A.C. manifold gauge

[Troy Ahner]

I have a set of A.C. manifold gauges (for R134). The last time I had them hooked up to the car I was getting a reading on the high side gauge when only the low side was hooked up. Earlier today I took out the valves from both sides and there is a hole straight through from one side to the other. This doesn't seem right. Is it supposed to be this way? How am i supposed to get readings from each side of the system at the same time if they are connected?

[Bruce Wrenn]

With both valves closed, only the gauge on that side sees pressure. Opening either valve allows for connection to center hose, which is used for vacuuming, and charging system.

[Ronald Blue]

That's normal and correct. When doing an initial charge on an evacuated system you will usually add liquid on the high side with the vehicle or whatever it happens to be shut off. Once you have equalized pressure between the Freon canister and the system you close the high side valve and start the engine and then finish charging through the low side port.You should have both valves closed on the manifold when just checking pressures. Were both gauges reading pressure? Can you attach a photo or two showing the gauges with hoses attached?

[Troy Ahner]

Thank you guys. Now I understand. I didn't realize the valves are only for connecting to the center port. The valves were not closed so the low side pressure was reading on both gauges. (High side wasn't hooked up)
Thanks Again

[Jason Roehl]

There are also more advanced uses for those valves when it comes to system diagnosis--you can crack both valves to allow some refrigerant to pass from the high side to the low side on a running system. I've only seen brief reference to it, and I'm not HVAC certified, so I really couldn't explain the usefulness of it. I think it might have had something to do with finding a plugged orifice.