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)

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.

A pro won't work on a R12 system without doing a conversion to R134a.

R12 systems should run about 30 on the low side and 150-180 on the high side when the compressor is running. Cold static pressure should be about 70 on both sides.

I have about 20 lbs of R12 I need to sell. Bought it a long time ago.

You are in a catch-22, Roadie. Not only is it illegal for anyone to buy R-12 without a license, the seller also has to have a license. The price of R-12 has come way down now that there is almost no demand for it any more.

You've summed up my thoughts. Want to cut this one loose, but hate to do it with a/c in need of attention...in Phoenix. 2 Jeeps, an F150, and an Explorer make this one a bit odd in the bunch. Hoped to make it a sleeper/project car one day. Like many before me who thought similarly, reality with all it's entanglements is making that look like a pipe dream.

Thanks though- the reassurance was helpful. :beers:

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".

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.

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.

Thanks-

I think I will just invest in a good set of gauges to get a better read of both high and low sides. Really had no interest in pursuing an a/c thing, but it's gnawing at me.

Going to price out an evac too, convert over to R134a. Why not.

Looking at craigslist, around here you can buy R12 as cheaply as R134a. Unless you buy in bulk at Sams Club or similar place.

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.

^^^

Hence the conversion.

If you're going to do a conversion, do it right.

Flush the system thoroughly to get all the old mineral oil out, replace the orifice and accumulator, put in the correct oil, evacuate (check for leaks), and charge to proper pressures and for optimal performance.

Can't argue that! Thumbs up for ya.