Section 10-03: Speed Control System
1994 Mustang Workshop Manual
DIAGNOSIS AND TESTING

Pinpoint Tests

  

PINPOINT TEST A: SPEED CONTROL INOPERATIVE

 

A1 CHECK CONNECTIONS

Yes
GO to A2.

No
SERVICE or REPLACE as required.


 

A2 CHECK STOPLAMPS

Yes
GO to A3.

No
SERVICE stoplamp circuit.

 

A3 CHECK SPEED CONTROL ACTUATOR CABLE CONNECTION TO THROTTLE BODY AND SPEED CONTROL SERVO

  • Check to see if speed control actuator cable is attached to throttle body accelerator linkage.
  • Check to see if speed control actuator cable is attached to speed control servo linkage.
  • Is speed control actuator cable attached to both?
Yes
GO to A4.

No
ADJUST or SERVICE as required.

 

A4 CHECK LINKAGE OPERATION

  • Check the throttle linkage for proper operation.
  • Does linkage operate properly?
Yes
GO to A5.

No
GO to A16.

 

A5 CHECK VACUUM HOSES

Check to see if:

  • Vacuum supply hose is tightly connected to VAC port on speed control vacuum valve and to vacuum outlet manifold, and free of cuts, cracks and kinks.
  • Vacuum hoses are tightly connected between speed control vacuum valves and speed control servo , and free of cuts, cracks and kinks.
  • Vacuum hose is tightly connected between speed control vacuum valve and speed control vacuum reservoir , and free of cuts, cracks and kinks.
  • Speed control dump valve hose is tightly connected to the speed control servo and speed control dump valve , and free of cuts, cracks and kinks.
  • Are all checks successful?
Yes
GO to A6.

No
SERVICE as required.

 

A6 CHECK SPEED CONTROL VACUUM VALVE AND SPEED CONTROL VACUUM RESERVOIR

  • Disconnect the hose between speed control vacuum valve and speed control servo , at the speed control servo end.
  • Apply 60.6 kPa (18 in-Hg) vacuum to open end of hose.
  • Can vacuum be pumped to and held at 60.6 kPa (18 in-Hg) vacuum?
Yes
GO to A7.

No
SERVICE as required.

 

A7 CHECK SPEED CONTROL DUMP VALVE

  • Disconnect the hose (white stripe) between speed control dump valve and speed control servo at the speed control servo end.
  • Apply 60.6 kPa (18 in-Hg) to open end of hose.
  • Can vacuum be pumped up and held at 60.6 kPa (18 in-Hg) vacuum?
Yes
GO to A8.

No
SERVICE or ADJUST as required.

 

A8 PERFORM SPEED CONTROL ACTUATOR SWITCH TEST

  • Perform speed control actuator switch test as outlined.
  • Are tests successful?
Yes
GO to A9.

No
SERVICE circuits or REPLACE speed control actuator switch assembly or air bag sliding contact. Refer to Section 11-04.

 

A9 CHECK SPEED CONTROL SERVO

  • Separate speed control servo from speed control amplifier .
  • Connect the Circuit 144 (O/Y) lead of the speed control servo to the battery positive terminal.
  • Connect the Circuit 146 (W/PK) lead of the speed control servo to ground, and momentarily touch the Circuit 145 (GY/BK) lead of the speed control servo to ground.
  • The throttle actuator arm should pull in and the engine speed should increase. The arm should hold in that position or slowly release.
  • When Circuit 146 (W/PK) is removed from ground the throttle actuator arm should release.
  • Does speed control servo operate properly?
Yes
GO to A10.

No
REPLACE speed control servo.

 

A10 CHECK VSS

  • Disconnect 6-pin connector at speed control amplifier .
  • With an ohmmeter measure resistance between Circuit 150 (DG/W, VSS signal) and Circuit 57 (BK, ground).
  • Is the resistance approximately 200-300 ohms?
Yes
GO to A11.

No
REPLACE vehicle speed sensor.

