My heater badly needed replacement (leaking a fine mist of coolant inside the cabin), the A/C hadn't held a charge for years, and they're both in the same box that's a pain to get to and you have to break both systems to get the box out. So I replaced both at the same time. New heater core, and a completely new A/C system except for one hose that I didn't realize was absent from both kits that I thought would cover everything. Oh well, it's just an open hose, nothing special about it so long as it doesn't leak.

Anyway, I put it all together, vacuumed it out, put the first can on, and...what?!? BOTH gauges spike way up when the compressor engages, and quickly fall back down when it stops. Different angle for each needle because of the different scales, but the same pressure always. Perfect tracking between them.

Thinking through how the plumbing works, I figured there must be a plug somewhere between the low-pressure port and the compressor. The orifice tube becomes moot because there's no flow, so both sides have the same pressure. So I took it back apart, immediately downstream of the low-pressure port and off of the compressor, and blew some dry shop air into it, expecting nothing from the other end. But I got a lot! In both directions! And I didn't see anything blocking the compressor's input either, like maybe a plastic dust cover that I forgot to take out. Nope, that's open too.

Okay, so the new evaporator and dryer are good, which means I don't have to dig the box back out again. YAY! But I still have this problem. Why do BOTH sides increase, exactly the same, when the compressor runs?

1 Answer 1


As we say in the software world, this turned out to be a PEBKAC error. (Problem Exists Between Keyboard And Chair)

I put it back together and vacuumed it out again, not expecting anything different, but maybe just messing with it might have dislodged something. Then I got to thinking about how the manifold gauges work.

If each tap has its own gauge that is always connected, and there's a valve to connect each one to the common manifold or not, which is where the new can or the vacuum hook up...<mull around for a while>...D'OH!!!

I had just finished vacuuming it out with both valves open, and I hadn't closed the high-pressure valve to fill it! So both ports were connected through the manifold! No wonder they both went together!

Closed the high-pressure valve, so that side was just an isolated monitor, and it filled right up and did exactly what it's supposed to!

It would sure be nice if these things came with better explanations of how they're designed and how to use them, so I wouldn't have to practically re-design one for myself and then guess that that's how it's actually designed. 3 different manuals say nothing about HOW to use a manifold gauge set, only that you do...(somehow?)

Well, I guess now I know...having sent one 12oz can entirely into the atmosphere, because once you open one it can't be re-sealed, and I needed to take it off to get the vacuum back on again. (It would also be nice if ALL cans would re-seal...)

  • Once the system is running you see a difference across the throttle.
    – Solar Mike
    Apr 11, 2021 at 5:26
  • There are non puncturing cans available. They take a different inexpensive tap. Where I live the puncture type is mandated obsolete.
    – Jupiter
    Apr 11, 2021 at 11:40
  • @Jupiter That's good to know, but it doesn't seem to have made it to my part of the world yet...or maybe it has? I bought two 18oz cans of refrigerant with a leak-stopper included, as a preventative measure, and they each came with their own tap, hose, low-side gauge, and quick-connector. But they don't fit my manifold gauge, which was still needed for the vacuum. So I used a 12oz puncture can after the vacuum so I could hook it up to the manifold and avoid spoiling that vacuum with regular air, and then finished it off with an 18oz self-contained can with the leak-stopper in it. (continued)
    – AaronD
    Apr 11, 2021 at 18:12
  • It took all of that too, and came out about right, and then I noticed some instructions about attaching the self-contained hose to the can. Why would it say that if it came already attached? So I tried to take it off of the empty can, and sure enough, it unscrewed to reveal a rubber seal, and a plunger that pushed a button on the can itself. And the can itself looks an awful lot like my pile of cheap puncture cans. Could it be that I just don't have the right fitting to use them that way with the manifold gauge? All I have that fits the manifold is a puncture needle.
    – AaronD
    Apr 11, 2021 at 18:12
  • (I put that self-contained hose in a ziplock bag and only threw the can away)
    – AaronD
    Apr 11, 2021 at 18:18

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