A/C - Poor Performance in High Heat/Humidity Conditions
Information IB05-12-S001Intermittent A/C Performance Concerns
in High Ambient Temperatures and/or
High Humidity Conditions, Especially at
Low Engine Speeds
Light Duty ASCENDER 2003 Information HVAC 02/01/05
Light Duty ASCENDER 2004 Information HVAC 02/01/05
Light Duty ASCENDER 2005 Information HVAC 02/01/05
Intermittent A/C Performance Concerns in High
Ambient Temperatures and/or High Humidity
Conditions, Especially at Low Engine Speeds
ISSUE DATE: FEBRUARY 2005
Affected Vehicles
2003-2005 Isuzu Ascender (US/UT).
Service Information
When diagnosing a customer concern of poor air conditioning (A/C) system performance, the following information should be considered. The performance of the A/C system may not meet customer expectations. A customer may comment on high A/C outlet temperatures or that the A/C system is not cooling sufficiently, especially in high ambient temperatures. This may be an intermittent condition. The operating characteristics of the heating, ventilation and air conditioning (HVAC) system can allow a delay in cooling performance especially on initial start up after a hot soak in high ambient temperatures with a high sun load. This lack of A/C system performance can occur for longer periods of time if the vehicle is allowed to idle or is driven in low speed conditions. The A/C system performance is also related to airflow across the condenser. The response of the Electro-Viscous (EV) fan clutch will affect A/C system performance. As higher road speeds are attained~ the increased airflow across the condenser will usually improve A/C system performance and the response of the EV fan clutch.
At this time, only minor improvements in certain operating conditions can be made by using a new Powertrain Control Module (PCM) software calibration that was developed to improve coolant temperature gauge readings reference Technical Service Bulletin SB04-01-S005. This new calibration will cause the PCM to command small amounts of additional fan engagement to better control engine coolant temperatures. The new calibration is available for 2004 model year and older vehicles only. Model year 2005 vehicles already have this calibration installed. The increase in airflow that will be provided by this calibration may only improve A/C system performance by a small amount, approximately 3°C (5°F) at the center A/C vent outlet. This calibration will provide the most A/C system performance improvement as engine temperature increases. It may take the EV fan clutch a few minutes to respond and provide additional airflow across the condenser. Engineering is continuing to investigate additional updates that may be made available in the future.
Proper diagnosis of any customer concern is essential. Refer to the Diagnostic Starting Point -- Heating, Ventilation and Air Conditioning procedure in the Service Manual to begin a diagnosis of any HVAC concerns. Refer to the Diagnostic Starting Point -- Engine Cooling procedure in the Service Manual to begin a diagnosis of any engine cooling or engine cooling fan concerns. Unless a specific fault is found, do not replace any parts (for example the low pressure cycling switch or the A/C compressor). If an attempt is made to diagnose an HVAC system concern, all parameters must be measured. As an example, both the low and high side refrigerant pressure readings must be measured. If only the low side pressure reading were measured, it would appear that the A/C compressor is disengaging at a higher pressure than expected. If both the low and high side pressures were being measured, it would be noted that the A/C compressor is probably disengaging due to the maximum high side pressure cut out point is being reached. This occurs to protect the HVAC system from damage. The updated PCM software calibration will improve this condition due to increased air flow across the condenser.
Another area of concern may be the low engine cooling fan speed on initial vehicle start up. The engine cooling fan speed may be as low as approximately 250 rpm. When this occurs, the A/C system performance may be reduced until the engine cooling fan speed has increased. It may be possible to improve the engine cooling fan speed response. Advise the customer to slowly raise the engine rpm to 1500 every 10 seconds 4 times. Operation above 1500 rpm will not improve A/C system performance and will only allow the A/C compressor to reach the high pressure cut out point faster. Until the engine cooling fan is responding, the maximum engine idle speed should be limited to 1000 rpm while the vehicle is stationary. Any higher than this and the A/C compressor will cycle on and off with little affect on A/C system performance. As additional airflow through the condenser is created, the engine idle speed can be increased without the A/C compressor cycling off due to the high pressure cut out point being reached. Some additional engine cooling fan noise can be expected with the updated PCM software calibration.
The last area of customer concern is the operation of the HVAC system blower motor. For vehicles equipped with automatic control HVAC systems (RPO CJ2), the blower motor fan speed will be limited to 80% of maximum when the HVAC system is operating in the automatic mode. To obtain maximum blower motor speed, instruct the customer to manually adjust the blower motor speed to the highest setting.
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