Section 12-00: Climate Control System—Service
1994 Mustang Workshop Manual
DIAGNOSIS AND TESTING

Pinpoint Tests

  

PINPOINT TEST A: VENT DOES NOT OPERATE

 

A1 NO VENT OPERATION

Yes
GO to A2.

No
SERVICE vacuum source tube (black). RESTORE vehicle. RETEST system.

 

A2 CHECK VEHICLE FOR VACUUM

Yes
RECONNECT vacuum tube. GO to A3.

No
SERVICE vacuum distribution block. RESTORE vehicle. RETEST system.

 

A3 CHECK VENT VACUUM TUBES FOR LEAK

  • Check in-line and control assembly multiple connectors for proper installation and connection.
  • Does vacuum hiss stop?
Yes
RESTORE vehicle. RETEST system.

No
GO to A4.

 

A4 CHECK FUNCTION CONTROL FOR LEAK

  • Actuate function knob to all positions to determine which selector positions are leaking.
  • Do all positions leak?
Yes
GO to A5.

No
GO to A6.

 

A5 CHECK FUNCTION CONTROL VACUUM SUPPLY

  • Pinch off source tube (black) at control assembly.
  • Did vacuum hiss stop?
Yes
REPLACE function control selector knob. RESTORE vehicle. RETEST system.

No
SERVICE source tube, vacuum harness. RESTORE vehicle. RETEST system.

 

A6 DETERMINE LEAKING HOSE(S)

  • Pinch off suspect hose near respective vacuum control motor .
  • Did vacuum hiss stop?
Yes
REPLACE vacuum control motor . RESTORE vehicle. RETEST system.

No
GO to A7.

 

A7 CHECK VACUUM HARNESS FOR LEAK

  • Carefully listen for location of vacuum leak.
  • Pinch off suspect hose(s) one at a time, near control assembly, and/or in-line connector.
  • Did vacuum hiss stop?
Yes
SERVICE hose for cut or damage. RESTORE vehicle. RETEST system.

No
REPLACE function control knob. RESTORE vehicle. RETEST system.

  

PINPOINT TEST B: HEAT DOES NOT OPERATE

 

B1 CHECK FOR VACUUM TO FUNCTION CONTROL

  • Start engine and allow engine to idle.
  • Disconnect A/C, heater vacuum source tube (black), check for vacuum.
  • Is vacuum present?
Yes
GO to B2.

No
SERVICE vacuum source tube (black) as required. RESTORE vehicle. RETEST system.

 

B2 CHECK FLOOR PANEL DOOR OPERATION

  • Actuate function control switch to the floor position.
  • Locate heater air damper door vacuum control motor.
  • Check for vacuum at blue tube and red tube.
  • Is vacuum present at each vacuum tube?
Yes
REPLACE vacuum control motor . GO to B3.

No
SERVICE suspect hose for cut or damage. RESTORE vehicle. RETEST system.

 

B3 CHECK HEATER FOR PROPER TEMPERATURE

  • Allow engine to run until operating temperature is achieved.
  • Move A/C damper door switch to FLOOR position.
  • Actuate temperature knob to full heat position.
  • Actuate blower motor switch .
  • Is heater temperature OK?
Yes
GO to B4.

No
GO to Heater Core Test and SERVICE as outlined. RETEST system.

 

B4 CHECK BLOWER MOTOR OPERATION 

  • Actuate heater blower motor in all positions.
  • Did heater blower motor operate properly?
Yes
RESTORE vehicle. RETEST system.

No
GO to Pinpoint Test E.

  

PINPOINT TEST C: DEFROST DOES NOT OPERATE

 

C1 CHECK FOR VACUUM TO FUNCTION CONTROL

  • Start engine and allow engine to idle.
  • Disconnect A/C, heater vacuum source tube (black), check for vacuum.
  • Is vacuum present?
Yes
GO to C2.

No
SERVICE vacuum source tube (black) as required. RESTORE vehicle. RETEST system.