 

A11 CHECK SPEED CONTROL AMPLIFIER

  • Disconnect 8-pin connector at speed control amplifier .
  • Measure resistance between the following circuits of the connector and check against the normal values.

    Circuits Normal Resistance
    144 (O/Y) and 145 (GY/BK) 40-75 ohms
    144 (O/Y) and 146 (W/PK) 100-150 ohms
    147 (P/LB) and 148 (Y/R) 20k-30k ohms
    147 (P/LB) and 149 (B/LG) 40k-60k ohms

  • Are all resistance values within limits?
Yes
GO to A12.

No
EXAMINE all connectors carefully for proper contact. SERVICE as required. REMOVE substitute speed control amplifier.

 

A12 CHECK INTERMITTENT ACTION

  • Note carefully when intermittent action occurs.
  • Determine whether intermittent action occurs while cruising or if it occurs while using control buttons or turning steering wheel.
  • Does intermittent action occur while cruising?
Yes
GO to A13.

No
GO to A15.

 

A13 CHECK VACUUM TO SPEED CONTROL SERVO

  • Check vacuum supply to speed control servo .

    NOTE: 8.42 kPa (2.5 in-Hg) is minimum vacuum for normal speed control servo operation. The vacuum source hose is attached to the 7.9mm (5/16 inch) engine vacuum-fitting port.

  • Is vacuum supply OK?
Yes
GO to A14.

No
SERVICE vacuum supply.

 

A14 CHECK SPEED CONTROL SERVO

  • Separate speed control servo from speed control amplifier .
  • Connect the Circuit 144 (O/Y) lead of the speed control servo to the battery positive terminal.
  • Connect the Circuit 146 (W/PK) lead of the speed control servo to ground, and momentarily touch the Circuit 145 (GY/BK) lead of the speed control servo to ground.
  • The throttle actuator arm should pull in and the engine speed should increase. The arm should hold in that position or slowly release.
  • When Circuit 146 (W/PK) is removed from ground the throttle actuator arm should release.
  • Does speed control servo operate properly?
Yes
SUBSTITUTE known good speed control amplifier if OK — properly INSTALL speed control amplifier.

No
REPLACE speed control servo.

 

A15 PERFORM SPEED CONTROL ACTUATOR SWITCH TEST

  • Perform speed control actuator switch test as outlined.
  • Are tests successful?
Yes
SUBSTITUTE known good speed control amplifier if OK — properly INSTALL speed control amplifier.

No
SERVICE circuits. REPLACE speed control actuator switch assembly or air bag sliding contact as required. REFER to Section 11-04.

 

A16 VERIFY ACCELERATOR OPERATION

  • Verify that engine is properly tuned.
  • Check accelerator action and actuator cable adjustment.
  • Is accelerator operation OK?
Yes
GO to A17.

No
ADJUST or CORRECT as required. REFER to Section 10-02.

 

A17 CHECK SPEED CONTROL DUMP VALVE

  • Disconnect the hose (white stripe) between speed control dump valve and speed control servo at the speed control servo end.
  • Apply 60.6 kPa (18 in-Hg) to open end of hose.
  • Can vacuum be pumped up and held at 60.6 kPa (18 in-Hg) vacuum?
Yes
GO to A18.

No
ADJUST or SERVICE as required.

 

A18 CHECK VACUUM HOSES

Check to see if:

  • Vacuum supply hose is tightly connected to VAC port on speed control vacuum valve and to vacuum outlet manifold , and free of cuts, cracks and kinks.
  • Vacuum hoses are tightly connected between speed control vacuum valves and speed control servo , and free of cuts, cracks and kinks.
  • Vacuum hose is tightly connected between speed control vacuum valve and speed control vacuum reservoir , and free of cuts, cracks and kinks.
  • Dump valve hose is tightly connected to the speed control servo and speed control dump valve , and free of cuts, cracks and kinks.
  • Are all checks successful?
Yes
GO to A19.

No
SERVICE as required.

