2002 Civic A/C trouble shooting
#21
To help understand the diagram, when you press the A/C button the control panel grounds the blue wire on pin 4 (if the interior fan is on and the evaporator temperature is not below the freezing point). If the pressure switch is closed (pressure OK), the multiplex control unit detects that the blue/white wire has been pulled down to ground. This causes it to send a coded signal over the yellow wire to the ECU. The ECU decodes this signal and increases the idle speed and turns the fan relay on. After a short time delay (for the engine to stabilize at higher idle speed) it also grounds the red wire, engaging the compressor clutch relay. Power from the relay contact passes through the thermal switch to the clutch coil.
Since (I think you said) you can make the compressor run by grounding the blue/white wire, that means the MCU, ECU, relay, and thermal switch are all OK. Check that the pressure switch is closed and that the control panel is grounding the blue wire. If it is not, run the control panel self-test.
Since (I think you said) you can make the compressor run by grounding the blue/white wire, that means the MCU, ECU, relay, and thermal switch are all OK. Check that the pressure switch is closed and that the control panel is grounding the blue wire. If it is not, run the control panel self-test.
#23
Plug everything in like stock. Start engine and measure voltage from the blue wire at the pressure switch to ground. Should be some positive voltage (5 volts or 12 volts I'm not sure). Turn interior fan on and activate A/C button. Voltage should go to near zero (and compressor should start...)
If the voltage doesn't go down this means the control panel is not driving the blue wire. This is probably a bad control panel though it could also be the evaporator temperature sensor. I would think the control panel self-test would check the temperature sensor though. Also measure at pin 4 of the control panel (the other end of the blue wire) to rule out a break in the wire in the car.
If the voltage doesn't go down this means the control panel is not driving the blue wire. This is probably a bad control panel though it could also be the evaporator temperature sensor. I would think the control panel self-test would check the temperature sensor though. Also measure at pin 4 of the control panel (the other end of the blue wire) to rule out a break in the wire in the car.
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