 

C2 CHECK DEFROST DOOR OPERATION

  • Actuate function control switch to the defrost position.
  • Locate windshield defroster door vacuum control motor.
  • Check for vacuum at yellow vacuum tube.
  • Is vacuum present at each vacuum tube?
Yes
REPLACE defrost door vacuum control motor . GO to C3.

No
SERVICE suspect hose for cut or damage. RESTORE vehicle. RETEST system.

 

C3 CHECK DEFROSTER TEMPERATURE

  • Allow engine to run until operating temperature is achieved.
  • Move A/C damper door switch to FLOOR position.
  • Actuate temperature knob to full-heat position.
  • Actuate blower motor switch .
  • Is defrost temperature OK?
Yes
GO to C4.

No
GO to Heater Core Test and SERVICE as outlined. RETEST system.

 

C4 CHECK BLOWER MOTOR OPERATION

  • Actuate A/C blower motor in all positions.
  • Did A/C blower motor operate properly?
Yes
RESTORE vehicle. RETEST system.

No
GO to Pinpoint Test E.

  

PINPOINT TEST D: A/C DOES NOT OPERATE

 

D1 CHECK A/C COMPRESSOR CLUTCH

  • Prepare vehicle as follows:
    • Connect manifold gauge set.
    • Set function control at MAX A/C.
    • Set blower motor switch on HIGH.
    • Set temperature lever at full-cold.
    • Close doors and windows.
    • Run engine at approximately 1500 rpm.
  • Does A/C clutch operate?
Yes
GO to D2.

No
GO to D11.

 

D2 CHECK VOLTAGE SUPPLY AT SWITCH

  • Turn engine off. Leave ignition switch in RUN position.
  • Using a test lamp, check for voltage at Pin 2 Circuit 296 (W) at blower motor switch .
  • Did test lamp illuminate?
Yes
GO to D5.

No
GO to D3.

 

D3 CHECK VOLTAGE SUPPLY TO SWITCH

  • Using test lamp, check voltage at output side of fuse 6 (15A).
  • Did test lamp illuminate?
Yes
SERVICE Circuit 296 (W) for open circuit. RESTORE vehicle. RETEST system.

No
GO to D4.

 

D4 CHECK CIRCUIT 296 (W) FOR SHORT

  • Using an ohmmeter, connect one lead to output side of fuse 6 (15A). Connect second lead to a known good ground.
  • Disconnect A/C damper door switch .
  • Is resistance 5 ohms or less?
Yes
SERVICE Circuit 296 (W) for short to ground. REPLACE fuse 6 (15A). RESTORE vehicle. RETEST system.

No
GO to D5.

 

D5 CHECK A/C, HEATER CONTROL SWITCH

  • Reconnect A/C, A/C damper door switch .
  • Using a test lamp, check voltage at Pin 4 Circuit 348 (P).
  • Did test lamp illuminate?
Yes
GO to D6.

No
REPLACE A/C, A/C damper door switch . RESTORE vehicle. RETEST system.

 

D6 CHECK CLUTCH CYCLING PRESSURE SWITCH

  • Disconnect A/C cyclic switch .
  • Start engine, A/C should be in MAX A/C mode.
  • Using a jumper wire, connect one end to Pin 1 at A/C cyclic switch connector. Connect second lead to Pin 2 at A/C cyclic switch .
  • Observe A/C clutch .
  • Did A/C clutch engage?
Yes
REPLACE A/C cyclic switch . Do not discharge system. A/C cyclic switch has a Schrader valve. RESTORE vehicle. RETEST system.

No
GO to D7.

 

D7 CHECK A/C INPUT TO CONSTANT CONTROL RELAY MODULE (CCRM)

  • Turn engine OFF, leave ignition switch ON. Gain access to CCRM as outlined.
  • Using test lamp, check Pin 21 Circuit 198 (DG/O) at CCRM connector for voltage.
  • Did test lamp illuminate?
Yes
GO to D8.

No
SERVICE Circuit 198 (DG/O) for open circuit. RESTORE vehicle. RETEST system.

