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All auto A/C service starts with pressure gauge readings. In order to get the pressure gauge readings you require a set of manifold gauges that will connect to the high and low pressure sides of the A/C system.
Illustrated below is a typical manifold gauge set for testing and recharging any auto A/C system. Almost all pressure gauges will conform to the standard color codes (as illustrated) being RED for high pressure; BLUE for low pressure and YELLOW for the common hose that’s used for vacuum and refrigerant supply. Also note that the BLUE or low side gauge is usually what is known as a compound gauge. It reads both pressure as well as vacuum. (The vacuum section of the pressure gauge is shown as the shaded area in the illustration below).
Typical manifold gauges also include ‘on-off’ control valves. Those valves allow you complete control over all procedures including vacuum and recharging. Using the valves you can recover refrigerant from both sides of the system (both control valves open); vacuum the entire system (both control valves open); recharge or add refrigerant to the system on the low pressure side (low side valve open; high side valve closed) or simply get the pressure readings of the A/C system (both control valves closed). Whenever taking gauge readings, be sure that both high and low side control valves should always be closed.
Before connecting the manifold gauges, be sure that 1- all valves are closed 2- Engine and A/C system are off 3- Proper couplers are attached to the manifold gauge hoses
With the required couplers attached to the charge hoses on the manifold gauge set and all control valves closed, attach the RED hose to the HIGH pressure side charge port and the BLUE hose to the LOW pressure side charge port. For testing purposes, the YELLOW or CENTER hose is not required or connected.
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