Section 10-03: Speed Control System
1993 Mustang Workshop Manual
DIAGNOSIS AND TESTING

VSS Test

 

Disconnect the connector to the VSS and connect an ohmmeter between the VSS terminals at the VSS end. A reading of approximately 200-300 ohms should be obtained. A reading of 0 ohms indicates a shorted coil and a maximum reading indicates an open coil. Replace the VSS in either case.

If the ohmmeter records 200-300 ohms and the speedometer operates properly within needle waver, the VSS is probably good. A speed sensor of known good quality can also be substituted in place of the existing VSS to check for proper operation.

 

Servo Assembly Test

 
  1. Separate the 8-pin connector at the amplifier.
  1. Connect an ohmmeter between the Circuit 144 (O/Y) and Circuit 145 (GY/BK) leads at the 8-pin connector. A resistance of 40/75 ohms should be obtained.
  1. Connect the ohmmeter between the Circuit 144 (O/Y) and Circuit 146 (W/PK) leads. A resistance of 100/150 ohms should be obtained.
  1. Connect the ohmmeter between Circuits 147 (P/LB) and 148 (Y/R). A resistance of 20 K ohms to 30 K ohms should be obtained.
  1. Connect the ohmmeter between Circuits 147 (P/LB) and 149 (BR/LG). A resistance of 40 K ohms to 60 K ohms should be obtained.
  1. If proper resistance is not obtained, check the wiring and servo separately for damage and replace or service as required.

    Start the engine, and with the servo disconnected from the amplifier, connect the Circuit 144 (O/Y) lead of the servo to the battery positive terminal. Connect the Circuit 146 (W/PK) lead of servo to ground, and momentarily touch the Circuit 145 (GY/BK) lead of the servo to ground. The servo throttle actuator arm should pull in and the engine speed should increase. The arm should hold in that position or slowly release, when Circuit 145 (GY/BK) lead is removed from ground. When Circuit 146 (W/PK) is removed from ground, the servo should release. Replace the servo if it fails any part of the preceding test.

    If the Circuit 144 (O/Y) lead is shorted to either the Circuit 146 (W/PK) or Circuit 145 (GY/BK) leads, it may be necessary to replace the amplifier.

 

Amplifier Test

 

 CAUTION: Do not use a test lamp to perform the following tests as excessive current draw will damage electronic components inside the amplifier. Use only a voltmeter of 5,000 ohm/volt rating or higher.

 

ON Circuit Test

 

Turn ignition switch to the RUN position. Connect voltmeter between Circuit 296 (W/P) and ground in 6-pin connector at amplifier. Voltmeter should read battery voltage. If battery voltage is not present, check the fuse voltage and service as required.

Connect voltmeter between Circuit 151 (LB/BK), and ground in 6-pin connector at amplifier. The voltmeter should now read battery voltage when the ON switch on the steering wheel is depressed and held. If voltage is not available perform control switch test.

Release the ON button; the voltmeter should read approximately 7.8 volts. The voltage should remain approximately 7.8 volts until one of the speed control buttons is depressed and held. If the meter reads zero, check the ground connections on the amplifier (either black wire on the 6-pin connector). If there is still no ground on the amplifier, check the system ground connection and wiring. Also check the 20-amp fuse and/or substitute (but do not install) a known good amplifier and recheck for a properly operating ON circuit.

 

Brake Circuit Test

Connect voltmeter between Circuit 511 (LG) on the 6-pin connector and ground. The voltmeter should read battery voltage with brake pedal depressed, and less than one volt with brake pedal released. If voltage is other than above, check for improper wiring, burned out stoplamps, burned out stoplamp fuse, clutch switch malfunction or stoplamp switch malfunction.

 

OFF Circuit Test

With the ignition switch in the RUN position and the voltmeter connected between Circuit 151 (LB/BK) on the 6-pin connector and ground, depress and hold the OFF switch on the steering wheel. The voltmeter should indicate 0 volts. If the voltage does not drop to zero, perform control switch test. If the switches check OK, temporarily connect a known good amplifier and recheck the OFF circuit as outlined.

 

SET ACCEL Circuit Test

With the ignition switch in the RUN position and the voltmeter connected between Circuit 151 (LB/BK) in the 6-pin connector and Circuit 57 (BK) ground, depress and hold the SET ACCEL button on steering wheel. Voltmeter should indicate approximately 4.5 volts. Rotate the steering wheel back and forth and watch the voltmeter for fluctuations. If the voltage varies more than 0.5 volt, perform the control switch test.

 

COAST Circuit Test

With the ignition switch in the RUN position and the voltmeter connected between Circuit 151 (LB/BK) in the 6-pin connector and ground, depress and hold the COAST button on steering wheel. The voltmeter should indicate approximately 1.5 volts. Rotate the steering wheel back and forth and watch the voltmeter for fluctuations. If the voltage varies more than 0.5 volt, perform the control switch test.

