1 Operating Principle of the AEC System
Caterpillar ET 2026A & 2019C Electronic Technician Diagnostic Software Download and Installation
The AEC (Automatic Engine Control) system is an automatic throttle control system that limits the engine to high and low speed settings. It utilizes a worm gear mechanism to shift the throttle limit linkage, thereby controlling the engine speed: the throttle is set to the high-speed position during operation and to the low-speed position during no-load conditions.
1.1 Acceleration When the control lever is moved, a hydraulic signal is transmitted to the AEC pressure switch (8), causing it to close. With the AEC switch (9) set to ON and the work mode switch (10) set to
position I (or II/III), three signals are input to the controller (7). After processing these signals, the controller outputs current to the AEC relay (6). The relay activates: normally open contact (a) closes, and normally closed contact (b) opens. A closed circuit is formed: Battery → Power Switch (2) → Fuse Box (3) → Limit Contact (c) → Relay Contact (a) → AEC Motor (5) Winding → Ground. Motor (5) rotates, driving the worm gear 180 degrees to push the throttle to the high-speed position, accelerating the engine to 2380 rpm. Due to the mechanical linkage of motor (5), once the high-speed position is reached, contact (c) opens and contact (d) closes; this breaks the circuit driving the motor's acceleration, causing the motor to stop while the engine throttle remains in the high-speed position. The engine then operates atits rated speed.
1.2 Deceleration When the control lever returns to the neutral position, no hydraulic signal is sent to the AEC pressure switch (8), causing it to open. If switch (8) remains open for 3 seconds after the controller (7) receives this signal, the controller cuts off the output current to the SOL.B terminal. Relay (6) de-energizes; contact (a) opens, and normally closed contact (b) closes. 1 Current flows from the battery through contact d to contact b, then through the AEC motor 5 winding to ground, forming a closed circuit.
Motor 5 rotates, driving the worm gear to continue turning by 180° and pushing the throttle from the high-speed position to the low-speed position, thereby reducing the engine speed to 1300 r/min. Due to the mechanical linkage, contact d opens and contact c closes; this breaks the closed circuit that caused motor 5 to decelerate, causing motor 5 to stop operating. The engine then runs at low speed to conserve fuel.
2 Fault Diagnosis Common faults in the automatic engine throttle control system of the Caterpillar E240 excavator include:
CAT Comm Adapter3 478-0235 Diagnostics for Caterpillar
(1) After the control lever returns to the neutral position, the engine throttle fails to automatically drop to low speed and remains in the high-speed position.
(2) When the control lever is operated, the engine throttle fails to automatically accelerate and remains in the low-speed position. Based on the AEC system's circuit principles, fault diagnosis can be performed using the following steps:
① First, check whether the AEC motor and controller fuse in fuse box 3 has blown.
② Check the AEC pressure switch 8. When the engine is running and the control lever is in the operating position, the AEC pressure switch should be closed; when the control lever is in the neutral position, the AEC pressure switch should be open. If there is no response, first check the hydraulic signal. The signal oil pressure for the AEC pressure switch is the pilot oil pressure; during normal operation, the oil pressure should be between 4.25 and 4.55 MPa. When the control lever returns to the neutral position, the signal oil pressure drops to the return oil pressure level. The AEC pressure switch closes when the signal oil pressure is ≥2.45 MPa and opens when it is <2.45 MPa. If the detected signal oil pressure is normal but the AEC pressure switch shows no response, the switch is faulty and should be replaced. If the detected signal oil pressure is abnormal, further inspect the pressure in the signal oil circuit.
③ Check the AEC switch (9), backup switch (4), and operating mode switch (10) for damage. Check the continuity of the corresponding contacts and look for any short-to-ground faults; replace if damaged.
④ Check the AEC relay (6). The AEC relay coil resistance is 170Ω; this can be measured. Check the closing and opening of the AEC relay contacts by directly applying and cutting off power. Replace if damaged.
⑤ Check the AEC motor (5). This can be tested by momentarily applying power: ground the 'i' wire and momentarily apply power to the 'f' wire; if the AEC motor operates, it is functional; otherwise, disassemble/inspect or replace it. Check the linked contacts 'c' and 'd': when the AEC motor is in the high-speed position, wires 'e' and 'h' should be connected (0Ω resistance) and wires 'e' and 'g' disconnected (infinite resistance); when the AEC motor is in the low-speed position, wires 'e' and 'h' should be disconnected (infinite resistance) and wires 'e' and 'g' connected (0Ω resistance). If these conditions are not met, inspect the contacts and the linkage mechanism.
⑥ Check the connections of all wires and plugs for open circuits or short circuits; ensure proper connection.
⑦ Test the controller (7). Measure the voltage parameters at the controller terminals according to Table 1. (Referred to as the AEC system).
If the test results do not match specifications, it indicates internal damage to the controller's control circuitry (ECU board). Options include replacing the entire controller, using the backup switch for emergency operation, or modifying the AEC control system. 3 Emergency Modification of the AEC Control System
If a fault in the AEC motor relay control circuit of the Caterpillar E240 excavator engine throttle system is confirmed to be caused by an internal controller failure, a simple time-delay relay control circuit can be designed as an emergency measure. For the time-delay relay (7), a DS22C electromagnetic time-delay relay can be selected; specifications: 24V DC, normally open contact, adjustable delay range of 1.5–5 seconds. The DS22C relay is configured to cut off power after a 3-second delay, utilizing the controller's function for controlling the AEC relay; the delay setting is adjusted to the 3-second mark. A mounting bracket is fabricated to securely install the DS22C time-delay relay inside the electrical component box on the right side of the cab. The AEC pressure switch, AEC switch, AEC relay coil, controller fuse (e), cab lighting fuse (f), and DS22C time-delay relay are connected as shown in Figure 2, while all other existing wiring remains unchanged. It should be noted that this modification is limited to replacing the controller's function of controlling the AEC relay; all other original functions of the controller remain intact. Two units have been modified according to this scheme, yielding excellent operational results. The AEC function is implemented by installing a time-delay relay (7)—featuring a 3-second power-off delay—to replace the function of the controller (7) in controlling relay (6) as shown in Figure 1.
