Downshift Switch Circ Interm/Erratic
The PCM has detected a malfunction in the electrical circuit of the downshift switch circuit. The switch, system wiring and connectors, and mode actuator are all included in this circuit. The upshift/downshift switch is located near the shifter in the passenger compartment. Upshift/downshift switches are used on vehicles with manually shiftable automatic transmissions.
Code Set Parameters
If the PCM detects a voltage or resistance signal (in the downshift switch circuit) that is not within the manufacturer’s recommended specifications, a code will be stored and a malfunction indicator lamp may be illuminated. Some vehicles will require multiple failure cycles in order for the malfunction indicator lamp to be illuminated. Some vehicles will never illuminate the service engine soon lamp for this code.
It is very likely that there will be no distinguishable symptoms aside from the manual shifter function being disabled. The transmission controller may place the transmission in limp-in mode. In this mode, the transmission will shift harshly and torque converter lockup will normally be eliminated. A code will be stored and a service engine lamp may be illuminated after multiple failures. In some applications, the overdrive light may flash, as well.
The most common cause for this type of failure is due to liquids spilled on the upshift/downshift switch. Open or shorted electrical wiring, damaged, corroded, or disconnected wiring or connectors are also a viable possibility.
Techs report that liquids spilled onto switches are often overlooked in preference of transmission wiring malfunctions.
- 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.
Some applications utilize multiple transmission modes
- This allows the driver to choose between two (or more) shift strategies
- Typical transmission modes include a “Normal” mode, in which the transmission shifts softly and smoothly, and a “Performance” mode
- In the performance mode the transmission will feature a more high-performance oriented shifting strategy by adapting higher fluid pressure and stiffer shifting.
Manually shiftable automatic transmissions allow the driver to downshift or upshift very suddenly, by depressing a button (switch) located near the shift lever
- Some upshift/downshift switches are mounted on the steering wheel (paddle shifters)
Visually inspect all 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 there
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 code immediately returns, and the transmission seems to operate normally, check for reference voltage and ground signals at the downshift switch and actuator
- 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.
Remember to use caution when testing for resistance and continuity in wiring circuits and always disconnect all related controllers prior to testing
- Using the manufacturer’s wiring diagram for the downshift selector system, test all related circuits and switches 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 upshift switch
- If the upshift 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 transmission mode selector fails to activate manually, replace the normal/performance switch and retest the transmission control system.