 

A19 CHECK SPEED CONTROL VACUUM VALVE AND SPEED CONTROL VACUUM RESERVOIR

  • Disconnect the hose between speed control vacuum valve and speed control servo , at the speed control servo end. Apply 60.6 kPa (18 in-Hg) vacuum to open end of hose.
  • Can vacuum be pumped to and held at 60.6 kPa (18 in-Hg) vacuum?
Yes
GO to A20.

No
SERVICE as required.

 

A20 CHECK SPEED CONTROL SERVO

  • Separate speed control servo from speed control amplifier .
  • Connect the Circuit 144 (O/Y) lead of the speed control servo to the battery positive terminal.
  • Connect the Circuit 146 (W/PK) lead of the speed control servo to ground, and momentarily touch the Circuit 145 (GY/BK) lead of the speed control servo to ground.
  • The throttle actuator arm should pull in and the engine speed should increase. The arm should hold in that position or slowly release.
  • When Circuit 146 (W/PK) is removed from ground the throttle actuator arm should release.
  • Does speed control servo operate properly?
Yes
GO to A11.

No
REPLACE speed control servo.

  

PINPOINT TEST B: SPEED CONTROL OPERATES BUT DOES NOT RESUME, ACCELERATE OR COAST DOWN PROPERLY

 

B1 CHECK THE FOLLOWING SWITCHES AND CIRCUITS

  • Check the SET ACCEL switch, RESUME switch, COAST switch and air bag sliding contact circuits.
  • Are all circuits and switches OK?
Yes
GO to B2.

No
SERVICE the circuit as required.

 

B2 CHECK VACUUM HOSES

Check to see if:

  • Vacuum supply hose is tightly connected to VAC port on speed control vacuum valve and to vacuum outlet manifold, and free of cuts, cracks and kinks.
  • Vacuum hoses are tightly connected between speed control vacuum valves and speed control servo , and free of cuts, cracks and kinks.
  • Vacuum hose is tightly connected between speed control vacuum valve and speed control vacuum reservoir , and free of cuts, cracks and kinks.
  • Dump valve hose is tightly connected to the speed control servo and speed control dump valve , and free of cuts, cracks and kinks.
  • Are all checks successful?
Yes
GO to B3.

No
SERVICE as required.

 

B3 CHECK SPEED CONTROL VACUUM VALVE AND SPEED CONTROL VACUUM RESERVOIR

  • Disconnect the hose between speed control vacuum valve and speed control servo , at the speed control servo end.
  • Apply 60.6 kPa (18 in-Hg) vacuum to open end of hose.
  • Can vacuum be pumped to, and held at, 60.6 kPa (18 in-Hg) vacuum?
Yes
GO to B4.

No
SERVICE as required.

 

B4 CHECK SPEED CONTROL

  • Separate speed control servo from speed control amplifier .
  • Connect the Circuit 144 (O/Y) lead of the speed control servo to the battery positive terminal.
  • Connect the Circuit 146 (W/PK) lead of the speed control servo to ground, and momentarily touch the Circuit 145 (GY/BK) lead of the speed control servo to ground.
  • The throttle actuator arm should pull in and the engine speed should increase. The arm should hold in that position or slowly release.
  • When Circuit 146 (W/PK) is removed from ground the throttle actuator arm should release.
  • Does speed control servo operate properly?
Yes
GO to B5.

No
REPLACE speed control servo .

 

B5 CHECK SPEED CONTROL AMPLIFIER

  • Disconnect 8-pin connector at speed control amplifier .
  • Measure resistance between the following circuits of the connector and check against the normal values.

    Circuits Normal Resistance
    144 (O/Y) and 145 (GY/BK) 40-75 ohms
    144 (O/Y) and 146 (W/PK) 100-150 ohms
    147 (P/LB) and 148 (Y/R) 20k-30k ohms
    147 (P/LB) and 149 (B/LG) 40k-60k ohms

  • Are all resistance values within limits?
Yes
REPLACE speed control amplifier .

