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Rack Signal Invalid (Diagnostic Code: 24-8)

Dynamic Rack Control System

Figure 1.1 - Rack Position (Module and Sensor)

#System Operation
The primary function of the ECM is to electronically govern the engine. The ECM senses engine rpm using the Engine Speed (RPM) Sensors and controls the Rack Actuator to achieve the desired engine rpm. This test determines if the ECM is properly controlling the Rack Actuator. 

Variables displayed on CAT ET that are used in this test are:
  • Desired Engine RPM is an input to the electronic governor within the ECM. Desired engine speed is based on the throttle input or PTO Switches.
  • Fuel Ratio Control (FRC) Rack is a fuel to air ratio limit used for emission control purposes. When the ECM senses a higher inlet manifold (Boost) pressure (more available air for combustion), the FRC limit is increased to allow more fuel.
  • Rated Rack is a limit on rack position that provides the specified power and torque curves. The Rated Rack value comes from maps programmed into the personality module.
  • Actual Rack Position is the ECM interpretation of the signal from the Rack Position Sensor. The ECM uses the Rack Position Sensor signal as feedback to allow precise control of rack position. 
Under a steady state condition, the Actual Rack Value should not exceed either FRC Rack or Rated Rack values. When the engine is in a steady state lug condition and Actual Engine RPM is less than Desired Engine RPM, Actual Rack Position will equal FRC Rack or Rated Rack, whichever is lower.

Under a transient condition, when a large load is suddenly applied to the engine, the Actual Rack Value can momentarily exceed both FRC Rack and Rated Rack. This condition typically lasts no longer than two seconds as the rack controls compensate for the sudden load increase.
Figure 1.2 - Rack Control System Schematic

#Functional Test
Test Step 1 - Check Electrical Connectors and Wiring
A. Turn the Engine Control Switch to the OFF position.
B. Check that ECM Connector J14/P14, MUI Interface Module Connector J15/P15, Rack Actuator Connector J1/P1, Customer Connector J31/P31, and Rack Position Sensor J11/P11 are securely mated and free of damage or corrosion.
C. Check connector Allen Screws for proper torque.
D. Check the harness and wiring for abrasion and pinch points from the Rack Actuator and Rack Position Sensor back to the ECM.
E. Perform a 45 N (10 lb) Pull Test on each wire.
If Not OK - Repair as required, if conditions are not resolved, proceed for next step.

Test Step 2 - Verify Normal Rack Operation at Idle
A. Connect CAT ET to the Service Tool Connector. Read More: How to Connecting Caterpillar ET Software to the ECM.
B. Start the Engine.
C. Monitor Desired Engine RPM, Rated Rack Position, and Actual Rack Position on CAT ET. Desired Engine RPM should be equal to the programmed low idle setting and remain steady. Read More: Status Tool on Caterpillar ET Software.
If Not OK - The Desired Engine RPM is unstable or too high. STOP - Check the Throttle Sensor, if any Diagnostic Codes are active: 91-8, 91-10, or 91-13.
Figure 1.3 - Idle Speed (Equal - OK)

Test Step 3 - Verify Normal Rack Operation Under Load
A. Load the Engine.
B. Monitor Actual Rack, Rated Rack, and FRC Rack Position on CAT ET. When the Engine is in lug condition, Desired Engine RPM should exceed Actual Engine RPM by at least 10 rpm, and Actual Rack should equal FRC Rack or Rated Rack, whichever is lower.
OK - The ECM is moving the rack to the specified position. If the problem persists, the cause is not the rack controls.
Desired Rack = FRC Rack - The Actual Rack is limited to FRC Rack indicating low boost pressure.
Actual Rack Low - Actual Rack is limited to some value below Rated Rack, the ECM may be limiting engine power or rpm.


Rack Position Sensor

#System Operation
The Rack Position Sensor is spring loaded to the fuel rack. The sensor is supplied with +Battery (24 VDC) and provides a PWM type Rack Position Signal to the ECM.

