AC question (Page 1/2)
cam-a-lot AUG 16, 10:39 AM
Hello

Recently had my AC charged, R134 and seems to be working. 3800 SC motor

Works fine when I start the car. But after running 10-15 min, the AC cuts out intermittently and won't work for a few minutes. Then it starts working again

Could this be too much refrigerant? Or more likely too little? The guy who topped it up only had a tank and didn't accurately weigh the refrigerant

I don't think it is leaking, since it works great next day when engine is cold

Will try to measure low and high side with engine running and AC on. Where should I be with pressure at summer temps (around 26-30 C?)
82-T/A [At Work] AUG 16, 11:18 AM

quote
Originally posted by cam-a-lot:

Hello

Recently had my AC charged, R134 and seems to be working. 3800 SC motor

Works fine when I start the car. But after running 10-15 min, the AC cuts out intermittently and won't work for a few minutes. Then it starts working again

Could this be too much refrigerant? Or more likely too little? The guy who topped it up only had a tank and didn't accurately weigh the refrigerant

I don't think it is leaking, since it works great next day when engine is cold

Will try to measure low and high side with engine running and AC on. Where should I be with pressure at summer temps (around 26-30 C?)




Do you have a set of A/C hoses? The kind that has the dual gauges and cables for connecting to both the high and low side at the same time? They generally have an indicator on them that should tell you where the pressure should be.

A lot of these places do a half-ass job... (not saying this is the case, but it's a common problem). What should happen is all the Freon that's already in there, should be pulled out. You then put a vacuum motor on both the high and low (using the gauge set I talked about), and it should be able to maintain a constant vacuum of -5 mercury (or ~-5 PSI). With the valve closed to the pump (and the pump turned off), the system should be able to maintain that -5 PSI. If it cannot... then you have a leak somewhere, which would typically be one of the valve seals.

Unfortunately, because you're connecting to both the low and high side valves, you can't actually TEST to see if those valves are even good... these valves themselves actually have a little plastic O-ring as well, and should be changed out too. When you drain entirely a system, they should also change out the expansion tube, which is located in the front of the Fiero... and if I'm being honest, the dryer (the steel cyl in the front compartment) should also be changed out. This has dessicant in it, and if it's allowed to be exposed to the open air (especially if you live by the coast or live in Florida), it's going to absorb water from the ambient temperature. It's supposed to pull out all the water in the lines.

THEN... when all this is done... you should run a vacuum and add the correct amount of PAG oil through the high side vacuum port. This is a good video to explain how it's done:
https://www.youtube.com/watch?v=uqu8Q_p91oY

... and then... seal it back up again and pull a vacuum again. At that point, you recharge with new freon... making sure you shake the R134a cans really well. Some with oil need to be turned upside down, while others can be upright if they don't come with oil).


Anyway, to answer your question... it could be that your low-pressure switch is seeking a lack of pressure, and is temporarily shutting off the compressor. This can also make it really hard to actually fill. In some cars, I've had to take the low-pressure switch connector, and complete the circuit with a paper clip into the connector... that way the compressor keeps moving, and you can correctly add freon. Otherwise you're adding too much pressure before the compressor and then it's forcing the freon into the other side of the line that might be filled with air that came in. This is one of the main reasons why a vacuum should always be done.


It sucks, but if you feel like doing it yourself... expect to pay $200 for every tool you could possibly need (vacuum pump, a good set of gauges and hoses), and then all the O-rings, a new dryer, and a new expansion tube for the Fiero should run you about ~$60 shipped. And then you can literally just buy cans of freon.

It's so much more satisfying when you do it yourself... it's actually really easy once you've done it a couple of times...
Dennis LaGrua AUG 16, 02:18 PM
I was told by a retired mechanic to evacuate the A/C system before recharging to 32 in.Hg that is equivalent to a negative pressure of 15psi. Then monitor if the systems holds for 2 hours. A vacuum pressure of 5inHg = 2.4558psi seems too low. That might work but I believe 32 in Hg may be better. I'll check out my gauge sets later to confirm.

------------------
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cam-a-lot AUG 16, 05:19 PM
system was vacuumed properly. Orifice tube and accumulator dryer was also changed. My question is , is there any chance that there is too much or too little R134A to cause the system to stop working when it gets warmed up?
82-T/A [At Work] AUG 16, 08:52 PM

quote
Originally posted by cam-a-lot:

system was vacuumed properly. Orifice tube and accumulator dryer was also changed. My question is , is there any chance that there is too much or too little R134A to cause the system to stop working when it gets warmed up?




Yes... too little Freon will cause the compressor to cycle because the low-pressure switch will cause the clutch to disengage until pressure equalizes and builds back up. I said that at the end of my long post.
cam-a-lot AUG 17, 04:44 AM
Ok, thanks. I do have a high pressure/low pressure gauge that I will hook up to it and turn on the AC. Need to figure out what my pressures "should" be at current ambient temperatures I guess, then see if I should add some more R134
cam-a-lot AUG 17, 10:38 AM
I connected pressure gauge. Turned the engine on , AC on max

low pressure reads as 110 PSI and high pressure reads as 105 PSI??

How is this possible? Air inside the car was blowing cold. Could this mean I have too much refrigerant and the compressor cuts out because pressure is too high, instead of too low?

Also, on the high side, to the right of the port (toward the brake booster) the fitting actually was warm to the touch, not like the rest of the orifice tube area which was cold. So the fitting on the high pressure side was actually warm


so this is weird

[This message has been edited by cam-a-lot (edited 08-17-2025).]

cam-a-lot AUG 19, 06:54 AM
Could it be I have a blockage in the system? I did change the orifice tube. Is it possible to have put in the orifice tube the wrong way (backwards?)
RWDPLZ AUG 19, 11:32 AM
Since we're not sure how much refrigerant is in the system, compare the pressure to a static pressure chart with the system off for a few hours. This will tell us if it's over or undercharged. 110psi on both gauges at an ambient temp of 80F for example would be highly over-charged. Total R-134A capacity in a Fiero should be about 2.25lbs



The same pressure on the high and low side means the system was just turned on and isn't running yet, or the compressor isn't running or bad. Is the clutch at the compressor cycling on and off at all? You can disconnect ad short the terminals on the accumulator mounted cycling switch to force the compressor on, and see what the gauges read then.

It's technically possible to install the orifice tube backwards, but unlikely. Should look like this installed:



Here's a video with examples of different readings and failure scenarios:

[This message has been edited by RWDPLZ (edited 08-19-2025).]

cam-a-lot AUG 19, 07:59 PM
Thanks for the reply

Here is the weird part. When I took the photo of the gauges, the car was running AC, was blowing cold. So compressor was running and working

I will check the pressure with the engine cold and off and report back. Thank you