Low side pressure swings
#1
Low side pressure swings
General A/C question here, it's actually for another vehicle I own, still an R12 system.
No cold air is blowing. That's the symptom.
I tested the low side pressure at idle and it sits right at 50 with an ambient temp of about 95 degrees.
Under sustained RPM's at 2K, the pressure swings wild from about 35 to 100 as the compressor cycles on and off. Hmmm...that has me concerned.
I've checked blend door operation and it is normal, but the compressor is making some noise- no surprise on a 93- it is probably going bad, but I've heard compressors make noise when low on refrigerant too if I recall.
My biggest concern is that pressure fluctuation- I can't find anything on that particular thing, but I think it's not right. Normal in a low refrigerant situation? Maybe a clogged orifice tube?
Thanks in advance for any insights!
(in case it matters, the car is a 93 Bird LX 5.0)
No cold air is blowing. That's the symptom.
I tested the low side pressure at idle and it sits right at 50 with an ambient temp of about 95 degrees.
Under sustained RPM's at 2K, the pressure swings wild from about 35 to 100 as the compressor cycles on and off. Hmmm...that has me concerned.
I've checked blend door operation and it is normal, but the compressor is making some noise- no surprise on a 93- it is probably going bad, but I've heard compressors make noise when low on refrigerant too if I recall.
My biggest concern is that pressure fluctuation- I can't find anything on that particular thing, but I think it's not right. Normal in a low refrigerant situation? Maybe a clogged orifice tube?
Thanks in advance for any insights!
(in case it matters, the car is a 93 Bird LX 5.0)
Last edited by Simply67X; 08-15-2015 at 01:35 PM.
#2
You should put a full gauge set on it and see what the high side pressure is. However, based on what you have seen so far, I would expect the orifice needs cleaning and the dryer/reservoir needs replacement. Usually when the dessicant in the dryer/reservoir starts breaking up, it clogs the orifice. You might want to take it to a pro.
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#6
You should put a full gauge set on it and see what the high side pressure is. However, based on what you have seen so far, I would expect the orifice needs cleaning and the dryer/reservoir needs replacement. Usually when the dessicant in the dryer/reservoir starts breaking up, it clogs the orifice. You might want to take it to a pro.
Thanks though- the reassurance was helpful.
#7
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Your symptoms in post #1 is of a very low charge in the system.
You should read near ambient temperature (in degrees F) static pressure when the system isn't engaged (Phoenix in the summer). When the clutch engages in a system with an inadequate charge, the low side pressure will drop to the lower trip point of the cycling switch, usually around 25 psi. The clutch will disengage and the pressure will rise rapidly to the upper trip point around 45 psi and the clutch will re-engage. This is called "short-cycling".
You should read near ambient temperature (in degrees F) static pressure when the system isn't engaged (Phoenix in the summer). When the clutch engages in a system with an inadequate charge, the low side pressure will drop to the lower trip point of the cycling switch, usually around 25 psi. The clutch will disengage and the pressure will rise rapidly to the upper trip point around 45 psi and the clutch will re-engage. This is called "short-cycling".
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#8
He said it cuts off at 35psig and that should be too high for the low pressure cutoff to activate, which made me think that high discharge pressure was causing the cutoff of the compressor.
I kept my old 86 Monte Carlo SS for years intending on fixing it up and building the performance but by the time I had time to do it, I had lost interest. So, I passed it on to a young kid who had dreams of building a hot rod out of it. He and his Dad fixed up old Chevelles, Camaros and such, so it went to a good place.
I kept my old 86 Monte Carlo SS for years intending on fixing it up and building the performance but by the time I had time to do it, I had lost interest. So, I passed it on to a young kid who had dreams of building a hot rod out of it. He and his Dad fixed up old Chevelles, Camaros and such, so it went to a good place.
Last edited by Roadie; 08-16-2015 at 11:49 AM. Reason: corrected the year
#9
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Since the gauge isn't seeing the exact pressure as the switch due to several possible variables, there's a potential for a lot of variability if the gauge reading. We don't know his specific hookup.
Since it's an R12 system, it does not have a high-pressure containment switch, so that effectively eliminates cycling due to high side pressure.
Since it's an R12 system, it does not have a high-pressure containment switch, so that effectively eliminates cycling due to high side pressure.
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Several tidbits:
Purchasing R12 requires a MACS 609 license, R134a doesn't.
R134a by the can is often half the price at Walmart as it is at the auto parts stores.
The gauge fittings for an R12 set will be different than for a set intended for R134a. You'll need adapters if you want to check both types of systems.
Purchasing R12 requires a MACS 609 license, R134a doesn't.
R134a by the can is often half the price at Walmart as it is at the auto parts stores.
The gauge fittings for an R12 set will be different than for a set intended for R134a. You'll need adapters if you want to check both types of systems.