2002 GMC 2500HD D/A combo Crew cab.

1) What PSI does the high pressure switch cut in and stops the compressor?

2) What PSI does the low pressure (clutch cycle switch) cut in and stops the compressor?

3) What is the test procedure to tell if the Radiator Fan clutch is working properly or not. Mine seems to be way late engaging and I've noticed that the AC clutch is cycling on/off alot when I start it up and OAT is above 95F. so my air vent temps go up / down / up / down until I can get out on the highway and get some speed up to get the air moving.

Thanks

A/C Pressure Sensors
The A/C system is protected by two pressure switches. The A/C high pressure switch interrupts the A/C request signal when the A/C line pressure exceeds 2896 kPa (420 psi). The A/C low pressure switch interrupts the A/C low pressure switch signal when the A/C line pressure falls below 145-172 kPa (21-25 psi). When the PCM sees an open in either signal, the A/C clutch relay control circuit is no longer grounded, thus shutting off the compressor. The low pressure switch will close when pressure reaches 262-290 kPa (38-42 psi).

Fan Clutch Diagnosis
Step
Action
Yes
No

1
Were you sent here from Symptoms or another diagnostic table?
Go to Step 2
Go to Symptoms - Engine Cooling

2
Is there excessive fan air noise?
Go to Step 3
Go to Step 4

3
Fan air noise is normal during cold engine start up.

Does the fan noise go away at normal engine operating temperature?
Go to Step 13
Go to Step 4

4

Important
The engine must be turned OFF and the engine temperature should be cold.


Rotate the fan clutch.

Does the fan clutch rotate?
Go to Step 5
Go to Step 14

5
Visually inspect the fan blades for cracks, looseness or damage.

Are the fan blades in good condition?
Go to Step 6
Go to Step 15

6
Visually inspect the fan clutch for signs of silicone leakage.

Slight silicone leakage may not effect the fan clutch engagement.
Excess leakage will prevent the fan clutch from engaging.
Is the silicone fluid leakage excessive?
Go to Step 14
Go to Step 7

7
Inspect the fan clutch for proper installation.

Move the fan blade back and forth in a lateral motion.
Inspect for fan blade to fan clutch movement.
Is the fan blade loose at the fan clutch?
Go to Step 10
Go to Step 8

8
Inspect the fan clutch for wear.

Move the fan blade back and forth in a lateral motion.

Important
Approximately 6.5 mm (¼ in) movement at the tip of the fan blade is normal.


Inspect for fan clutch lateral movement.
Is the fan clutch lateral movement excessive?
Go to Step 14
Go to Step 9

9
The fan clutch should have more turning resistance when the engine is at or above normal operating temperature.

Does the fan clutch have more resistance when the engine temperature is raised?
Go to Step 11
Go to Step 14

10
Tighten the fan. Refer to Fastener Tightening Specifications .

Is the repair complete?
Go to Step 16
--

11
Perform a fan clutch engagement test.

Ensure the engine coolant level is full.
Ensure the cooling fan drive belt tension is correct and not slipping.
Position and secure a thermometer between the fan clutch and the radiator.
Ensure the cooling fan is disengaged before starting this test.
Sufficiently cover the radiator grille to restrict the air flow.

Important
Do not allow engine temperature to exceed 121°C (250°F).


Start the engine.
Turn the A/C ON, if equipped.
Operate the engine at approximately 2,000 RPM.
Inspect the thermometer reading when the fan clutch engages.
Do not continue this test if the fan clutch does not engage between 85-96°C (185-205°F).
Fan clutch engagement will be indicated by an increase in fan air noise, fan speed, and a drop of about 3-10°C (5-15°F) on the thermometer reading.

Did the fan clutch engage between 85-96°C (185-205°F)?
Go to Step 12
Go to Step 14

12
Once the fan clutch engages, perform the following steps:

Uncover the radiator grille.
Turn the A/C OFF, if equipped.
Operate the engine at approximately 2,500 RPM to reduce the engine operating temperature.
Remove the thermometer.
Did the engine return to normal operating temperature?
Go to Step 13
--

13
As the engine temperature returns to normal, the fan clutch will disengage, indicated by a reduction in fan air noise and fan speed.

Did the fan clutch disengage?
Go to Step 16
Go to Step 14

14
Replace the fan clutch. Refer to Fan Clutch Replacement .

Is the repair complete?
Go to Step 16
--

15
Replace the fan blades. Refer to Fan Replacement .

Is the repair complete?
Go to Step 16
--

16
Operate the fan clutch to verify proper operation.

Did you find and correct the condition?
System OK
Go to Step 2

Thanks Max!! Now I just need to go hook up the gauges and see which switch might be acting up......... my money is on the high pressure side. Either a defective switch or maybe just not enough air flow across the evaporator to keep the head pressure in check.

Again thanks for the quick response :ro)