A brief A/C question.

[celticwarlock]

I have a '69 NY with the stock A/C system that I successfully converted to 134a about a month and a half ago. Things are working well, and the air is cold, but I noticed something while it was idling in my driveway before leaving for Carlisle yesterday morning. Every 2 minutes or so, there would appear to be a bit of additional drag on the 2 belts. It only happened with the A/C on. This would last less than 30 seconds, then disappear. It was happening at regular intervals, so it didn't freak me out, but I thought I'd ask about it. Some background on the conversion for those who have not read my post from a few weeks ago: The RV2 was drained of mineral oil and cleaned out, the dryer was changed, the lines and condenser were blown out, the proper amount of ester oil was added to the compressor, all 3 Schrader valves were changed, the system was vacuumed down, and it was then filled with 134a at 70-75% of the original capacity for R12. The EPR valve was not removed because I do not have a thermostat probe to put into in the evaporator. I should also mention that this was originally an AutoTemp 1 car. The flow of antifreeze into the heater core has been disabled by a manual valve so I could get cold air when running the A/C.

Is this additional resistance normal? The compressor is on continuously (as it should be on this car), but as I said, there is this rhythmic cycle going on here that I'd like some insight on. It's not terribly noticeable, but it is there.

 

[70bigblockdodge]

I believe that is the ERP valve closing to keep evaporator pressure high enough, or opening because the evaporator pressure is rising allowing more freon to flow into compressor. As long as your air is cold and you know you have enough oil in the system I would not worry, I left my ERP and never had a problem with the refrigerant side.

 

[twostick]

I've never seen an A/C system that didn't cycle the compressor. I hotwired the compressor on a Kenworth I used to own because of a faulty temp switch on the evaporator. It blew 27 deg for about 10 minutes until it froze the evap coil up solid.

Are you sure the clutch doesn't disengage and re-engage? That would explain what you are hearing.

I don't see how a compressor can run constantly without a 3 way valve to control evap temp like transport reefers do but I'll admit I'm not intimately familiar with AutoTemp systems.

Kevin

 

[celticwarlock]

I've never seen an A/C system that didn't cycle the compressor. I hotwired the compressor on a Kenworth I used to own because of a faulty temp switch on the evaporator. It blew 27 deg for about 10 minutes until it froze the evap coil up solid.

Are you sure the clutch doesn't disengage and re-engage? That would explain what you are hearing.

I don't see how a compressor can run constantly without a 3 way valve to control evap temp like transport reefers do but I'll admit I'm not intimately familiar with AutoTemp systems.

Kevin

Actually, such compressors are very common. Both my Saturns, for example, have what are known as variable displacement compressors, which stay on constantly. I am sure that the RV2 in the New Yorker is very different internally than those Sanden units on my Saturns, but nevertheless, it is supposed to stay on all the time. I am absolutely certain it is not cycling. I, too, am not terribly well versed in AutoTemp systems, but I am aware at the very least that the clutch on this compressor is supposed to remain engaged. I think all Mopar A/C systems with an EPR valve behave this way, but I might be mistaken.

 

[celticwarlock]

I believe that is the ERP valve closing to keep evaporator pressure high enough, or opening because the evaporator pressure is rising allowing more freon to flow into compressor. As long as your air is cold and you know you have enough oil in the system I would not worry, I left my ERP and never had a problem with the refrigerant side.

This gives me some comfort. I hope this is correct. It was a bit unnerving watching the two belts react even slightly to the change in resistance (or whatever was causing them to twitch a bit). I suppose I will have to keep them more firmly taut.

 

[70bigblockdodge]

That dual belt big block setup is a PITA, and don't shine the timing light on them at 2500-3000 rpm you will swear they are going to fly right off. I guess that is why they put a flat idler pulley on them in the 70s