#Diagnostic Codes
  • 24-2 (Flash Code 22) - The mechanical system or Rack Position Sensor signal is lost.
  • 24-8 - (No Flash Code) - Rack Signal Invalid.
  • 24-10 (Flash Code 22) - The Rack Position Sensor signal is changing at an abnormal rate.
System Response - The diagnostic code is logged. The ECM uses engine speed to govern the engine and the rack sensor signal is ignored. Rack position is controlled by the Rack Actuator Current Drive Level only (open loop mode).

Test Step 4 - Check for Active Rack Position Sensor Diagnostic Codes
A. Turn the Engine Control Switch to the ON position, and Engine OFF.
B. Observe the Active Diagnostic Codes screen and check for an active 24-2, 24-8, or 24-10. In this case, we get an active 24-8 (Rack Signal Invalid).
Figure 1.4 - Rack Signal Invalid (24-8)
C. Disconnect the Rack Position Sensor connector, and we get an active 24-2 (Rack Loss of Signal), it means the ECM and wiring harness are OK, proceed for next step.
Figure 1.5 - Rack Loss of Signal (24-2)

Test Step 5 - Check Resistance Through Wiring
A. Turn the Engine Control Switch to the OFF position.
B. Disconnect the Rack Position Sensor Connector J11/P11 and the ECM Connector J14/P14.
C. Measure the resistance between P11 socket-C to P14 socket-37. The resistance should be less than 10 Ohms.
If Not OK - There is an open circuit or excessive resistance in the harness. Repaired as requires.

Test Step 6 - Check for Shorts in Harness
A. Measure the resistance between P11 socket-A and P11 socket-C, P11 socket-B and P11 socket-C.
B. Measure the resistance between P11 socket-C and Engine ground.
If Not OK - There is short circuit in the harness. Repair as requires.

Test Step 7 - Check Supply Voltage to the Rack Position Sensor
A. Turn the Engine Control Switch to the ON position, and Engine OFF.
B. Disconnect the Rack Position Sensor connector, and measure the sensor supply voltage between terminal-A and terminal-B. The voltage should be 24.0 +/- 4.0 VDC.
If Not OK - There is a problem in the supply voltage from the batteries.

Test Step 8 - Check the Rack Position Reading on CAT ET
A. Reconnect all connectors.
B. Move the rack back and forth by hand and observe Actual Rack Position on CAT ET. The Rack Position reading should be approx. 5.0 mm (0.20 inch) in the Fuel OFF position and increase to a limit of approx. 38 mm (1.50 inch). But in this case, the Rack Position is stuck on 2.70 mm (refer to Figure 1.4 above).
Figure 1.6 - Rack Actuator
C. If compares with other equipment (refer to Figure 1.7 below), the Rack Position reading 10-37 mm on CAT ET. Read More: Using Data Log Recorder on Caterpillar ET Software.
Figure 1.7 - Data Log Viewer (Rack Position)
If Not OK - Replace the Rack Position Sensor (with CAT P/N: 6T-9593) and recalibrate. If the problem is not resolved, reinstall the old Rack Position Sensor and replace the ECM.


Rack Actuation System

#System Operation
Rack Control with this engine is accomplished as follows:
  • The ECM first determines the Desired Engine RPM from throttle inputs, and the Actual Engine RPM from the Engine Speed (RPM) Sensors. Next, a Desired Rack Position is calculated using fuel map data stored in the Personality Module, Engine rpm error (Desired Engine RPM minus Actual Engine RPM), and FRC and Rated Rack Limits. The ECM then calculates a Desired Rack Actuator Current using Desired Rack Position and Actual Rack Position (from the Rack Position Sensor signal) values and converts it to a 1 to 99 percent PWM Actuator Drive signal.
  • The MUI Interface Module receives this signal and produces a Rack Actuator current signal between 0 and 200 mA to position the rack. The MUI Interface Module also produces a Measured Actuator Milliamp Feedback signal to allow the ECM to accurately regulate Actual Rack Current by adjusting the PWM Actuator Drive signal.
  • Additional signals between the MUI Interface Module and the ECM include: Actuator Enable - This signal is produced by the ECM. If the Actuator Enable signal voltage is greater than 0.5 VDC at pin-5 of the MUI Interface Module, the MUI Interface Module will interrupt the Rack Actuator current and cause the rack to return to the Fuel OFF position.


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