AC Systems 101 - Causes of No Refrigerant Pressure?

Ahoy !
This is a learning situation for me - AC 101 - so, I apologise if I become trying.
(I don’t know history of car. 'Twas not abused but more the case of being overlooked.)

The gauge on the auto parts store can of refrigerant (low side fill) did not register any pressure at all. Needle didn’t even blip.

Since the pressure gauge needle did not move,
a) If system is compleatly void of refrigerant, then, one 20 ounce can of refirgerant may not be sufficient to obtain a measureable amount of pressure given the “can” equipment I had. Nope - not at this time do I want to spend money of additional refrigerant. Maybe some diagnosis, first.
b) I am guessing the compressor is not working because the few leaks my cars have had were slow leaks. So, a working compressor should have shown a pressure bleed-down reading on the gauge. Also guessing it would have to be a massive leak - unusual ?- to allow the can of refrigerant to vacate to atmosphere seemingly immediately.

  1. Is there a way to attempt to test-pressurize the system - say by compressing all that free air in the atmosphere - to determine if there is a leak ?
  2. In my ignorance of AC systems, I a thinking the lack of pressure is either i) failed AC compressor or, ii) leak to atmosphere. So then, I should press ahead and replace the compressor (and receiver/dryer ?) and attempt to fill the system with refrigerant-leak dye and see what’s what. Question: Is there any component other than the AC compressor the failure of which would cause zero pressure ?
  3. I am going to continue poking around the internet for S Type specific diagrams and trouble-shooting procedures but, if anyone can point me to information, I’d appreciate it.

nb: I had to engage the clutch by by-passing Relay No. 8 and jumping terminals 3 & 5.

All the best,
Richard Cielec
Greater Chicago Metro, U.S.A.

My past experience on a a/c issue on my last X300 proves that spending a £100 having the system vac’d down, diagnosed and recharged by a Specialist is the CHEAPEST way to go!

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That’s probably where this will end up but, I feel the need to give my friends the opportunity to say “I told you so.” Ha!
All the best,
Richard Cielec
Greater Chicago Metro, U.S.A.

Hi, this is not S-type specific but generally for an
R134 system. 20oz can should be plenty to get a pressure reading if it’s holding pressure. Obviously to start the compressor you’re jumping past the pressure switch so you have no pressure or less likely it’s overchargrd.

If you really want to diagnose this yourself you need a real gauge manifold. Not super expensive from HF. Those can gauges are notoriously un-reliable. I just was working on another car’s A/c and just for fun put the can gauge on it. It showed all kinds of ridiculous readings. My manifold gauges showed what was actually going on.

Also to answer your question about refrigerant loss. It could literally be anywhere in the system. Any joint or component.

Thank you, Mike
Can gauges not reliable - Yes, agree. I was looking for nothing more that a something or nothing reading. Yes, agree, I should get a propere gauge manifold.
Repeating and rephrasing to make sure I understand: Pushing aside external leakage for the moment, the AC compressor is the only component involved with producing pressure. That is: if there are no external leaks, a compressor in good working condition will pressurize the system. If a compressor in good working condition does not pressurize the system then, the cause is an external leak (failed O-rings, failed gaskets, broken hoses or lines, rusted out receiver/drier or accumulator/drier or condenser or evaporator (Ugh!). The expansion valve, if failed will still allow the system to be pressurized but, may not allow the system to produce a cooling effect. All this about right ?
Thank you.
Richard Cielec
Greater Chicago Metro, U. S. A.

Richard, yup, about right. the compressor is the only component that pressurizes the system but there will be static pressure (when it’s charged) with the system off. If you hook up a gauge manifold to the high and low sides when it’s off there should be equal pressure on both sides. The thermal expansion valve can cause issues if it sticks open or closed but the symptoms of that will show on a gauge manifold. Modern cars have a pressure switch that will prevent the compressor from operating without pressure in the system but if put a 20oz can of pressurized refrigerant into it and the system isn’t leaking there should be enough pressure for it to activate the switch.
If there is no pressure it’s leaking. Next step is to diagnose where it’s leaking from.

The car I was just working on had a completely sealed and pressurized system but the a/c didn’t work because the compressor had failed internally. It was pretty obvious with the gauge manifold… The can gauge showed nothing useful… The can gauge actually would drop to zero so it looked like it was leaking out - not at all what was happening. So if you’re sure you have no pressure after squirting in that can it’s time to figure out where it’s leaking. You’d need to get the tools though.

I see you’re in the Chicago area so you have the winter salt to deal with… Might have a rotted condenser… Or really anything else. I hope it’s not the evaporator core.


Yes, I, also, hope it’s not the evaporator core. Rusted evaporator is my fear, too.
I am assuming no leak nor assortment of leaks can be so great as to cause imeediate zero pressure reading (unless, say, massive (collision) puncture of the condensor or a broken line. (I would expect bleed-down). So, I’ll replace the compressor, the gasket and “O” rings on the piping that attaches to it and, see what is the result.
Richard Cielec
Greater Chicago Metro, U.S.A.

Well like I said, those can gauges are pretty much useless so zero pressure on that thing may be meaningless… Hard to say. If it is leaking it wouldn’t neccessarily take much time to get to zero. Even schrader valves can leak.

Compressors are expensive though. Might be worth having an a/c specialist diagnose the problem before spending a few hundred on a compressor if you don’t wanna invest in the gauge manifold and vacuum pump. If you get an a/c guy to tell you what needs replacing then you can save the $$ and do the parts hanging yourself.

Also if you’re not aware there is r134a in a can with a glowing dye in it. You install it and then use a black-light flash light to find the leak. Not super expensive and may help you find it.

BTW you should always replace the receiver/drier if you open the system.

Good luck. Fingers crossed your evap core is okay

As I suggested earlier, save yourself aggro & $$$$$$’s go to a Specialist!