[Repair Case] Lumina can’t drive at high speed

[Repair Case] Lumina can’t drive at high speed

Model: 3.1L engine and automatic transmission.

Issue: When the car is operating, the driver reports that it becomes harder to increase the speed even if the accelerator pedal is depressed. The vehicle's top speed is limited to 44 miles per hour. After inspection by maintenance staff the defect is assumed to be the automatic gearbox lock. The automatic transmission system is in good working order, transmission oil is adequate, and the internal friction components are not overly worn or burned.

Diagnosis: A stall test was performed. The drive gear has a stall speed of 2250r/min. Electrical failures appear to be more prevalent within the normal range. The power system control module is the main control module of the automatic transmission, and it performs self-diagnosis on the power system. Short-circuit the diagnostic sockets A and B pins, turn on the ignition, and read the problem code from the engine trouble light's flashing frequency; there are three defects. Codes: 12, the engine speed signal (ignition pulse signal) was not received; 31, the parking/neutral position switch signal was not received; 43, the electronic ignition control was not received, and the ignition signal delay was too long. The fault code can be regarded as a normal code because the self-diagnosis is performed while the engine is not running. The error persists after unplugging the PCM fuse, clearing the code, and then performing a road test. When you read the fault code again, you'll see fault codes 12 and 31. The automated transmission gear switch is more likely to fail since fault code 31 is directly tied to the automatic transmission system.

The automatic transmission's exterior control circuit is quite basic, it’s housing contains three electrical components: a solenoid valve assembly, an automatic transmission gear switch assembly, and a vehicle speed sensor assembly. In addition, under the brake pedal is an electrical component relating to the automatic transmission: the TCC brake switch.
Pay special attention to the automatic transmission gear switch assembly, which has two harness connectors: a 6-pin and a 4-pin harness connector. The 6-pin harness plug and its wiring are not connected to the power system control module, although the 4-pin harness plug and wiring are. Black/white, yellow, gray, and white wires are available. Measure the voltage after unplugging the 4-pin harness plug and turning on the ignition switch. The voltage on all four wires is 12V, suggesting a normal connection between the harness and the power system control module. Insert the 4-pin harness plug, turn the shift lever, and check the gear signal output status with the diode test light. The diode test light in the P gear is half-bright when the yellow wire is examined, indicating that the working voltage is too low. When you gently touch the shift fork with your hand, the diode test light begins to flicker, indicating that the automatic transmission gear switch's internal circuit has a poor connection. The gear switch for the automatic transmission was disassembled and inspected. The circuit board was found to be corroded in many places and was unrepairable. There was a hose for ventilation at its end, which caused water to enter inside owing to the fracture, and the circuit board was corroded in many places and was of no repair value. Replace the automatic transmission gear switch, test drive, and the failure symptom

Summary: The lock problem produced by damage to the automatic transmission gear switch is relatively rare. The Lumina model's automatic transmission gear switch is a part of the parking/neutral position switch. It is made up of four transfer switches (four-wire gear switch). The logic of the four-wire gear switch signal is used to manage the power system. The combined state determines the shift mode by judging the location of the shift lever. Table 4-4-1 shows the four-wire gear switch signal logic.

 Table 4-4-1 Four-wire gear switch signal logic table

 Clarification: o: High electric potential. *: Low electric potential

 The logical combination signal of a four-wire gear switch gets jumbled when it is damaged. Even if the throttle position sensor, vehicle speed sensor, TCC brake switch, and other signals are normal, the power system control module will set the corresponding error code, locking the automatic gearbox and preventing the car from being driven.