 

RESUME Circuit Test

With the ignition switch in the RUN position and the voltmeter connected between Circuit 151 (LB/BK) in the 6-pin connector and ground, depress and hold the RESUME button on the steering wheel. The voltmeter should read approximately 6.5 volts. Rotate the steering wheel back and forth and watch the voltmeter for fluctuations. If the voltage varies more than 0.5 volt, perform the control switch test.

If all the circuits check OK, perform the servo assembly test and temporarily connect a known good amplifier. Do not substitute a good amplifier until the servo assembly test has been successfully completed.

 

Simulated Road Test

Tools Required:

 CAUTION: When performing this simulated road test, the rear wheels of the vehicle must be raised clear of the floor. Block the front wheels securely and use only a suitable lifting device (such as a garage-type hoist) and support the rear axle with one jack stand on each side. Never attempt to use the vehicle bumper jack for tests of this type.

  1. Raise vehicle on hoist. Refer to Section 00-02.
  1. Start the engine.
  1. Shift the transmission gear selector to DRIVE.
  1. Turn on the speed control.

     CAUTION: If any time during the following Steps the system should appear to go out of control and overspeed, be prepared to turn the system off at once with the OFF switch or the ignition switch.

  1. Accelerate and hold at 56 km/h (35 mph).
  1. Press and release the SET ACCEL button. Hold foot pressure very lightly on accelerator pedal. Normally the speed will continue at 56 km/h (35 mph) for a short period of time and then gradually start surging because the engine is not loaded.
  1. Press the OFF button. The engine should drop back to idle. Stop the rear wheels with the brakes.
  1. Press ON button, accelerate and hold the speed at 56 km/h (35 mph).
  1. Press and hold the SET ACCEL button. Slowly remove the foot from the accelerator. The engine speed should gradually increase.
  1. When the speed reaches 80 km/h (50 mph), release the SET ACCEL button. The vehicle should maintain 80 km/h (50 mph) for a short time before the surging begins.
  1. Press the COAST button and hold. The engine should idle. Slow the rear wheels to 56 km/h (35 mph).
  1. Release the COAST button. Speed should maintain 56 km/h (35 mph). Surging should soon start.
  1. Press and release the brake pedal. The system should shut off and the engine should slow to idle.
  1. Accelerate engine and set the speed at 80 km/h (50 mph). Brake to 56 km/h (35 mph) and maintain 56 km/h (35 mph) with the accelerator. Depress and release the RESUME button. The speed should return to 80 km/h (50 mph).

    NOTE: When performing these tests, keep the vehicle speeds within reasonably low limits. If the system does not perform as normally expected, make note of the malfunction.

 

Stoplamp Switch and Circuit Test

To be performed when brake application will not disconnect the speed control:

  1. Check for stoplamp operation with a maximum brake pedal effort of 3 Kg (6 lb). If more than 3 Kg (6 lb) effort is required, check stoplamp fuse and lamps, check the brake actuation and stoplamp switch. Service or replace as required.
  1. If stoplamps do not work, the stoplamp switch, supply circuit fuse, or bulbs must be checked.
  1. If stoplamps work properly check for battery voltage on Circuit 296 (W/P) at the 6-pin connector. Depress the brake pedal until the stoplamps are lit. Check voltage on the Circuit 511 (LG) lead at the 6-pin connector. The difference between the two voltage readings must not exceed 1.5 volts. If a voltage difference greater than 1.5 volts is found, the high resistance in the stoplamp circuit must be found and corrected. There should be no voltage present on the Circuit 511 (LG) lead with the brakelamps off.
  1. Perform the vacuum dump valve test.
 

Vacuum Dump Valve Test

Tools Required:

The vacuum dump valve releases the vacuum in the servo assembly whenever the brake pedal is depressed, and thus acts as a redundant safety feature. It should be checked whenever brake application does not disconnect the speed control.

Disconnect the vacuum hose (white stripe) from the dump valve to the servo at the servo.

NOTE: LH inner front fender splash shield must be removed. Refer to Section 01-02.

Connect a hand vacuum pump such as Rotunda Vacuum Tester 021-00037 or equivalent, to the hose, and pump up a vacuum. If a vacuum can not be obtained, the hose or the dump valve leaks and should be replaced or adjusted. Step on the brake pedal. The vacuum should be released. If it is not, adjust or replace the dump valve.

 

Clutch Switch Test

 

Automatic Transmission

Vehicles equipped with automatic transmissions use a shorting plug instead of a clutch switch.

