AutoFocus: The Honda (!) may tell the A/C clutch to stop working!
In the late 1990’s, Honda automobiles started using a blower motor power transistor to control the speeds of the blower motor instead of the traditional blower motor resistor. Now, when the blower motor power transistor fails, the air conditioning (A/C) compressor will become inoperable. When the blower motor does not operate on later model A/C systems, the heater control panel will set a code which in turn shuts off the request for A/C compressor operation. If you are not familiar with this problem, you may waste time trying to diagnose the compressor operation.
When a later model Honda comes into your shop with no power to the A/C compressor clutch, verify that the relay and fuses are good, and then check to see if a scan tool indicates the A/C request is off. To save time, first check the heater control panel for codes. When a code is indicated, diagnose the code before testing the A/C request circuit (our Direct-Hit Hotline Archives explain how to check the A/C system for codes).
If there are no codes present, continue testing the A/C request circuit as usual. If there is a code for the blower motor circuit and the blower motor does not work on any or all of the speeds, testing the blower motor power transistor and the blower motor circuit is the first step in repairing the A/C problem. The blower motor power transistor is located in the same place as the blower motor resistor used to be. Most blower motor power transistors have four wires, but there are some systems that use three wires and have a separate relay for high speed. With either system, the blower motor circuit testing is the same.
To test the blower power transistor circuit:
1. Verify there is battery voltage to the blower motor on the blue/white wire. If so, ground the blue/black wire at the blower motor and verify the blower motor operates.
2. If the blower motor is good, jump the blue/black wire at the blower motor power transistor to the black wire and verify the blower motor operates; this tests the blower motor wiring and also indicates the black wire of the power transistor has a good ground.
3. Next, test the blue/yellow wire at the blower motor transistor; this is the command from the heater control panel/climate control unit. When the heater control panel is on and is trying to operate the blower motor, it should output a voltage on the blue/yellow wire to the base of the blower motor power transistor. The transistor then turns on and provides a ground for the blower motor. The more voltage the heater control panel outputs, the faster the blower motor should operate.
4. If there is voltage on the blue/yellow wire and the blower motor does not operate, the blower motor power transistor is faulty (some heater control panels will only output voltage for a few seconds and then set a code).
5. Another test is to remove the blue/yellow wire from the power transistor connector and apply battery voltage to the blower motor power transistor for a very short period of time. If the blower motor does not operate, the power transistor is faulty.
If there are no blower motor control problems, testing of the A/C request circuit will be necessary. On 2001-2005 Civics, the A/C request circuit is a reference voltage that is sent out from the multiplex control unit to the heater control panel. The request voltage passes through the A/C pressure switch (and on some model years, an A/C thermal protector) and then to the heater control panel. The reference voltage is five to 12 volts and will not light a test light. The heater control panel grounds the request circuit to indicate A/C compressor operation has been requested. Once the multiplex control unit sees the voltage go below one volt (or to ground), it will then send a signal to the engine control module (ECM) to turn on the A/C compressor clutch relay.
To correctly test the circuit, start at an easily accessed point in the circuit like the A/C pressure switch.
1. Unplug the switch and use a digital volt ohm meter to test for the reference voltage on the blue/white wire (2001 Civic) or the red wire (2002-05 Civic).
2. If the reference voltage is present, ground the circuit with a jumper wire with the engine running to see if the A/C compressor and fan operate. If so, the circuit is good to that point.
3. If there is no voltage, check for an open wire in the harness or a poor connection at the multiplex control unit. On a 2002-2005 Civic, also check for an open circuit in the A/C thermal protector.
4. If the A/C compressor operated when the request circuit wire was grounded, verify the A/C pressure switch is good and that there is pressure in the system.
5. Then check the blue wire to the heater control panel in the dash by grounding it at the heater control panel and checking for A/C compressor operation.
6. If grounding the blue wire did not make the compressor operate, check it for an open between the pressure switch and the control panel. If grounding the blue wire at the control panel did make the compressor operate, the control panel may be faulty.
7. Test all the inputs to the heater control panel before replacing it.
The most important thing to remember when testing the A/C request circuit is never apply battery voltage to this circuit. Damage to the heater control panel or the A/C thermostatic switch may result -- and in some cases, even the engine control module could be damaged.
Hopefully, this will help simplify some air conditioning diagnostics on the next Honda Civic that lands in your shop.
(For information on Identifix, visit (www.identifix.com.)