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2001 Chevy Tahoe: AC Issue - Rear Air..cooling..low side…

Customer Question
2001 Chevy Tahoe - AC...
2001 Chevy Tahoe - AC Issue - Rear Air not cooling, Front air cools, however not cold.

What should the low side pressure be on an 01 chevy tahoe with rear air? I put approximately 2.7 lbs of 134a in and the cheapo guage i am using says around 42 lbs low side. It is about 90 degress F outside.... thanks...
Submitted: 8 years ago.Category: Chevy
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Answered in 10 minutes by:
6/26/2010
Chevy Mechanic: Mark, ASE Certified Technician replied 8 years ago
Mark
Mark, ASE Certified Technician
Category: Chevy
Satisfied Customers: 847
Experience: 34+ yrs Dealership Exp. - Fully State & ASE Certified - GM Master & GM World Class Tech
Verified
Greetings, my name is Mark.

Below I have included a table GM uses for system pressures related to temperature.

Below that is a system performance flow-chart test we sometimes use. Don't bother clicking on any of the URL links. You won't go anywhere.

I might add that it is EXTREMELY important to have the proper and most exact charge. In the olden days of R-12, AC system pressures were 1/3 what they are with R-134a. Because of the extreme pressure, a slightly under or over charge could be significant in AC system performance.

A/C Performance Table -- with Rear A/C

Ambient Air Temperature

Relative Humidity

Service Port Pressure

Maximum Discharge Air Temperature

Low Side

High Side

Left Center

Rear Center

13-18°C (55-65°F)

0-100%

175-205 kPa (25-30 psi)

420-860 kPa (61-125 psi)

6°C (43°F)

9°C (48°F)

19-24°C (66-75°F)

Below 40%

175-240 kPa (25-35 psi)

(NNN) NNN-NNNNkPa (89-157 psi)

8°C (46°F)

11°C (52°F)

Above 40%

175-267 kPa (25-39 psi)

(NNN) NNN-NNNNkPa (99-170 psi)

10°C (50°F)

13°C (55°F)

25-29°C (76-85°F)

Below 35%

184-284 kPa (27-41 psi)

(NNN) NNN-NNNNkPa (136-193 psi)

11°C (52°F)

14°C (57°F)

35-50%

197-293 kPa (29-43 psi)

(NNN) NNN-NNNNkPa (144-197 psi)

12°C (54°F)

15°C (59°F)

Above 50%

207-310 kPa (30-45 psi)

1020-1430 kPa (148-208 psi)

13°C (55°F)

16°C (61°F)

30-35°C (86-95°F)

Below 30%

233-339 kPa (34-49 psi)

1210-1640 kPa (176-238 psi)

15°C (59°F)

18°C (64°F)

30-50%

241-348 kPa (35-51 psi)

1240-1680 kPa (180-244 psi)

16°C (61°F)

19°C (66°F)

Above 50%

253-362 kPa (37-53 psi)

1280-1730 kPa (186-251 psi)

18°C (64°F)

21°C (70°F)

36-41°C (96-105°F)

Below 20%

289-392 kPa (42-57 psi)

1520-1940 kPa (221-282 psi)

19°C (66°F)

21°C (70°F)

20-40%

294-399 kPa (43-58 psi)

1540-1980 kPa (224-287 psi)

20°C (68°F)

22°C (72°F)

Above 40%

303-407 kPa (44-59 psi)

1580-2010 kPa (229-294 psi)

21°C (70°F)

24°C (75°F)

42-46°C (106-115°F)

Below 20%

348-440 kPa (51-64 psi)

1840-2210 kPa (267-321 psi)

23°C (73°F)

24°C (75°F)

Above 20%

352-446 kPa (51-65 psi)

1860-2240 kPa (270-325 psi)

24°C (75°F)

26°C (79°F)

47-49°C (116-120°F)

Below 30%

400-472 kPa (58-69 psi)

2130-2380 kPa (309-345 psi)

25°C (77°F)

27°C (81°F)

 



The numbers below refer to the step numbers on the diagnostic table.

  1. This step determines if the A/C system has at least the minimum refrigerant charge required to operate the system without damage.

  2. This step measures the performance of the A/C system.

  3. This step is to allow for vehicle variations as well as high ambient air temperatures.

Step

Action

Values

Yes

No

Important:

The ambient air temperature must be at least 16°C (60°F).
Do not induce additional air flow across the front of the vehicle during the test.
If you were sent here from a DTC diagnostic table, clear the DTC upon completion of this test.

