How to Diagnose ISUZU Truck Air Conditioning AC Compressors System

An investigation of the air conditioning compressors returned as warranty parts has shown that compressors are being unnecessarily replaced. The common complaints of noise and/or poor cooling are most likely a result of low refrigerant levels. In order to help technicians diagnose A/C system problems, Isuzu Commercial Truck of America, Inc. (Isuzu) is providing the diagnostic procedure below to help differentiate between a low refrigerant condition and a mechanical issue. A check sheet is also provided at the end of this bulletin to assist you in keeping track of your test procedure results. This check sheet must be attached to the repair order for any compressor replacement made under warranty.


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It is important to understand that a properly performing A/C system will only provide outlet temperatures up to about 20°F (6.7°C) lower than ambient temperatures.


Service Procedures:

Step Description Yes No
Step 1 Visually inspect the following.

Check the climate controls for proper operation. Be sure all vents and blend doors activate correctly.

Inspect the inside and outside HVAC air filters for debris and obstructions

Check that the A/C drive belt is tensioned correctly.?Check the condenser fins for debris and obstructions.

Check for obvious damage to the A/C system(broken hoses, bent or damaged pipes, road damage to the condenser, etc.)

Attempt to turn the compressor inner hub by hand. If it will not turn by hand the compressor may be seized.

Was a problem found and corrected?


Go to Step 2 Go to Step 2
Step 2 Perform the following /WC performance test.

Keep engine at 1500 RPMs

Select max A/C and recirculation

Keep the windows and doors shut

Place a thermometer in the center A/C duct

Put the A/C blower fan on maximum

Run 5 minutes and check the duct outlet temperature Check the vent temperature and compare it to the current ambient temperature.

Is the vent temperature 20F(6.7C) lower than the ambient temperature?


System Normal Go to Step 3
Step 3 Check that the magnetic clutch engages when the system is turned on.

If the clutch does not engage there may be an electrical problem.Check the wiring to confirm power is supplied to the clutch coil when the A/C button is pushed ON.

A low or empty refrigerant charge may also prevent the magnetic clutch from engaging.

Was a problem found and corrected?


Go to Step 2 Go to Step 4
Step 4 Remove the front panel and radiator grill in order to access the A/C service fittings. Connect a set of A/C gauges, start the engine, turn the A/C ON and check the pressures at IDLE and at 1500 RPM. Review the pressure readings of the vehicle against the pressure chart provided below. Check the

“possible causes”based on the pressure readings of the vehicle.

Was a possible cause low refrigerant?


Go to Step 5 Go to Step 6
Step 5 It is normal in a system with a low refrigerant charge for the compressor to make some noise. This is because oil is not being provided to the compressor by refrigerant. Once the refrigerant level is brought up to normal, the noise should decrease or cease.

If a low refrigerant charge is determined to be the source of the complaint, make sure to inspect the entire A/C system for leaks. Look for oil stains and/or missing A/C caps and use an electronic refrigerant leak detector to determine the source of a leak.

Was a problem found and corrected?


Go to Step 2 Go to Step 6
Step 6 Diagnose the possible cause. If the compressor is determined to be inoperable and will be covered under warranty, complete the Air Conditioning System Operation Check Sheet and attached it to the repair order. It will need to be returned with the compressor if the part is collected.

Was a problem found and corrected?


Go to Step 2 Go to Step 1


Pressure Chart

At Ambient Temperatures of 86 – 95°F (30 – 35°C)



Complete and attach to the repair order


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