 

D8 CHECK (CCRM) CLUTCH OUTPUT

  • Using test lamp, check Pin 23 Circuit 347 (BK/Y) for voltage.
  • Did test lamp illuminate?
Yes
GO to D9.

No
REPLACE constant control relay module. RESTORE vehicle. RETEST system.

 

D9 CHECK SUPPLY TO A/C CLUTCH

  • Disconnect A/C clutch .
  • Using a test lamp, check Circuit 347 (BK/Y) at A/C clutch connector.
  • Did test lamp illuminate?
Yes
GO to D10.

No
SERVICE Circuit 347 (BK/Y) for open circuit.

 

D10 CHECK A/C CLUTCH GROUND CIRCUIT

  • Using an ohmmeter connected to ground, connect second lead to Circuit 57 (BK) at A/C clutch connector.
  • Measure resistance.
  • Is resistance 5 ohms or less?
Yes
REPLACE A/C clutch as outlined. RESTORE vehicle. RETEST system.

No
SERVICE Circuit 57 (B/K) for open circuit. RESTORE vehicle. RETEST system.

 

D11 CHECK COMPRESSOR CLUTCH CYCLING 

  • Use refrigerant system pressure/clutch cycle rate and timing evaluation charts. Refer to Service Procedures.
  • After preparing vehicle as follows:
    • Hook up manifold gauge set.
    • Set function control at MAX A/C.
    • Set blower motor switch on HIGH.
    • Set temperature knob full COLD.
    • Close doors and windows.
    • Use a thermostat to check temperature at center discharge register, record outside temperature.
    • Run engine at approximately 1500 rpm with A/C clutch engaged.
    • Stabilize with above conditions for 10-15 minutes.
  • Check A/C clutch OFF/ON time with watch. Refer to charts for normal A/C clutch cycle timing rates.
  • Did A/C compressor cycle very rapidly?
Yes
GO to D12.

No
GO to D13.

 

D12 CHECK CLUTCH CYCLING SWITCH

  • Using a jumper wire, bypass A/C cyclic switch .
  • Hand feel A/C evaporator inlet and outlet tubes.
  • Is outlet tube approximately same temperature between -2°C and -4°C (28° F-40°F) or slightly colder than inlet tube?
Yes
REPLACE A/C cyclic switch . Do not discharge system. A/C cyclic switch has a Schrader valve. RESTORE vehicle. RETEST system.

No
Discharge system and SERVICE as outlined. RETEST system.

 

D13 CHECK A/C SWITCH OUTPUT 

  • Turn ignition switch to RUN.
  • Turn A/C damper door switch to the OFF position.
  • Using a test lamp, check for voltage at Pin 4 Circuit 348 (P) at A/C, heater switch connector.
  • Did test lamp illuminate?
Yes
REPLACE A/C, A/C damper door switch . RESTORE vehicle. RETEST system.

No
GO to D14.

 

D14 CHECK FOR VOLTAGE TO A/C CLUTCH

  • Using a test lamp, check Circuit 347 (BK/Y) for voltage.
  • Did test lamp illuminate?
Yes
REPLACE constant control relay module. RESTORE vehicle. RETEST system.

No
Condition does not exist at this time. RETEST system.

  

PINPOINT TEST E: HEATER BLOWER MOTOR DOES NOT OPERATE

 

E1 CHECK VOLTAGE SUPPLY

  • Remove A/C heater control .
  • Turn ignition switch to RUN.
  • Using a test lamp connected to a known good ground, check Pin 3 Circuit 181 (BR/O) at A/C heater control connector.
  • Did test lamp Illuminate?
Yes
GO to E4.

No
GO to E2.

 

E2 CHECK VOLTAGE SUPPLY CIRCUIT FOR VOLTAGE

  • Using a test lamp, check output side of fuse 9 (15A).
  • Did test lamp illuminate?
Yes
SERVICE Circuit 181 (BR/O) for open circuit. RESTORE vehicle. RETEST system.

No
GO to E3.