 

Manual Transmission

The speed control system is designed to disengage when the clutch pedal is depressed. This is accomplished via a clutch disable switch mounted to the clutch pedal.

The disengage function operates by opening the circuit between the speed control module and the stoplamps. This prevents engine overspeed when the clutch is depressed and the speed control is engaged.

The switch is a plastic plunger-type mounted to the brake and clutch pedal support on the side near the accelerator pedal.

The switch is activated by a bar that is part of the clutch pedal mechanism.

 CAUTION: Do not use a strong magnet close to the clutch switch. The switch operation can be affected by strong magnetic fields.

If the switch is open when the clutch pedal is released, the speed control will not operate. This must be corrected before making other tests.

 CAUTION: Do not use a test lamp to perform the following tests. A light cannot properly indicate condition of switch. Use only a voltmeter of 5,000 ohm/volt rating or higher.

To check the switch, disconnect switch pigtail connector from speed control harness connector and connect an ohmmeter to the two switch connector terminals. With the clutch pedal in the full-up (released) position, the resistance should be less than 5 ohms. With the clutch pedal depressed (switch plunger extended), the circuit should be open.

If the switch does not function as described, remove and replace by removing one sheet metal screw and installing a replacement switch.



 

Speed Control Tester

The speed control system can be operated and diagnosed using the Rotunda Speed Control Tester 007-00013 or equivalent. The tester works on vehicles with all combinations of control switches, transmissions, speedometers (electronic and mechanical), brakes and clutches. With three modes of operation: Automatic, Single Scan, and Scan Probe, the technician can pinpoint any failure including wiring, vacuum hoses, switches and servo. The tester includes an instruction manual with its own specially developed diagnostic tree charts for identifying any failed or marginal component in the speed control system.



 

PINPOINT TEST A: SPEED CONTROL DOES NOT WORK

 

A1 VERIFY CONDITION
GO to A2.

 

A2 CHECK CONNECTIONS

Yes
GO to A3.

No
SERVICE or REPLACE as required.

 

A3 CHECK STOPLAMPS

Yes
GO to A4 if manual transmission.

   
    If automatic transmission, GO to A5.
No
SERVICE stoplamp circuit.

 

A4 CHECK CLUTCH SWITCH (MANUAL TRANSMISSION)

  • Check clutch switch for proper operation.
  • Does switch operate properly?
Yes
GO to A5.

No
SERVICE as required.

 

A5 CHECK ACTUATOR CABLE CONNECTION TO THROTTLE BODY AND SERVO

  • Check that actuator cable is attached to the throttle body accelerator linkage.
  • Check that actuator cable is attached to servo linkage.
  • Are cables attached?
Yes
GO to A6.

No
ADJUST or SERVICE as required.

 

A6 CHECK LINKAGE OPERATION

  • Check the throttle linkage for proper operation.
  • Does linkage operate properly?
Yes
GO to A7.

No
SERVICE as required.

 

A7 CHECK VACUUM

  • Check vacuum at servo.
  • Is vacuum OK?

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

Yes
GO to A9.

No
GO to A8.

 

A8 CHECK DUMP VALVE

  • Check vacuum dump valve.
  • Is valve OK?
Yes
GO to A11.

No
SERVICE or ADJUST as required.

 

A9 PERFORM CONTROL SWITCHES AND CIRCUIT TEST

  • Are circuits and switches OK?
Yes
GO to A10.

No
SERVICE or REPLACE switches or circuits as required.

 

A10 PERFORM SERVO TESTS

  • Are tests successful?
Yes
GO to A11.

No
REPLACE servo.

 

A11 PERFORM VSS TEST

  • Is test successful?
Yes
GO to A12.

No
REPLACE VSS.

 

A12 PERFORM AMPLIFIER TEST

  • Is test successful?
Yes
INSTALL a new amplifier.

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

 

PINPOINT TEST B: SPEED CONTROL OPERATION IS INTERMITTENT

 

B1 VERIFY THE CONDITION

  • Note carefully when intermittent action occurs.

GO to B2.

 

B2 INSPECT VISUALLY

Intermittent action occurs while cruising
GO to B3.

Intermittent action occurs while using control buttons or turning steering wheel
GO to B5.

 

B3 CHECK VACUUM TO SPEED CONTROL SERVO

  • Check vacuum supply to speed control servo (9C735).

    NOTE: 8.42 kPa, (2.5 in-Hg) is minimum vacuum for normal speed control servo (9C735) operation. The vacuum source hose is attached to the 7.9mm (5/16 inch) engine vacuum-fitting port.

  • Is vacuum OK?
Yes
GO to B4.

No
SERVICE vacuum supply.

