4-5 Shift Malfunction
The PCM has detected an abnormality in 4-5 shift function. The computer controlled automatic transmission uses shift solenoids to manage the transfer of fluid between hydraulic circuits and actuate a change in transmission gear ratio. Varying gear ratios are needed to increase and decrease vehicle speed, maximize engine performance, and optimize fuel efficiency.
Code Set Parameters
Although code setting parameters vary greatly from one manufacturer (and model) to another, virtually every application utilizes calculations based upon throttle position, engine load, engine speed, and vehicle speed to determine the desired gear ratio. If the desired gear ratio fails to coincide with the actual gear ratio detected, a code will be stored and a malfunction indicator lamp may be illuminated. Some applications require multiple failure cycles in order for a service engine soon lamp to be illuminated.
These may vary from no detectable symptoms at all to a failure of the transmission to shift from (or into) any particular gear. Other symptoms may include harsh shifting, transmission slippage, transmission overheating, decreased fuel efficiency, a stored code, and possibly an illuminated malfunction indicator lamp. Other shift solenoid codes often accompany this code. The transmission controller may also enter “limp-in” mode when this code is stored.
Common causes include a faulty shift solenoid, hydraulic blockages inside the internal transmission fluid passages, mechanical internal transmission failures, low transmission fluid, dirty or contaminated transmission fluid, a faulty PCM or transmission control module.
Technicians report that more frequently than not, faulty shift solenoid circuit malfunctions cause this code, and shift solenoid malfunctions are diagnosed and replaced in error.
- A scanner (or code reader) and a digital volt/ohmmeter will be required to successfully diagnose this code.
Virtually every vehicle (with an automatic transmission) produced since the mid-eighties is equipped with some type of computer controlled automatic transmission system
- OBD-II equipped vehicles utilize a sophisticated computerized control system that calculates various engine drivability inputs with vehicle speed inputs, engine/turbine speed inputs, and transmission output speed inputs to determine the desired gear ratio necessary to achieve maximum fuel efficiency, to optimize engine performance, and provide the greatest engine and transmission longevity
- The PCM calculates input data and initiates upshifting and downshifting by using strategically designed shift solenoids to actuate fluid transfer between the hydraulic circuits of each set of corresponding gears.
Multiple stored shift solenoid codes usually indicates that a transmission component slippage or delayed engagement condition has occurred.
The automatic transmission uses a high pressure pump (driven by the engine via the torque converter) to circulate fluid through the transmission hydraulic circuits in the transmission housing and valve body
- If transmission pressure is insufficient to actuate gear changes, this type of code may be present
- Insufficient fluid pressure is most often caused by a low or dirty (contaminated) transmission fluid condition
- Begin any transmission diagnosis with an inspection of the transmission fluid dipstick (if equipped)
- At any rate, make sure that the fluid level is within manufacturer’s specified recommendations and that the fluid is in good condition
- Fluid that smells burned or abnormal in any way should be considered inadequate
- Burned fluid is a certain indication that the transmission has been operated during a low fluid level condition and may have suffered catastrophic internal mechanical failure
- If the fluid smells or appears burned, remove the transmission pan and check for the presence of clutch material and other debris
- If such contaminants are found in the transmission pan, a total rebuild, fluid flush and replacement, and a new torque converter will be required before the transmission will function properly.
If no debris is found in the transmission pan, visually inspect all internal wiring and connectors
- Look for shorted or burned wiring and replace circuitry and connectors as required
- Most applications will require replacement of the full internal transmission harness if discrepancies are noted
If the system wiring, connectors, and components appear to be in normal working order, connect the scanner to the diagnostic connector and record all stored trouble codes and freeze frame data
- This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
- After the codes are cleared, operate the vehicle to see if the code returns
- If the code fails to immediately return, you may have an intermittent condition
- Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made
If the transmission fluid appears to be in decent shape, but the fluid level is low, add enough fluid to locate a leak and repair it
- Fill the transmission with fluid, allow the vehicle to reach normal operating temperature, and test drive it to see if the transmission operates normally and to determine whether the code will return
- If the transmission functions normally and the code doesn’t return, then it was likely caused by transmission slippage due to a low fluid level/pressure condition.
If the code returns and the transmission slips or suffers from delayed engagement, check pump pressure using a manual pressure gauge
- Obtain a hydraulic pressure diagram pertaining to the vehicle in question and thread the end of the gauge into the appropriate opening on the transmission housing to record pump pressure
- Compare your findings with manufacturer’s specifications and make repairs accordingly
- Typical causes of low pump pressure include clogged hydraulic passages, a defective pump, faulty shift solenoids, or a faulty electronic pressure regulator
- Pump replacement will require transmission removal and partial disassembly
If the code immediately returns, and the transmission seems to operate normally, check for reference voltage and ground signals at the affected shift solenoid
- If either the reference voltage or ground circuits are open, use the digital volt/ohmmeter to check for continuity (disconnect all related control modules from the circuit before checking circuit resistance or controller damage may occur) and resistance in all circuits
- Repair or replace system circuits/connectors as required and retest the system to ensure that repairs were successful.
Using the manufacturer’s wiring diagram for the affected shift solenoid, test all related circuits and the solenoid itself for resistance and continuity and compare your findings with manufacturer’s specifications
- Repair or replace system circuitry, connectors, and/or components that fail to coincide with manufacturer’s specs
- Always retest the system to ensure a successful repair.
If all system circuits are intact connect the scanner to the diagnostic connector and attempt to manually activate the shift solenoid/s in question
- If the shift solenoid functions properly and all circuits coincide with manufacturer’s specifications, suspect a defective PCM
- Remember that PCM failure is rare and replacement will require reprogramming
- If the shift solenoid fails to activate manually, replace the solenoid (or solenoid pack) and retest the transmission control system.