 

E3 CHECK CIRCUIT 181 (BR/O) FOR SHORT

  • Remove fuse 9 (15A).
  • Disconnect A/C, heater control connector.
  • Using ohmmeter connected to a known good ground, connect second lead to Pin 3 Circuit 181 (BR/O). Measure resistance.
  • Is resistance 5 ohms or less?
Yes
SERVICE Circuit 181 (BR/O) for short to ground. REPLACE fuse 9 (15A). RESTORE vehicle. RETEST system.

No
REPLACE fuse 9 (15A). GO to E4.

 

E4 CHECK A/C, HEATER SWITCH, BLOWER OUTPUT

  • Disconnect A/C blower motor.
  • Reconnect A/C, A/C damper door switch .
  • Actuate A/C, A/C damper door switch to any ON position.
  • Using a test lamp, check Pin 1 Circuit 249 (DB/P) for voltage.
  • Did test lamp illuminate?
Yes
GO to E5.

No
REPLACE A/C, A/C damper door switch . RESTORE vehicle. RETEST system.

 

E5 CHECK CIRCUIT 249 (DB/P) FOR OPEN

  • Using a test lamp, check A/C blower motor end of Circuit 249 (DB/P) for voltage.
  • Did test lamp illuminate?
Yes
GO to E6.

No
SERVICE Circuit 249 (DB/P) for open circuit. RESTORE vehicle. RETEST system.

 

E6 CHECK A/C BLOWER MOTOR

  • Reconnect A/C blower motor.
  • Using a No. 10 gauge jumper wire, connect one end to battery negative. Connect other end of jumper to black wire negative lead at A/C blower motor.
  • Did A/C blower motor operate?
Yes
GO to E7.

No
REPLACE A/C blower motor. RESTORE vehicle. RETEST system.

 

E7 CHECK CIRCUIT 261 (O/BK)

  • Disconnect A/C blower motor.
  • Disconnect heater blower motor switch resistor .
  • Disconnect blower motor switch .
  • Using an ohmmeter, check resistance of Circuit 261 (O/BK) at blower moto r harness connector, to Pin 4 Circuit 261 (O/BK) at heater blower motor switch resistor .
  • Is resistance 5 ohms or less?
Yes
GO to E8.

No
SERVICE Circuit 261 (O/BK) for open circuit. RESTORE vehicle. RETEST system.

 

E8 CHECK CIRCUIT 261 (O/BK) AT SWITCH

  • Leave ohmmeter connected to A/C blower motor harness connector.
  • Disconnect second lead at heater blower motor switch resistor and connect it to Pin B Circuit 261 (O/BK) at heater blower motor switch connector. Measure resistance.
  • Is resistance 5 ohms or less?
Yes
GO to E9.

No
SERVICE Circuit 261 (O/BK) for open circuit. RESTORE vehicle. RETEST system.

 

E9 CHECK CIRCUIT 754 (GN/W)

  • Using an ohmmeter, check resistance of Circuit 754 (GN/W) from blower motor switch to heater blower motor switch resistor .
  • Is resistance 5 ohms or less?
Yes
GO to E10.

No
SERVICE Circuit 754 (GN/W) for open circuit. RESTORE vehicle. RETEST system.

 

E10 CHECK CIRCUIT 752 (Y/R)

  • Using an ohmmeter, check resistance of Circuit 752 (Y/R) from blower motor switch to heater blower motor switch resistor .
  • Is resistance 5 ohms or less?
Yes
GO to E11.

No
SERVICE Circuit 752 (Y/R) for open circuit. RESTORE vehicle. RETEST system.

 

E11 CHECK CIRCUIT 57 (BK) SYSTEM GROUND

  • Using an ohmmeter connected to a known good ground, connect second lead to Circuit 57 (BK) at blower motor switch .
  • Is resistance 5 ohms or less?
Yes
GO to E12.

No
SERVICE Circuit 57 (BK) for open circuit. RESTORE vehicle. RETEST system.

