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A/C will hold a vac, but won't take freon!

91502 Views 8 Replies 6 Participants Last post by  dsfloyd
86 w/a 350. Converted to R134a years ago. Been charged and working since then. Had a small leak recently. Usually I can charge it this time a year and only have to add a can or so through out the summer. Went to pull a vac on it, but wouldn't take the freon. Here's what I've done:

- Borrowed mastercool pump (1.5 CFM Single stage) and manifold set from AZ
- Hooked up according to instructions(blue to low, red to high, yellow to pump)
- Turned on pump
- Opened low side
- Opened high side
- Let it pull to 29hg then let it run for another 30 mins
- Closed off valves
- Set for 10 mins to check vacuum (it held)
- Hooked up freon to yellow hose
- Opened low side (needle didn't move)
- ?????

That's when nothing happpened. No compressor engagement, no can turning cold, no sucking of the freon.

Don't know if it's the pump or not, but seems like when it was pulling you could here it, but when it got to 29, it just kinda sat there, running, but didn't seem like it was still pulling. Took the hoses off to feel the pressure, and it didn't seem like it was sucking that much. When I pushed on the schrader valve to check it, it just kinda did a "slight breeze" vs. a hiss.

Am I missing a step?
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Does it have any pressure on the high side? I've found in a couple vehicles in the past that they would not pull any freon on the low side until the high side got "jump-started". If the high side has any pressure at all DO NOT OPEN THE HIGH SIDE VALVE. But, if it's still in a vacuum or no pressure, dump a little freon into the high side. It might be just enough to get the compressor engaging and pulling freon in on the low side. Again, NEVER open up the high side valve if there's any pressure over there. The pressure will backfeed into the freon can and turn it into a grenade, complete with schrapnel. If in doubt, don't do it.
Needle doesn't even attempt to move on the high side. Thought about jumping the cycling switch to see if that'll jump start it. May try to put a little over there and see if that will work.

Thanks for the input. Wasn't this much trouble in the past..but not doing it that often, was wondering if I missed a step.
Is it possible that the schrader valves aren't being depressed by the pins in the manifold hoses? This would still allow the vac pump to pull down vacuum but it would only be in the manifold hoses & not be pulling vac in your AC system. Then when you open the manifold valves it only charges the manifold hoses, but not your trucks AC system. Many times with borrowed/rented tools the previous user has lost pieces & parts. I would double check to see if the pins are even in there, & make sure that they are actually depressing the schrader valves in your AC ports.
Thought about that too. Took the quick connect off the low side to relieve the pressure back to zero. Hooked it back up, turned the valve, and presto...a vacuum. I know the needle is supposed to be around 40 when charging, but it won't budge and the sight glass isn't showing a thing.
My gauges have a feature that allows one to extend or retract the pins
If you can,

Hook up the can to the compressor, and jumper the compressor so it turns on.
Make sure you have oil in the system. ;)

This should force it to suck in the freon.....worth a shot.

But if your system is empty (which it sounds like it is the case), then your compressor may not kick on....thus not suck in the freon.
Don't know if this had anything to do with it...when I got the kit, the yellow hose was on the bottom which looks to be controlled by the knob. Did some experimenting and found that if I hooked it up to the top, it seemed to pull better. Sure enough, vapors started coming out of the pump, vacuum pulled, compressor kicked on, and so began the cool down process!!!

Thanks for all the advice!!! Gauge Automotive tire Auto part Gas Cable


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Glad you fixed the issue.
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