89 SHO Speed Control Diagnostics

Discussion in 'Gen 1 & 2 - Guides (For How-to guides, NOT how-to ' started by projectSHO89, Aug 25, 2006.

  1. projectSHO89

    projectSHO89 SHOless In St L Staff Member

    Nov 7, 2001
    Likes Received:
    St. Louis, MO
    I have been asked by another member to put something together for this, so I'll just do it for all to see. This addresses only the 89 but may pertain to later years. I do not have factory documentation for either 90 or 91 but I do have full documentation for 92-on. Much of the 92 system appears to be the same as the 89's, so I will be relying on both the 89 and 92 documents

    The ECA (PCM) in the 89 fully controls the speed control function based on various inputs and uses a vacuum operated servo to operate the throttle as described in the 92 docs:

    How the Circuit Works (SHO)

    The Taurus SHO does not use a Speed Control Amplifier. Instead, all Speed Control functions are handled by the Electronic Engine Control (EEC) Module.
    When activated, the Speed Control System maintains vehicle road speed at a driver-selected rate. Using input information from the Vehicle Speed Sensor (VSS) and the Throttle Position Sensor, the EEC Module operates the Speed Control Servo Vacuum and Vent Valves to control the level of vacuum applied to the Speed Control Servo Motor. The Servo Motor, in turn, operates the engine throttle linkage through the Actuator Cable to speed up or slow down the vehicle.
    Voltage is present at the Speed Control Switch Assembly ON contact at all times through Circuit Breaker 12, the Horn Relay coil and the Clockspring Assembly. <89 - Fuse F12, Horn Relay coil, column wiring, sliprings.> With the Ignition Switch in START or RUN, voltage is present at pins 37 and 57 of the EEC Module through circuit 361 (R). (See Section 24 for more Electronic Engine Control Information.)

    EEC Module Inputs

    Clutch Switch
    With the Clutch Pedal depressed, the Clutch Switch contacts close, applying an input to pin 30 of the EEC Module. This input causes the EEC Module to disengage Speed Control.

    Speed Control Switch Assembly
    The Speed Control Switch Assembly signals the EEC Module to turn the system on and off, to set and hold the vehicle speed, to resume and to accelerate or coast. With the Switch held ON, voltage is applied to pin 28 of the EEC Module. In OFF, pin 28 is grounded. With the Switch held in SET/ACCEL, a 680 ohm resistor is placed in the ground circuit. In RESUME, a 2200 ohm resistor is placed in the ground circuit; in COAST, a 120 ohm resistor is used.

    Stop Lamp Switch
    With the brake pedal depressed, the Stop Lamp Switch contacts close, applying voltage to pin 5 of the EEC Module. This input causes the EEC Module to disengage Speed Control.

    Throttle Position Sensor
    The Throttle Position Sensor is a potentiometer connected to the throttle linkage. The EEC Module applies a reference voltage to the Sensor and monitors the return signal voltage. The feedback circuit provides information about throttle linkage position.

    Vehicle Speed Sensor (VSS)
    The Vehicle Speed Sensor (VSS) is a small, permanent magnet generator driven by a transaxle gear. Sensor output is proportional to vehicle road speed. With Sensor output indicating that vehicle road speed is above 30 mph, the EEC Module enables the Speed Control System. The Vehicle Speed Sensor (VSS) signal is used to maintain vehicle speed when the System is operating.

    Other System Inputs

    Vacuum Dump Valve
    The Vacuum Dump Valve provides a mechanical means of disengaging the Speed Control System. With the brake pedal released, the Valve is closed, and the vacuum level in the System is determined by the Speed Control Servo Vacuum and Vent Valves. With the brake pedal depressed, the Dump Valve opens, disabling the vacuum circuit.

    EEC Module Outputs

    Speed Control Servo
    With the Speed Control System turned on and vehicle road speed above 30 mph, the EEC Module energizes the Vacuum and Vent Valves in the Speed Control Servo. The Vacuum and Vent Valves control the level of vacuum applied to the Servo Motor.

    Items in RED are those which I have identifies to pertain to the 92, not the 89. I added the 89 info in <> symbols.

    Verify power to circuit.

    Inspect electrical connections at all connectors.

    Inspect vacuum supply lines to servo and vacuum dump valves for leaks.

    Inspect throttle linkage.

    Verify brake lamps work properly.

    Verify clutch pedal position (CPP) switch operates correctly.

    Perform resistance checks of control input switches, the solenoids in the servo, and the VSS sensor.

    Perform voltage check of the input switches to verify circuit function through whole steering column, etc.

    This should get most owners into the correct part of the system to diagnose the fault. I do not claim that it is comprehensive or even correct, so, if you find an error or omission or just need something else added, pleas notify me by PM of what is needed. Please do not reply in this thread with additional information or questions, this is to be a information only tutorial. Use the PM feature.

    I have attached to this post a pdf file (235K) that contains the electrical schematic of the 89 SHO SC circuit, a physical layout of the underhood components, and a couple of hint sheets from the 89 and 92 service manuals. Additional data may be added later if needed.


    Attached Files:

  2. Mels


    Jul 16, 2003
    Likes Received:
    Brookfield CT
    That's a lot of valuable information right there. Thanks for putting that together, Steve. :thumb:
  3. SHON7

    SHON7 SHO Member

    Oct 3, 2011
    Likes Received:
    Hello I can't seem to open your attached file??? It just appears blank

    JSHDBAAOK 89SHO since new

    Mar 1, 2012
    Likes Received:
    not any more
    Missing schematic

    Comes up blank for me too...is there a reboot somewhere?

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