|Code||Fault Location||Probable Cause|
|P0900|| Clutch actuator -circuit open |
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|Wiring, clutch actuator, ECM/PCM/TCM|
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What Does Code P0900 Mean?
OBD II fault code is defined as “Clutch Actuator – Circuit Open”, and is set when either the PCM (Powertrain Control Module), or TCM Transmission Control Module) detects an open circuit in the clutch actuator’s control circuit. The TCM interprets the actuator output voltage as it engages and disengages the clutch, along with that of the clutch stroke/movement sensor.
Much like fully automatic vehicles, the TCM on a SAT (Semi-automatic) vehicle uses information from a variety of sensors, such as the throttle position, engine speed, engine load, and others to calculate the correct moment when to initiate a gear change.
Gear changes are accomplished by a command from the TCM to energize the clutch actuator to release the clutch; after which various actuators inside the transmission disengage the current gear, and depending on current operating conditions, engage the next higher or lower gear ratio. All of this happens much faster than a human driver could change gears, and in most applications, gear changes are as smooth and seamless as on any fully automatic transmission.
Some types of actuators involve solenoid-type mechanisms to operate a clutch master cylinder in which power is transferred by brake fluid, such as in some Toyota models, while still other designs use electronic sensors, pneumatic and or hydraulic actuators, or a combination of sensors and different types of actuators that are controlled by microprocessors.
Regardless of the design though, the purpose of all clutch actuating systems is to improve the driving experience by removing the need for manual clutch operation by the driver. Computer-controlled clutch actuating systems work particularly well in dual clutch systems where one clutch controls gear changes of even-numbered gear ratios, and a second clutch that controls gear changes of odd-numbered ratios.
The image below shows one example of a clutch actuator. This particular unit is from Smart car, and uses an electric motor to rotate a crank-type mechanism to activate the clutch. Note that this is only one example of a clutch actuator; there are many other, different designs in use today.
What are the common causes of code P0900 ?
A common cause of code P0900 is the failure of the selector switch on the gear selector, but there are several other possible causes as well, such as-
- Open circuits caused by damage to wiring.
- