Troubleshooting a Jeep Cherokee ACby Don Bowman
An air conditioning system contains many sections. It starts at the compressor that compresses the Freon to a temperature warmer than the atmosphere and pushes it through the condenser, which releases the heat to the atmosphere. From the condenser, the Freon moves through the dryer which contains a desiccant that removes any moisture in the system. From the dryer the Freon flows through an H-block, which is similar to an orifice tube in function, where it turns the liquid Freon into a fine spray or vapor. This vapor or spray moves through the evaporator and absorbs the heat within the cabin and returns to the compressor.
Check to make sure that the internal air conditioning fan is functioning on all speeds. If not, the fan motor and blower motor resister must be checked. Check for power to the blower motor. If the blower motor does not have power, check the fuses, and if they are good, check to make sure there is power to the blower motor resister. If there is power to the resister, the resister is bad; if not, the control switch is bad.
Check to see if the compressor comes on when the switch is activated. If the compressor comes on, feel the larger of the two lines off the compressor. The large line is the low side for the low pressure (suction side) which returns the Freon to the compressor. It normally carries 30 to 60 lbs. of pressure. The line should be cold. If it is, then there is a problem with the electric motor controlling the doors under the dash. Remove the glove compartment insert, operate the air conditioning and move the knob that controls the position of air and notice which motor is not moving the lever to operate the door and replace the motor. If the line is not cold, hook up the air conditioning gauges.
Hook up the red (high side) line to the valve on the small line and the blue or low side to the big line. Make sure that the knobs on the gauges are in the off position. The small line from the compressor that goes to the condenser and then to the firewall is the high side hose. This line contains from 200 to 350 lbs. of pressure. Always be careful with this line. If a bottle of Freon was hooked up to this line and the line opened to the bottle, it could explode since the bottle has a lot less pressure than the line.
Look at the gauges. They should read 29 to 30 lbs. on the low side and at least 150 on the high side (with the engine off). If they are lower and the compressor did not come on, then it is undercharged and needs Freon to make the low side switch come on and activate the compressor. Start the car and turn the AC on. Open the low side only valve and allow the charge to build to 30 lbs. on the low side and 150 lbs. on the high side. Listen for the click in the low side switch to turn the compressor on. Shut the valve off if the compressor does not come on. Turn the engine off. Rotate the compressor clutch be hand to see if it is loose enough, and check the wire connector on the compressor.
Start the engine and carefully check for voltage at the compressor. If there's no voltage, check the fuses in the relay box under the hood and the relays. If there is power to the compressor clutch, then the compressor is bad.
Check the gauges with the engine running and the AC on now that the system is charged and see if it is cold. If it is, look for a leak in the system with a leak detector. If it is not cold, notice the gauge readings. If the low side is high and the high side is also high, the H-block is plugged up and needs to be changed. If the low side is low and the high side is low and does not move or barely moves when the compressor comes on, the compressor is bad. If the compressor makes a lot of noise or the clutch will not engage properly, it also requires a compressor. If the high side is very high and the low side normal or low, the condenser is plugged up and needs to be replaced.
- The evaporator is in a box under the dash and uses electric motors to open and close door within the box to direct heat to a desired area. The blower motor and resistor, which controls the fan speed, is located behind and below the glove box. There is also a high side switch in the high side line which shuts the compressor off if the pressure gets to high do to a engine fan problem. There is also a low side switch which will shut the compressor off to prevent damage to it if the Freon becomes too low.
Things You'll Need
- Air conditioning gauges
- Bottle of 134 refrigerant
- Air conditioning reclaimer
- Federal laws require a reclaimer to contain the Freon. It is illegal to release it to the atmosphere.
Don Bowman has been writing for various websites and several online magazines since 2008. He has owned an auto service facility since 1982 and has over 45 years of technical experience as a master ASE tech. Bowman has a business degree from Pennsylvania State University and was an officer in the U.S. Army (aircraft maintenance officer, pilot, six Air Medal awards, two tours Vietnam).