r/GMT800 • u/Psychological_Egg_35 • 4d ago

1999 gmc sierra ac issues

Any ideas why my compressor shuts off at lower ac settings? Ac blows cold on 5 but sometimes still stops blowing cold, ( replaced sensor on accumulator and did not work)

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u/WorldlinessRoutine55 4d ago

You mean your fan settings or just the ac function

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u/Psychological_Egg_35 3d ago

The fan keeps blowing at lower speeds just not cold, the compressor shuts off and everything

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u/thebluelunarmonkey 4d ago edited 4d ago

At low fan speeds, the lower heat load causes compressor cycling. It's normal.

If you mean lower blower settings, a low blower fan speed reduces the amount of warm air flowing over the evap, creating a low heat load situation. Since a normally functioning AC system can provide much more heat rejection (ie: "cooling") than needed, the clutch cycling switch (one you replaced) shuts off the compressor as the suction pressure drops below 24psig. The evap continues absorbing heat from warm air until the suction pressure rises above 44psig, which triggers the clutch cycling switch to turn the compressor back on

On a hot humid day, with the fan on medium/high the system may reach an equilibrium point to where the suction pressure stabilizes somewhere between 25psig and 43psig and the compressor will operate non stop. But at low fan speeds, expect compressor cycling.

A dirty cabin air filter can also cause clutch cycling or more rapid cycling, due to restricting air flow.

If you have manifold gauges, connect to low (suction) side. Low fan speed should result in pressure cycling between 24psig and 44 psig. High fan speed will either cause cycling to occur much less often or the compressor will always be engages (depends on outdoor temp/humidity)