AC Manifold Gauges

Manifold Gauges are a tool commonly used in servicing air conditioning systems. They can be rented or purchased at many auto parts and tools stores.

Connecting & Use

This illustration from the FSM gives a brief overview on setting up manifold gauges. Click for full-size.

Setting manifold gauges is simple and straightforward.

• The low-pressure hose (usually blue) connects to the low pressure service port on the car. This is the larger pipe (see the Air Conditioning System page to see where this is on your car).
• The high pressure hose (usually red) connects to the high pressure service port on the car (next to the passenger side shock tower).
• The center connector (usually yellow) connects to a vacuum pump.
• The low-pressure service connector (usually green) connects to the refrigerant source. The port for this is usually next to the low-pressure gauge on the manifold gauge assembly.

Vacuuming

It is necessary to vacuum the system before charging.

Initial Vacuum

1. Completely tighten all fittings and adapter valves.
2. Open the high and low pressure valves on the manifold gauge assembly.
3. Open the vacuum port (center valve) on the manifold gauge assembly.
4. Run the vacuum pump.
5. Perform evacuation for more than five minutes to stabilize the vacuum in the system. Check to ensure the low pressure gauge indicates -29.13 to -29.92 inHg.
6. Shut off the high and low pressure valves and vacuum valve.
7. Shut off the vacuum pump.

Checking for Air Tightness

After performing the above...

1. Leave the system as-is for 5 to 10 minutes.
2. Make sure the needle of the low-pressure gauge does not move towards the atmospheric (gauge pressure 0) side at all.
   • If any reverse movement is noted, it indicates poor system airtighteness. Service the system until airtightness is complete. If pressure changes approx 3.94 inHg in 10 minutes, the refrigerant in the system will be exhausted in about one month.
3. Once the system is air tight, continue to fully evacuate the system.

Final Evacuation

Once the system is completely holding vacuum...

1. Run vacuum pump.
2. Open the high and low pressure valves on the manifold gauge assembly.
3. Open the vacuum valve on the manifold gauge assembly.
4. Continue to run for more than 20 minutes.
5. Close high and low pressure valves and vacuum valve on manifold gauge assembly.
6. Continue to the A/C Recharge article.

Reading Manifold Gauges

Reading manifold gauges can be a very effective way to quickly and correctly identify issues with the cooling system.

Condition		Probable Cause	Corrective Action
Insufficient Refrigerant Charge	Insufficient refrigerant charge. Bubbles appear in sigh glass.	Refrigerant is low; system is slightly leaky.	Leak test. Repair leak. Charge system.Evacuate as necessary, and recharge system.
Almost No Refrigerant	No cooling action. A lot of bubbles or something like mist appears in sight glass.	Serious refrigerant leak.	Stop compressor immediately. Leak test. Discharge system. Repair leak(s). Replace receiver/drier if necessary. Check Oil Levels oil level. Evacuate and recharge system.
Malfunctioning Expansion Valve	Slight cooling. Sweat or frosting on expansion valve inlet.	Expansion valve restricts refrigerant flow. Expansion valve is clogged. Expansion valve is inoperative. Valve stuck closed. Thermal bulb has lost charge.	If valve reveals sweat or frost: Discharge system. Remove valve and clean it. Replace if necessary. Evacuate system. Charge system. If valve does not operate: Discharge system. Replace valve. Evacuate and charge system.
	Insufficient cooling. Sweat on suction line.	Expansion valve allows too much refrigerant through evaporator.	Check valve for operation. If suction side does not show a pressure decrease, replace valve.
	No cooling. Sweat or frosting on suction line.	Malfunctioning expansion valve.	Discharge system. Replace valve. Evacuate and charge system.
Malfunctioning Suction Throttle Valve	Insufficient cooling. Frosted evaporator.	Suction throttle valve is inoperative.	Discharge system. Replace valve. Evacuate and charge system. Note: Some owners opt to gut the valve instead of replace it.
	Insufficient cooling.	Suction throttle valve restricts refrigerant flow.	Discharge system. Replace valve. Evacuate and charge system.Note: Some owners opt to gut the valve instead of replace it.
Malfunctioning Condenser	No cooling action; engine may overheat. Bubbles appear in sight glass. Suction line is very hot.	Usually a malfunctioning/dirty condenser.	Check fan belt and clutch. Check auxiliary fan motor. Check engine cooling system. Check refrigerant for overcharging.
High Pressure Line Blocked	Insufficient cooling. Frosted high-pressure liquid line.	Receiver/drier clogged, or restriction in high pressure line.	Discharge system. Remove receiver/drier and replace it. Evacuate and charge system.
Malfunctioning Compressor	Insufficient cooling.	Internal problem in compressor, or damaged gasket and valve.	Discharge system. Remove and check compressor. Repair or replace compressor. Check Oil Levels oil level. Replace receiver/drier. Evacuate and charge system.
Too Much Oil in System	Insufficient cooling.	Too much oil circulates with refrigerant, causing the cooling capacity of the system to be reduced.	Refer to the Oil Levels Compressor Oil section of the A/C System article.
Air in System	Insufficient cooling. Sight glass shows occasional bubbles.	Air mixed with refrigerant system.	Discharge system. Replace receiver/drier. Evacuate and charge system.
Moisture in System	After short operation, suction side may show vacuum reading. During this condition, discharge air will be warm. As a warning of this, the reading vibrates around 6 PSI.	Drier is saturated with moisture. Moisture has frozen in expansion valve. Refrigerant flow is restricted.	Discharge system. Replace receiver/drier (twice if necessary). Evacuate system completely. (Repeat 30 minutes evacuating three times). Recharge system.