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A/C working for short periods of time

SnowDrift

SnowDrift

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On our trip last weekend, the A/C, that had been working around home every time I drove my truck, quit about 5-7 minutes into the trip. Once we got there and it sat a bit, the A/C worked again. At our destination point, I put two cans (12 oz. ea.) of R134a in. On the return trip, it worked for 15-20 minutes and then quit. It worked one time after it quit when I was at idle about 5 minutes later, while still on our trip home. Now it works around home again on short trips. I had tried it occasionally on the 3 hour trip home and it didn't work at all, even at idle, after that one time I mentioned.

According to the gauge on the little recharge hose I bought, it reads low, but I didn't want to overcharge the system. It is a 2.0 lb. requirement. When it does work, it is ice cold. It doesn't act like I would think it would in that I would think it would go from cold to not-so-cold, and so on. This is either ice cold, or it's warm. It works every time I turn it on, around home, on demand.

Since I'm no A/C professional by any means, does this make sense? I don't recall the amount that they system holds, but I thought to put in one more can (12 oz.) and see what happens. I have an hour trip to make today, so it might be a good test for it before the weather starts to turn cold.
 
See if the compressor is running during the warm blow periods. If it doesint might be the cycling switch.
 
Don't keep adding cans. you can easily overcharge the system and then it definitely wont work. Think about what you could do to your engine if you keep adding oil without checking the dipstick simply because it was running rough.

When it does work you say it's ice cold. That means the proper amount of coolant is probably there. Don't trust the little gauge on the recharge "kit" you can buy. That only tells you half the story. You also need to know high pressure reading to know what's in the system. Takes a manifold setup to do this. At this point to get a true diagnosis you need to start with being able to read both low and high pressure with the system running. If we go by the assumption that there is enough refrigerant do this. Replace the relay for the AC in the fuse box. That may be your problem, they can go bad. The low pressure cutoff switch may be faulty too. Replacing that is generally fairly easy but you do need to evacuate the system first. I would hesitate doing this unless you knew that was the problem. I would start with the relay first and see what happens.
 
Checking relay is good idea also.
Cycling switch does not require evac, costs about $20ish IIRC. Its located on dryer mounted to firewall. About a 2 minute job.
 
Since you added 2 cans, I would have it evacced and charged back to the correct charge before you grenade your compressor and end up having to replace a bunch of things. You could also be icing up the orifice tube and tripping the high side safety switch if you over charged it. The low pressure cycling switch is a COMMON issue as GM decided it was a good idea to power the A/C clutch directly off of the switch instead of using the switch to trigger the A/C relay like everybody else did. And do not go by those cheap gauges as they can cost you a lot of money by being way off.
 
I made a 2 hour trip today - one hour each way - and the A/C worked the whole time, flawlessly. I've changed nothing since I charged it with the two cans I mentioned on our trip.
 
Low freon charge allows the oil to build up in the evaporator. The R4 compressor then puts all kinds of metal trash in the system while being starved of oil. It will cool just fine while doing so. In fact these systems work slightly better (colder) at critical low charge.

The only way to know you have the correct amount of freon in the system is to evac the system and charge by weight. Find a shop or friends with the proper equipment. Or rent a vac pump and add 3 12 oz cans to get 36 oz: 1/4 LB overcharged. Obtain a postal scale to use on the 3rd can to get it exactly.

Best to find the leak if it was charged less than 2 years ago.
 
I agree with getting it looked at properly. I have a shop I use for A/C locally. The last time it was touched was maybe 8+ years ago when I put on everything new, except the evaporator.

I guess I thought it wouldn't operate if the system was low on Freon, as a fail-safe. Is there a way to find out if there is debris in the system, or just evacuate, recharge and hope for the best?
 
Leak test the system with a sniffer esp around the compressor body. Recover the 134a. Remove and replace the orface tube and inspect it for the amount of debris it has caught. The debris amount will tell you how the system is doing. You could also pull the compressor hose manifold off and see if there is any dusting or debris coming out of the compressor.

Does the compressor still run quietly or is it noisy with a knock? Noise is a bad sign of course.