No
CHECK circuit connections for proper contact. SERVICE as required.

  

PINPOINT TEST C: SET SPEED FLUCTUATES

 

C1 CHECK THROTTLE LINKAGE

  • Check throttle linkage for proper operation and adjustment.
  • Is operation and adjustment OK?
Yes
GO to C2.

No
SERVICE or ADJUST as required. REFER to Section 10-02.

 

C2 CONTINUITY CHECK

  • Check continuity of Circuits 147 (P/LB), 148 (Y/R) and 149 (BR/LG).
  • Is there continuity in all circuits?
Yes
GO to C3.

No
SERVICE or REPLACE wiring as necessary.

 

C3 CHECK VACUUM HOSES

Check to see if:

  • Vacuum supply hose is tightly connected to VAC port on speed control vacuum valve and to vacuum outlet manifold, and free of cuts, cracks and kinks.
  • Vacuum hoses are tightly connected between speed control vacuum valves and speed control servo , and free of cuts, cracks and kinks.
  • Vacuum hose is tightly connected between speed control vacuum valve and speed control vacuum reservoir , and free of cuts, cracks and kinks.
  • Dump valve hose is tightly connected to the speed control servo and speed control dump valve , and free of cuts, cracks and kinks.
  • Are all checks successful?
Yes
GO to C4.

No
SERVICE or REPLACE wiring as necessary.

 

C4 CHECK SPEED CONTROL VACUUM VALVE AND SPEED CONTROL VACUUM RESERVOIR

  • Disconnect the hose between speed control vacuum valve and speed control servo , at the speed control servo end.
  • Apply 60.6 kPa (18 in-Hg) vacuum to open end of hose.
  • Can vacuum be pumped to and held at 60.6 kPa (18 in-Hg) vacuum?
Yes
GO to C5.

No
SERVICE as required.

 

C5 CHECK SPEED CONTROL SERVO

  • Separate speed control servo from speed control amplifier .
  • Connect the Circuit 144 (O/Y) lead of the speed control servo to the battery positive terminal.
  • Connect the Circuit 146 (W/PK) lead of the speed control servo to ground, and momentarily touch the Circuit 145 (GY/BK) lead of the speed control servo to ground.
  • The throttle actuator arm should pull in and the engine speed should increase. The arm should hold in that position or slowly release.
  • When Circuit 146 (W/PK) is removed from ground the throttle actuator arm should release.
  • Does speed control servo operate properly?
Yes
GO to C6.

No
REPLACE speed control servo .

 

C6 CHECK SPEEDOMETER CABLES

  • Check speedometer cables, if so equipped, for proper routing no sharp bends or binding.
  • Are speedometer cables good?
Yes
GO to C7.

No
SERVICE as required.

 

C7 CHECK VSS

  • Disconnect 6-pin connector at speed control amplifier .
  • With an ohmmeter measure resistance between Circuit 679 (GY/BK), VSS signal and Circuit 676 (PK/O).
  • Is the resistance approximately 200-300 ohms?
Yes
GO to C8.

No
REPLACE vehicle speed sensor .

 

C8 CHECK SPEED CONTROL DUMP VALVE

  • Disconnect the hose (white stripe) between speed control dump valve and speed control servo at the speed control servo end.
  • Apply 60.6 kPa (18 in-Hg) to open end of hose.
  • Can vacuum be pumped up and held at 60.6 kPa (18 in-Hg) vacuum?
Yes
GO to C9.

No
SERVICE or ADJUST as required.

 

C9 CHECK SPEED CONTROL AMPLIFIER

  • Disconnect 8-pin connector at speed control amplifier .
  • With an ohmmeter, measure resistance between the following circuits of the connector, and check against the normal values.

    Circuits Normal Resistance
    144 (O/Y) and 145 (GY/BK) 40-75 ohms
    144 (O/Y) and 146 (W/PK) 100-150 ohms
    147 (P/LB) and 148 (Y/R) 20k-30k ohms
    147 (P/LB) and 149 (B/LG) 40k-60k ohms

  • Are all resistance values within limits?
Yes
REPLACE speed control amplifier .