1

  1. Park the vehicle inside or in the shade.
  2. Open the windows in order to ventilate the interior of the vehicle.
  3. If the A/C system was operating, allow the A/C system to equalize for about 2 minutes.
  4. Turn OFF the ignition.
  5. Install the J 43600 ACR 2000 Refrigerant Station.
  6. Record the ambient air temperature displayed on theJ 43600 .
  7. Record the low and high side STATIC pressure readings.

Are both the low and high side pressures within the specified value?

Greater than 16°C (60°F), 345 kPa (50 psi)
Greater than 24°C (75°F), 483 kPa (70 psi)
Greater than 33°C (90°F), 690 kPa (100 psi)

Go to Step 2

Go to Leak Testing

2

    Important: Record the relative humidity and the ambient air temperature at the time of the test.

  1. Close the vehicle doors and windows.
  2. Open the drivers door window 127-152 mm (5-6 inches).
  3. Select the following HVAC control settings:
  4. The A/C ON.
    The coldest temperature setting
    The maximum blower speed
    The IP panel outlet mode
    All IP panel outlets are OPEN
    Recirculation mode
  5. Install the temperature probes to the J 43600 in the left and right center A/C air outlets.
  6. Apply the parking brake.
  7. Place the transaxle/transmission in one of the following positions:
  8. PARK for Automatic
    NEUTRAL for Manual
  9. Start the engine.
  10. Operate the A/C system for 5 minutes.
  11. Inspect A/C components for the following conditions:
  12. Abnormal frost areas
    Unusual noises

    Important: Press the RESET button, before using the print function of theJ 43600 .

  13. Print the following information:
  14. The panel outlet air temperatures
    The low-side pressure
    The high-side pressure
  15. Compare the low and high side pressures and the panel output temperatures to the table below.

Does all the data recorded fall within the specified ranges of the table below?

--

Go to Step 8

Go to Step 3

3

If the pressures and temperatures recorded do not fall within the specified ranges:

  1. Continue to operate the A/C system for an additional 5 minutes.
  2. RESET theJ 43600 and record the pressures and temperatures again.
  3. Compare the low and high side pressures and the output temperature to the table below.

Does all the data recorded fall within the specified ranges of the table below?

--

Go to Step 8

Go to Step 4

4

Do the high and low side pressures fall within the specified ranges but the panel outlet temperatures do not?

--

Go to Air Conditioning (A/C) Diagnostics - Pressure Zone A

Go to Step 5

5

Is the low side pressure greater than the specified range but the high side pressure within or less than the specified range?

--

Go to Air Conditioning (A/C) Diagnostics - Pressure Zone B

Go to Step 6

6

Are the low and high side pressures both greater than the specified ranges?

--

Go to Air Conditioning (A/C) Diagnostics - Pressure Zone C

Go to Step 7

7

Is the high side pressure greater than the specified range, but the low side pressure is within or less than the specified range?

--

Go to Air Conditioning (A/C) Diagnostics - Pressure Zone D

Go to Step 8

8

Operate the system in order to verify the test results.

Did you find the same results?

--

System OK

Go to Symptoms - HVAC Systems - Manual in HVAC Systems - Manual or Symptoms - HVAC Systems - Automatic in HVAC Systems - Automatic

A/C System Pressure -- Zone Classification


Object Number: 693098  Size: MF
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Customer reply replied 8 years ago

I dont know the front is cooling and the back is not.....

Chevy Mechanic: Mark, ASE Certified Technician replied 8 years ago
Your original question was about AC system pressure vs temp. You did mention the front was cooling but not cold. But I assumed you only wanted to know what the pressures should be verses the temperature where you are at.

I know you're following the flow chart diagnostics and now things have changed. With you adding 2.7 lbs of Freon, I would have to conclude your system is probably not correctly charged. You said you replaced components some 12 months ago, and unless your system has a gross leak, you should have had some freon left in the system.

I also might want to ask how you could add exactly 2.7 lbs without the use of a charge station. ?

At this point, without the use of the proper diagnostic tools, I would ignore the flow-type diagnostics. I would use the chart for temperature, humidity, and pressure. If you are with-in the guidelines of that, then you have a mechanical issue. Either temp doors are not opening, the mechanics or wiring issues, your HVAC control head, the programmer, ... it could be any number of things.

If your pressures are not correct with the chart, then you have a leak. The most common leak that we repair almost weekly is the rear AC lines. They tend to corrode and leak. When you recharged your AC, you should have added some dye. Sometimes the freon will come with dye, most doesn't.

The rear lines can be repaired without replacing the whole assembly. The use of "swedgie" type connectors is very common.
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