 

E12 CHECK CIRCUIT 57 (BK) AT RESISTOR

  • Disconnect lead from switch connector and connect it to Circuit 57 (BK) at heater blower motor switch resistor connector.
  • Is resistance 5 ohms or less?
Yes
GO to E13.

No
SERVICE Circuit 57 (BK) for open circuit. RESTORE vehicle. RETEST system.

 

E13 CHECK HEATER BLOWER MOTOR SWITCH RESISTOR 

  • Remove heater blower motor switch resistor and inspect resistor coils.
  • Using an ohmmeter, check for continuity at all resistor terminals. Also check thermal limiter for continuity.
  • Is there continuity at all terminals?
Yes
GO to E14.

No
REPLACE heater blower motor switch resistor assembly. RESTORE vehicle. RETEST system.

 

E14 BLOWER MOTOR SWITCH TEST

  • Use chart below to test blower motor switch .

    TESTING PROCEDURE
    To Test Connect Self-Powered Test Light or Ohmmeter to Terminals Move Heater Blower Motor Switch to These Positions A Good Heater Blower Motor Switch Will Indicate
    Medium-Lo
    Speed    		 261 (O/BK)
    and
    754 (GN/W)    		 Lo
    Medium Lo
    Medium Hi
    Hi    		 Open Circuit
    Closed Circuit
    Open Circuit
    Open Circuit
    Medium-Hi
    Speed    		 261 (O/BK)
    and
    752 (Y/R)    		 Lo
    Medium Lo
    Medium Hi
    Hi    		 Open Circuit
    Open Circuit
    Closed Circuit
    Open Circuit
    Hi Speed 261 (O/BK)
    and
    57 (BK)    		 Lo
    Medium Lo
    Medium Hi
    Hi    		 Open Circuit
    Open Circuit
    Open Circuit
    Closed Circuit

  • Did heater blower motor switch test OK?
Yes
System operational. RESTORE vehicle. RETEST system.

No
REPLACE blower motor switch . RESTORE vehicle. RETEST system.

  

PINPOINT TEST F: ENGINE COOLING FAN DOES NOT OPERATE WHEN A/C IS ACTIVATED

 

F1 CHECK VOLTAGE SUPPLY TO COOLING FAN

  • Start engine and run until normal operating temperature is achieved.
  • Turn engine OFF, leave ignition switch in RUN position.
  • Actuate A/C damper door switch to MAX A/C position.
  • Using a test lamp, check Pin 1 Circuit 229 (R/O) and Pin 2 Circuit 229 (R/O) for voltage.
  • Did test lamp illuminate?
Yes
GO to F2.

No
GO to F3.

 

F2 CHECK COOLING FAN GROUND

  • Using an ohmmeter connected to a known good ground, connect second lead to black ground circuit at cooling fan motor connector.
  • Is resistance 5 ohms or less?
Yes
GO to F5.

No
SERVICE fan motor ground circuit for open circuit. RESTORE vehicle. RETEST system.

 

F3 CHECK FOR VOLTAGE SUPPLY TO CONSTANT CONTROL RELAY MODULE (CCRM)

  • Using a test lamp, check Pin 3 Circuit 38 (BK/O) for voltage.
  • Did test lamp illuminate?
Yes
GO to F5.

No
GO to F4.

 

F4 CHECK FUSE (60A)

  • Using test lamp, check both sides of fuse (60A).
  • Is fuse OK?
Yes
SERVICE Circuit 38 (BK/O) for open circuit. RESTORE vehicle. RETEST system.

No
SERVICE Circuit 38 (BK/O) for short circuit. RESTORE vehicle. RETEST system.

 

F5 CHECK COOLING FAN MOTOR

  • Disconnect cooling fan motor .
  • Using a No. 10 gauge jumper wire, connect one end to battery positive. Connect other end to cooling fan, low terminal at fan motor .
  • Did cooling fan run?
Yes
SERVICE Circuit 229 (R/O) for open circuit. RESTORE vehicle. RETEST system.

No
REPLACE cooling fan motor . RESTORE vehicle. RETEST system.