Rear Air Conditioning Troubleshooting Guide

This procedure assumes the person performing the service work is a qualified air conditioning service technician. If there is any question about the following procedures please call Manex Corporation prior to performing any service work.

Insufficient Rear Unit Cooling

1. Determine if the problem is low air output from the louvers or if the air temperature. If the problem is air temperature, proceed to step 2, if not proceed to step 7.

2. To determine the extent of the problem insert thermometers in the dash air outlet and the most distant rear air outlet and the rear evaporator. Set the air conditioning systems on maximum cooling and high blower with the windows up. Run the engine at high idle (1500 RPM) for 15-20 minutes. Under normal conditions the temperature differential between the dash and rear evaporator should be between 5 to 7 degrees. Any temperature gain between the rear evaporator and the most distant louver can only be corrected with proper insulation of the duct hose and the vehicle. The actual temperatures will rise and fall relative to the ambient temperature, however the temperature differential should remain the same. For heat/cool units proceed to step 3, for cool only units proceed to step 4.

3. If the rear unit is a heat/cool system, check the water valve controlling flow to the rear unit to be sure it is completely closed. If either hose at the rear unit is warm, a water bypass is occurring. Check the vacuum hoses that control the vacuum solenoid. The hose from the long port on the vacuum solenoid must go to the vacuum controlled water valve. The hose from the short port on the vacuum solenoid needs to go to the vacuum tee. Reversal of the connections will prevent the solenoid from venting in the cooling mode resulting in an open valve allowing the flow of HOT water to the rear unit during the cooling cycle. Adjust the control to the valve to insure full closure. If the problem persists, replace the valve. Proceed to step 4 if the water bypass was not the problem.

4. Check the system refrigerant charge with the engine running at high idle (1500RPM) set the air conditioning controls on maximum cooling and high blower with the windows up. The charge should be checked using a combination of gauge reading and louver outlet temperature to determine the charge that will yield the best temperature. In many cases this type of system has better performance when it is slightly undercharged. Proceed to step 5 if refrigerant charge was not the problem.

5. Evaluate the OEM portion of the air conditioning system to insure that all of the factory components are functioning properly and that the air discharge temperature of the dash louvers is adequate. Proceed to step 6 if the factory air conditioning system is functioning properly.

6. Check all electrical connections to the rear unit, especially the ground connections, to insure solid connections. Voltage at the rear unit must be between 12.5 and 13.5 Volts DC. If the system has a solenoid control valve in the line going to the rear unit, check for proper operation. If the valve is being properly energized when current is applied you will hear a “CLICK” sound as the coil energizes. (The coil will be magnetized at this point) If these characteristics are not present, be sure the circuit is correct and good contacts are made. If the electrical coil on the valve is defective (draws excessive amps or does not engage valve), the coil only can be replaced without recovering the refrigerant. If the valve still will not function, repair or replace the valve.

Check the liquid line from the condenser to the rear evaporator, if any portion of the line is cold to the touch or if it frosts up indicates a restriction. If a restriction is found, remove and clean or replace the component causing the problem. If the rear unit still does not cool repair or replace the expansion valve. It is important to remember that when an expansion valve or any other system component is replaced which requires the recovery of refrigerant; ALL ACCUMULATOR & DRIERS in the system must be replaced prior to recharging the system.

7. If the air output appears less then normal from the front and / or rear outlets, check to be sure at least 12.5 Volts DC is present at the blower motors. Check the duct hose connections at the louvers and evaporator to be sure they are properly attached. Check for collapsed duct hose to the air outlets and restricted return air ducts that could restrict air flow. A frozen evaporator coil will severely restrict air output. A heavily frosted or iced evaporator suction tube would usually indicate a frozen coil. An inoperative thermostat causes this condition.