 

B4 PERFORM SPEED CONTROL SERVO ASSEMBLY TEST

  • Is test successful?
Yes
SUBSTITUTE known good speed control amplifier (9D843) if OK — properly INSTALL speed control amplifier (9D843).

No
REPLACE speed control servo (9C735) assembly.

 

B5 PERFORM CONTROL SWITCHES AND CIRCUIT TEST

  • Are tests successful?
Yes
SUBSTITUTE known good speed control amplifier (9D843) if OK — properly INSTALL speed control amplifier (9D843).

No
SERVICE circuits. REPLACE horn pad assembly.

 

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

 

C1 VERIFY THE CONDITION
GO to C2.

 

C2 CHECK THE FOLLOWING SWITCHES AND CIRCUITS

  • Check the SET-ACCEL switch, RESUME switch, COAST switch, slip ring circuits and brush contacts.

  • Are circuits OK?
Yes
GO to C3.

No
SERVICE the circuit as required.

 

C3 TEST SPEED CONTROL SERVO

  • Is test successful?
Yes
GO to C4.

No
REPLACE speed control servo (9C735).

 

C4 TEST SPEED CONTROL AMPLIFIER

  • Does test correct condition?
Yes
REPLACE speed control amplifier (9D843).

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

 

PINPOINT TEST D: SPEED CONTINUOUSLY CHANGES UP AND DOWN

 

D1 VERIFY CONDITION GO to D2.  

D2 CHECK THROTTLE LINKAGE

  • Check throttle linkage for proper operation and adjustment.
  • Is operation and adjustment OK?
Yes
GO to D3.

No
SERVICE or ADJUST as required.

 

D3 CONTINUITY CHECK

  • Check continuity of Circuits 147, 148 and 149.
  • Is there continuity in all circuits?
Yes
GO to D4.

No
SERVICE or REPLACE wiring as necessary.

 

D4 TEST SERVO

  • Is test successful?
Yes
GO to D7.

No
REPLACE as required.

 

D5 CHECK SPEEDOMETER CABLES

  • Check speedometer cables for proper routing, no sharp bends or binding.
  • Are speedometer cables OK?
Yes
SERVICE as required.

No
GO toD6 .

 

D6 TEST VSS

  • With electronic instrument cluster perform speed signal test.
  • Is test successful?
Yes
GO to D7.

No
REPLACE VSS.

 

D7 CHECK DUMP VALVE

  • Check vacuum dump valve.
  • Is dump valve OK?
Yes
GO to D8.

No
SERVICE or ADJUST as required.

 

D8 TEST AMPLIFIER

  • Is test successful?
Yes
REPLACE amplifier.

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

 

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

 

E1 CHECK STOPLAMPS

  • Apply brakes and observe stoplamps.
  • Do stoplamps operate?
Yes
GO to E2.

No
SERVICE stoplamp circuit as required. VERIFY fuses are not open. GO to E3.

 

E2 CHECK SPEED CONTROL METER VALVE

  • Check speed control metering valve (9C727).
  • Is speed control metering valve (9C727) OK?
Yes
GO to E3.

No
ADJUST or SERVICE as required.

 

E3 CHECK SPEED CONTROL SERVO

  • Check speed control servo (9C735) operation and throttle linkage.
  • Are speed control servo (9C735) operation and linkage OK?
Yes
GO to E4.

No
REPLACE speed control servo (9C735).

 

E4 TEST SPEED CONTROL AMPLIFIER

  • Is test successful?
Yes
REPLACE speed control amplifier (9D843).

No
CHECK contacts of electrical connector. SERVICE as required.

 

PINPOINT TEST F: SPEED CONTROL SYSTEM DOES NOT DISENGAGE WHEN CLUTCH PEDAL IS DEPRESSED (MANUAL TRANSMISSION ONLY)

 

F1 VERIFY

  • Verify system disengages when stoplamp switch is activated.
  • Check clutch switch operation.
  • Do both operate properly?
Yes
SERVICE or REPLACE wire assembly 9A840 as required.

No
SERVICE or REPLACE as required.

 

PINPOINT TEST G: SPEED GRADUALLY INCREASES OR DECREASES AFTER SPEED IS SET

 

G1 VERIFY

  • Verify that engine is properly tuned.
  • Check accelerator action and actuator cable adjustment.
  • Is accelerator operation OK?
Yes
GO to G2.

No
ADJUST or CORRECT as required.

 

G2 CHECK SPEED CONTROL METER VALVE

  • Check speed control metering valve (9C727).
  • Is speed control metering valve (9C727) OK?
Yes
GO to G3.

No
ADJUST or SERVICE as required.

 

G3 TEST SPEED CONTROL SERVO

  • Is test successful?
Yes
PERFORM speed control amplifier test. REPLACE if required.

No
REPLACE speed control servo (9C735).