AC system diagnosing

Inside car

• Cabin Air Filter -Low air flow from vents - replace filter

• Climate Control module

  • Fuse & or Relay, check 12 volts

  • MIL or dash light blinking  (AC  failing) 

  • Turn on AC - Can you hear the AC clutch come on?

  • Turn on Blower - Can you hear the AC blower come on?

  • Listen for Low, Medium & High fan air flow.

    • No High fan – Replace Blower Resistor

    • No Noise – check blower motor, if ok, Replace Control mod.

• Blower Motor

  • Fuse & or Relay, check 12 volts

  • Listen for smooth movement, Possible Debree rubbing or smoke.

  • Feel for air flow at 3 speeds

  • Test motor 12 volt to positive, motor will run in high. Ok

  • Fail - Replace Blower Motor

• Blend Doors – Direct air flow, Normally  (2 doors)

  • Fuse & or Relay  check

  • Listen for motor or door & air movement. Ok

  • Repetitive clicking sound or stuck in place is a fail.

  • Fail – Replace bad blend door motor.

Outside the car

• Visual inspection

  • Check for connection and wire damage

  • Look for oily residue or freon dye residue

• Clutch pump

  • Fuse & or Relay  check

  • Clutch should turn on & off if system is working correctly.

  • Clutch should turn by hand in off mode checking for seized up pump.

  • Turn on clutch with hot wire or try defrost switch.

  • Sensors that allow the clutch to turn on. Control, Low PSI, High PSI & temp. sensors.

  • Front clutch will turn the same speed as the belt

  • Belt needs to be in good condition

Gauges Hookup

• Time to take a look at charges.

  • Compare to TAS chart.

  • If no PSI a leak is present.

  • If PSI is low or ok,  Check PSI Sensor

• PSI Switches

  • Low Pressure switch (on big line)

    • LPS failing 90% if ac clutch is cycling on & off to fast.

    • Bypass the switch if clutch turns on, the LPS is bad.

  • High Pressure switch check, smaller line

• If no charge

  • Fuse & or Relay  check

  • Runs in conjunction with temp sensor, fan mod & PCM

  • Fan runs with AC control on all the time.

• Fan high

  • Fuse & or Relay  check

  • Runs in conjunction with temp sensor, fan mod & PCM

  • Fan runs with AC control on under load only as needed.

Hints

1. 35° + 55° degrees Normal vent temperature at Max Cool

2. Vacuuming system down (replace with new Pag Oil w/UV dye)

3. Rule of thumb is replace 2oz Pag oil for every component changed.

4. Compressors are somtimes pre-loaded with oil, Find the recommended amount and adjust.

5. To warranty a compressor (flush system) also replace recommended components 

6. Low Side, 25psi to 45psi  – Thick tube

7. High Side, 225 – 300 psi  –  Thin tube

8. Leaks – inspect, damage, corrosion, residue, wet, use soap & water sprayer, UV Dye, Electronic Detector

9. Check fill ports for leaks always cap when done.

10.  Some compressors run constantly (no clutch)

 

Direction of freon flow. Starting at (Compressor), (High Side line) (thin line) (High pressure switch, anywhere in high side) (top of condenser) (bottom of condenser) (Drier) (High Press port anywhere in high side) (Expansion Valve or Orfice tube) (Low Press starts) (thick tube) (Low side Port & Switch) (Evaporator)  (Low Press tube Compressure)

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Ambient Temperature (examples -normal readings)

 Ambient Temperature in °F Low side Pressure Gauge reading

and High side Pressure Gauge reading
 

65°F Ambient temperature: 

Low side pressure 25-35 psi High side pressure 135-155 psi
 

70°F Ambient temperature: 

Low side pressure 35-40 psi High side pressure 145-160 psi
 

75°F Ambient temperature: 

Low side pressure 35-45 psi High side pressure 150-170 psi
 

80°F Ambient temperature: 

Low side pressure 40-50 psi High side pressure 175-210 psi
 

85°F Ambient temperature: 

Low side pressure 45-55 psi High side pressure 225-250 psi
 

90°F Ambient temperature: 

Low side pressure 45-55 psi High side pressure 250-270 psi
 

95°F Ambient temperature: 

Low side pressure 50-55 psi High side pressure 275-300 psi
 

100°F Ambient temperature: 

Low side pressure 50-55 psi High side pressure 315-325 psi

105°F Ambient temperature: 

Low side pressure 50-55 psi High side pressure 330-335 psi
 

110°F Ambient temperature: 

Low side pressure 50-55 psi High side pressure 340-345 psi 

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A/C Reading Pressures for (diagnosing)

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 Low side / High Side

 Low Side 25-40  / High Side 170-250 Normal Pressure affected by ambient temp- 

 

Low-Low Side /  Low-High Side,,  Under Charged - Freon -

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 Low-Low Side  / Low-High Side,,  expansion valve Restricted or closed -

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 Low-Low Side  / Low High Side,,  Orfice tube Restricted or blocked -

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 Low-Low Side  / Low-High Side,,  Suction line at pump Restricted or blocked -

 

 Low-Low Side  / Low-High Side,,  Condenser Line to Evaporator, Evaporator  Restricted -

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Low-Low Side  / High-High Side,,  Evaporator Restricted Air Flow -

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High-Low Side / Low-High Side,,   Compressor weak, High side needle will shake -

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High-Low Side / High High Side,,  Over Charged 

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High-Low Side / High High Side,,   Splash  water onto Condenser-if press drops,, Over Charged  -

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 High-Low Side / High High Side,,  Condenser Air Flow Restricted  -

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 High-Low Side / High High Side,, Engine overheating Temp High -

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 High-Low Side / High High Side,,  expansion stuck Open -

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 High-Low Side / High High Side,,  Air, Moister in freon