideally, you'd want 32--33 degree air at the vent (just warm enough that the evaporator doesn't freeze) and therefore you'd have a goal for the low-side pressure--about 30 psi.  In real life, I haven't seen air that cold in a VERY long time.  35--37 is about as good as it gets.  I've worked on many vehicles that won't go colder than 40 or 45--let's say 40 psi.  So your goal for low-side pressure is "between 30 and 40 psi, lower is better" with R-12 or R-134a when the compressor is running.  Anything above 45 degrees dash air temperature when the compressor is running is just plain unacceptable unless it's REALLY hot out.  Thousands and thousands of years ago, working with older cars and R-12 systems, we'd expect somewhere between 2x to 3x ambient air temperature for high-side pressure, usually toward the higher end of the pressure range.  The newer refrigerants will require different pressures than R-12.  I don't know what the new rule-of-thumb is for newer refrigerants.  It is probably higher than 3x ambient air temp, but you need to find someone who does a lot of work with the refrigerant you're using to know for sure.

 

http://airgasrefrigerants.com/high-press-refrigerants.php

 

If you need 2 1/2 cans of refrigerant, put in two cans.  I add to the low-side, and if the system is a cycling-clutch type, I close the valve on the "empty" can while the compressor is running--so the compressor "vacuum-cleans" the most refrigerant out of the can.  Dropping the can into a pot of hot water will also assist in getting the most refrigerant out of the can, and quickly.  There's always going to be some gas pressure (but NO liquid!) in the "empty" can, but you want to minimize the pressure before you remove the "empty" and install the next "full" can.

 

Once you have 2 of the 2 1/2 cans of refrigerant in the system, fan speed on "high", WATCH THE THERMOMETER in the dash vent.  Add 1/4 can of refrigerant at a time (more-or-less, you have to kind of guess at the quantity) and then let the temperature stabilize for a few minutes.  You're looking for the thermometer to show the LOWEST temperature.  If the temperature stabilizes lower than before...add another 1/4 can of refrigerant.  When the temperature begins to climb again, (and I mean by one degree, you're looking for the SLIGHTEST rise in temp) STOP ADDING REFRIGERANT.  This SHOULD be right at the 2 1/2 cans you've said is the system capacity--although I've seen minor variations.  Most shops now just look at the system capacity, and program the "charging machine" to dump that much refrigerant into the vehicle.  It works...but it's not optimum.

 

 

 

 

 

 

I'm pleased that you've taken the trouble to actually verify the amount of refrigerant in the can.  We used to call those cans "pounders" because the metal can plus the refrigerant was 16 ounces.  (MAYBE, before my time, the cans actually had 16 oz of refrigerant.  I wouldn't know.)  The can was about two ounces, so there was 14 oz. of refrigerant when I started my career.  The manufacturers decided to cheat the customer, so now most of 'em only have 12 oz. of refrigerant--but I still call them "pounder" cans.  I've seen several under-charged vehicles because the tech put "three cans" of refrigerant into a system--but forgot that a can of refrigerant doesn't have as much refrigerant inside as they used to.