A/C works after hooking up manifold gauges

I’ve been researching for about week now and finally broke down and decided to buy some manifold gauges to help troubleshoot my AC problems.

The a/c seemed to be doing nothing. The blower would work on all settings ac light came on but I couldn’t tell that the compressor was kicking on. Warm air blowing, no idle change.

So I brought the gauges home, hooked them up, turned on the vehicle and magically started blowing icy cold air. Yesterday I did put a new fuse in there even though the old fuse was reading good continuity but had no changes in performance. Also I released a very small amount of gas when hooking up the high pressure quick connect. I got 35-40 psi and 150 psi. The low pressure would drop intermittently then come back up. Would only drop 5 to 10 psi.

Could it be that the tiny amount release was enough to drop it down to shut off high pressure switch or did it just need to get the gas moving around? I can’t find anything related to this online.


I suspect this is more of an intermittent poor electrical connection than anything else. Compressor clutch connector or low side pressure switch would be my guess. Offhand, I might say the system could use a bit more refrigerant as both sides are a bit low.

Are those pressures at idle or at elevated RPMs?

That is at idle speed.

Ok I will try to find how to inspect both of those. I think I’ve ran across both somewhere.

Check the pressures with the engine at about 1800 RPM. A 150 is a bit low but will rise when the RPMs go up. This means the 35 to 40 on the low side will drop and that could lower it to the point where the compressor will not engage. Ideally at elevated RPMs (and there are variables) the 35 to 40 is fine but the high side should be up in the 220-230 range.

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I started the vehicle back up yesterday evening to see if by some stroke of luck it would still be working. Nope. I tried to look and see if I could see the clutch spinning but the sun was bright and I couldn’t see down in the dark hole. I tried looking from below but still couldn’t get a good glimpse of it. So I decided to take out the air cleaner box (not sure of the correct terminology but the place where the air filter is). One of the bolts was up under the box and there wasn’t enough room to get a wrench on the bolt and still be able to turn it so I had to cut a 10mm wrench in half. I got the nut loose and when I pulled it out the threads were nearly wore off about 1/4 inch from the end up the bolt. (More to come)

So now I could get a good look at the compressor and check the power running to the compressor. Positioned my voltmeter so as to have the truck running the least amount of time without a air filter. It was 14.3 volts according to my meter which is odd but it is power. Plug the power back to the compressor let it run for just a minute no cool air.

I go to put my air cleaner box back on and the boogered up bolt won’t start for anything. I decide, with my infinite wisdom, to drill a whole in the bottom of the box so I can get a socket and impact on the bolt. Well of course the hard plastic cracks and my whole becomes twice the size I had intended.

Should I patch this? And what recommendations for patch material would you have? Besides don’t be drilling holes in stuff.

I get everything back together. Hook up my manifold gauges and boom. Ice cold air. Low pressure drops from about 65 to 30 then back to 65 intermittently. High pressure around 170 on idle. So I rev it to 1800 rpms for about 2 - 3 mins. The low side top pressure is about 70 and the high about 200. The air is mysteriously colder when I’m NOT revving the engine.

This really seems as if the gauges are bypassing something or reducing the pressure enough on the low side to allow the pressure switch to disengage. Because when I was finished I did a check on the static pressure and the low was at 90 psi. D the high was at about 170 psi.

Thanks for the replies