The compressor cycling switch on the accumulator does two things. It keeps the evaporator from icing up by cycling the compressor off before it gets too cold. It also cuts the compressor off when the system is extremely low on freon. Known to rapidly short cycle the compressor under some low conditions. It doesn't protect the system when it is low on freon (aka below critical charge. Critical charge is the lowest charge where oil is still moving properly in the system.) I think Cadillac has a system that will protect the compressor. Everything else GM is bean counter torch the compressor when low on freon design. Esp. the R4 design that doesn't have an oil sump or pump. The R4 depends on a steady oil from from the accumulator/evaporator. The only way oil will flow well is with some liquid freon coming out of the evaporator. That liquid freon then boils off in the accumulator. Oil exits through a small hole in the bottom suction tube in the accumulator. The tube goes to the top of the accumulator to prevent any liquid from reaching the compressor.

When the system gets low liquid freon quits coming out of the evaporator because the liquid level has dropped. (This is below critical charge.) This starts to trap oil in the evaporator. The accumulator runs low on oil to deliver to the compressor. The compressor gets reduced or no oil flow under these conditions. Gas moves some oil, but, not as much as the compressor needs and not very steady. The compressor starving of oil puts the debris in the system. The little oil it does get keeps it from locking up completely, but, makes a bigger mess in the system. The cycling switch doesn't kick the compressor out at this point because the pressures are still over the equivalent of ~37 degrees.

No freon boiling out of the accumulator means it is all boiling out of the evaporator resulting in slightly colder air at the vents. You are moving the same BTU's just none from the accumulator. Interesting side effect while the oil starved compressor burns up.

The R4 is it's own worst enemy as the body seals leak causing the compressor to fail from low charge and no oil flow. Out here I simply have to check my AC systems once a year for leaks and making sure the charge is correct.
 
Thanks for the in-depth explanation. I'll print this out so I can study it.

The compressor has been noisier than it was when new for about the past 6-7 year, I suppose. I can hear it over the diesel engine and even inside the cab with the windows closed.

Did I read someplace that these compressors are not available new anymore and that only the reman. version is available?
 
They make them new. Quality sucks and really sucks for reman.

Sounds like you are looking at a system rebuild and flush.
 
This is depressing. WW, thanks for the best explanation I can remember ever seeing since high school auto shop back in the eeearly '70s. Mine lost the shrader valve last year, evac and pressure tested good, but is short cycling the compressor now.

SD, my condolences. Love your squirrel.
 
I think when I need a new compressor I'll be making a Sanden fit.

WW I think its a Sanden 709? you know anything about them? Would it be a good choice for dual air Burb?
 
Sounds... I get plenty of belt slap on my 1993 and 1995 at idle you can hear - more or less normal. Noise/knocking from the compressor is bad. So if you lost the vac pump you could be getting belt vibration. Look between the crank and compressor and see how much the belt is vibrating with the compressor on. It goes away at higher RPM.

Alternate compressors were discussed here. Sanden's site describes an extra high flow unit for buses etc. The thread has me thinking about brackets for my 1995... http://www.thetruckstop.us/forum/sh...xperience-Replacing-R4-Compressor-with-Sanden
 
Burning oil said:
I think when I need a new compressor I'll be making a Sanden fit.

WW I think its a Sanden 709? you know anything about them? Would it be a good choice for dual air Burb?
Click to expand...

SANDEN makes a drop in replacement for the 96-02 trucks, and it is a 709 compressor. I know I liked mine and will be getting another shaft seal so I can put it back on as the GPD replacement for the stock 96-02 compressor doesn't work as good. GPD does make a clone of the SANDEN for quite a bit less money than the SANDEN. The 507 SANDEN I don't think would work very well, but the 709 does do a good job.
 
Whats GPD?
I put a 508 on my 70 (reg cab) and it will freeze you out. Its a Vintage Air system. I made some brackets that went to the R4 mounts and to the Sanden. Works very well. I'll look into it again when the time comes for the 94 Burb.
 
GPD is an aftermarket manufacturer of A/C parts. They're Korean imports, but seem to be of better quality than the China crap nowadays.
 
WarWagon said:
They make them new. Quality sucks and really sucks for reman.

Sounds like you are looking at a system rebuild and flush.
Click to expand...

When you say system rebuild, do you mean new compressor, orifice tube, or do you mean evaporator and condenser, as well? I found a local shop that says they can do a system flush for $50, but what am I looking for, specifically? I will go to their shop first and have them show me what they do and what they use, but at this point, I wouldn't know what to look for.
 
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