No
CHECK circuit connections for good contacts. SERVICE as required.

  

PINPOINT TEST D: SPEED CONTROL DOES NOT DISENGAGE WHEN BRAKES ARE APPLIED

 

D1 CHECK STOPLIGHT SWITCH AND CIRCUITRY

  • Make sure the stoplamps operate properly.
  • Apply brake pedal until stoplamps are lit.
  • Check for battery voltage at speed control amplifier on Circuit 296 (W/P).
  • Measure voltage at speed control amplifier on Circuit 810 (R/LG).
  • The difference between the two readings should not exceed 1.5 V.
  • Is the difference between two voltage readings within limit?
Yes
GO to D2.

No
SERVICE stoplamp circuit as required. VERIFY fuses are not open. GO to D2.

 

D2 CHECK SPEED CONTROL DUMP VALVE

  • Disconnect the hose (white stripe) between speed control dump valve and speed control servo at the speed control servo end.
  • Apply 60.6 kPa (18 in-Hg) to open end of hose.
  • Can vacuum be pumped up and held at 60.6 kPa (18 in-Hg) vacuum?
Yes
GO to D3.

No
ADJUST or SERVICE as required.

 

D3 CHECK SPEED CONTROL SERVO

  • Check speed control servo operation and throttle linkage.
  • Are speed control servo operation and linkage OK?
Yes
GO to D4.

No
REPLACE speed control servo .

 

D4 CHECK SPEED CONTROL AMPLIFIER

  • Disconnect 8-pin connector at speed control amplifier .
  • Measure resistance between the following circuits of the connector and check against the normal values.

    Circuits Normal Resistance
    144 (O/Y) and 145 (GY/BK) 40-75 ohms
    144 (O/Y) and 146 (W/PK) 100-150 ohms
    147 (P/LB) and 148 (Y/R) 20k-30k ohms
    147 (P/LB) and 149 (B/LG) 40k-60k ohms

  • Are all resistance values within limits?
Yes
REPLACE speed control amplifier .

No
CHECK contacts of electrical connector. SERVICE as required.

  

PINPOINT TEST E: SPEED CONTROL SYSTEM DOES NOT DISENGAGE WHEN CLUTCH PEDAL IS APPLIED

 

E1 CHECK CIRCUIT 810 (LG/R)

  • Disconnect stoplight switch , and clutch pedal position switch .
  • Using an ohmmeter, check resistance of Circuit 810 (LG/R) between stoplight switch connector and pedal position switch connector.
  • Is resistance 5 ohms or less?
Yes
GO to E2.

No
SERVICE Circuit 810 (LG/R) for open. RETEST system.

 

E2 CHECK CLUTCH PEDAL POSITION SWITCH CONTINUITY

  • Gain access to the clutch pedal position switch .
  • Using an ohmmeter, check clutch pedal position switch continuity, with clutch pedal released.
  • Is resistance 5 ohms or less?
Yes
LEAVE ohmmeter connected. GO to E3.

No
REPLACE clutch pedal position switch . RETEST system.

 

E3 CHECK CLUTCH PEDAL POSITION SWITCH OUTPUT

  • Apply clutch pedal.
  • Measure resistance of clutch pedal position switch .
  • Does ohmmeter show open circuit?
Yes
GO to E4.

No
REPLACE clutch pedal position switch . RETEST system.

 

E4 CHECK SPEED CONTROL SERVO

  • Check speed control servo operation and throttle linkage.
  • Are speed control servo operation and linkage OK?
Yes
GO to E5.

No
REPLACE speed control servo .

 

E5 TEST SPEED CONTROL AMPLIFIER

  • Perform speed control servo amplifier test as outlined.
  • Is test successful?
Yes
REPLACE speed control servo .

No
CHECK contacts of electrical connector. SERVICE as required.