Question about 2003 Chevrolet S-10

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Just bought 2003 S-10 4-cy manual transmission. A/c compressor will never cycle off so the evaporator coil will ice up at highway speeds. The 3 wire sensor switch in the high pressure line was replaced. The voltage reading on the signal line from this switch is 3.1 volts when the a/c is first turned on and then after it has run for about 12 minutes it reads 2.4 volts. I?m not sure about the system being charged correctly. what voltages reading should I expect that would cause the pcm to turn off the compressor? thanks don T

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You will need to replace the compressor cycling switch, this part is screwed into a bung in the low side AC line under the hood.

Posted on Jul 07, 2010

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AC works for about 300 miles then stops and blows hot air. New AC compressor and freon charge. After sitting for a while it will again blow cold.

I think you have a similar problem to mine. At highway speeds is when it happens. And it is not blazing hot? Right?
Your evaporator is freezing up - as this happens the ice surrounding it prevents the flow of air over the evaporator coils and the result is warm air coming out of the registers. If I turn off the AC for about ten minutes and let unit melt the cold air comes back. This does not occur in stop and go driving.
I combat it by either running the fan at a higher than normal setting to increase air flow over the evaporator and then turning off when it gets too cold, OR using the air control to allow warmer outside air to enter into unit at highway speeds to prevent freezing up.
Newer cars have more sophisticated sensors and controls to prevent this but some older vehicles are subject to this anomaly.
Since it works fine under normal conditions I have not messed around with the freon level in system. If you release a tad of refrigerant the low pressure switch will shut off the compressor sooner and help minimize ice buildup. Good luck.

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Won't accelerate on highway? At what speed (estimate) are you traveling before you "punch it". Is the engine a 3.4 or a 3.8? Is the check engine light on? Is it flashing?

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Seems the AC is freezing up after about 30 or 40 minutes. After it unfreezes, it will work fine for another 30 or 40 minutes. I believe that the freon levels are correct.

If you have ice accumulating on the evaporator, there should be no air blowing out the registers because the air will be stopped at the evaporator.
If this is happening, the a/c is not cycling on and off as often as it should. At 70 degrees F, it should cycle on and off about 4 times/minute. Less when hotter.

If not cycling off at all, replace the screw-in pressure switch on top of the accumulator under the hood near the fire wall on passenger side.

report results of test.

The A/C cycling switch is mounted on a Schrader valve-type fitting on the suction accumulator/driver.
  • A valve depressor, located inside the threaded end of the A/C cycling switch, presses in on the Schrader valve stem.
  • This allows the suction or low side pressure to control the operation of the A/C cycling switch.
  • The electrical switch contacts open when the suction pressure drops. The contacts close when the suction pressure rises. For specifications regarding operating pressure(s), refer to Section 412-00 .
  • When the A/C cycling switch contacts close, the A/C clutch field coil is energized.
  • When the A/C cycling switch contacts open, the A/C clutch field coil is de-energized and compressor operation stops.
  • The A/C cycling switch will control the A/C evaporator core pressure at a point where the plate/fin surface temperature will be maintained slightly above freezing.
  • This prevents icing of the A/C evaporator core and blockage of airflow.
  • It is not necessary to discharge the refrigerant system to remove the A/C cycling switch.

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A c starts off cooling good then compressor starts kicking in and

Several possible causes. Knowing the state of refrigerant charge would be helpful. Assuming it has a 'full' charge (full charge 680 Grams = 1.50 Pounds) Most common causes of AC problems:
1. Thermostatic switch (sensor) malfunction in the evaporator - allowing the evaporator to ice up. Check the vent temperature (using a cooking thermometer) to see if the temperature gets near or below freezing (32 Deg F.). If so, the thermostatic switch in the evaporator is defective. Replace it.
2. Refrigerant UNDER-charge. If the temperature does NOT get near freezing, then you need a set of AC gauges to check the compressor head pressure (High side) and the suction (Low side). High side pressure (nominal) is 229 PSI (+ - 20 psi) @ 2000 RPM. Low side (nominal) is around 20 to 40 psi. NOT common problems:
1. The magnetic clutch coil on the compressor... as it heats/warms up, the coil windings could be going 'open" circuit which would turn the compressor 'off.'
2. Compressor inefficiency (worn out).
3. Expansion Valve
4. Clogged Condensor

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