89 S10 Compressor Cycle

[blockhead428]

Ive learned a lot searching this forum so this seems like a place I'll get a good answer to my questions. I have an 89 S10 Tahoe, 4.3, roughly 118000 miles on the clock. Previous owner converted to 134a, but the compressor sounded ready to die. Ive changed the compressor, drier, and orifice tube. The compressor is a new, not remanned, R4 ac delco. Oil was added before the install. Goodyear hooked their machine to it and charged it with 28oz of 134. Is this the correct amount for a system that originally took 2.5 lbs (40oz)? I should say that my ac works great. It blows cold and the lines seem to be sweating like a fat guy going up stairs. Im curious about the cycling operation. Mine doesnt cycle at idle, but it seems to cycle every 5-7 seconds while driving at roughly 1500 rpms. Last night via this forum i learned that the cyclimg switch is adjustable. I turned the screw out a whole turn and it seems to be the same as before. I did this yesterday. Weather this morning in memphis is 77 degrees with 90% humidity. I suppose the real test will be this afternoon when its closer to 100 outside. Like ibsaid, it seems to work great, it blows pretty cold, but im not sure what the normal cycling operation should be. Could my cycling switch be crapping out? I really appreciate any help.

[GM Tech]

In a cycling system, the reason the compressor shuts off is to avert evaporator freeze-up. So the switch works off low pressure- the lower the pressure, the less heat is being removed from the cabin, therefor the more likely the evap will turn to a block of ice. and you can't blow air through ice! So the switch is set to exactly the point where the evap won't freeze. Several factors come into play- such as ambient temperature, relative humidity (in Arizona, you can set the switch down to about 19 psi or so, because dry air is not likely to freeze so easy) - also engine speed and blower motor speed and air source (inside vs outside) affect the low side pressure.

Compressors rarely cycle when under high heat load- such as 80+ and high blower- to see it cycle, reduce the heat transfer load by slowing the blower motor (or unplug it entirely) put air source on Max or recirc, raise hood, or mist condenser with a garden hose- all these things will lower low side pressure. Compressor is much more likely to cycle at highway speeds, and at night.

Just because it is not cycling- is not a problem-- why oh why does everyone get their drawers in a wad if compressor does not cycle??? One must consider all the above variables and be happy....

hope this helps some..

[blockhead428]

I appreciate the response. Thats all I wanted was to make sure its working right. My last s10 had the 2.5 and i guess the compressor worked differently. With my current s10, i dont recall how often it cycled right after i first cranked it and began my commute to work. I just know the cycling becomes more noticeable after the first few miles down the road. I expect it to cycle more in the mornings when its cooler outside but it should run longer when its hotter in the afternoon. Or am i wrong?

Like i said in my previous post, im assuming the cycling switch is original. I imagine the previous owner did like most people and thats install retrofit fittings and charge with 134 and call it working. I never knew until yesterday that the cycling switch is adjustable. I noticed that the replacement switch on rockauto (part 36674) says its for r134. Im assuming this means its preset for the pressure difference in r134 vs the original switch being set for r12. Im still learning about ac systems. Im not one that just wants it fixed, i like knowing exactly how and why things work.

That being said, the cycling switch is supposed to cut the compressor off after the pressure on the low side gets too high correct? Turning the screw out (ccw) would let the pressure get a little higher before the compressor cycles. Am i right on that count?

[GM Tech]

The switch turns off compressor when pressure gets too LOW - and I forget which way to turn it- your gage set will tell you....

[blockhead428]

I was under the impression that the cycling switch wasnt really a lpco. Rather it cycled the compressor off until the pressure dropped back down. Ive always referred to it as a low pressure switch until i learned its true operation. Its the first thing we jumper whenever a compressor operation needs testing.

Im learning! Keep them replies coming. Ac is a necessity in memphis, damn near year round.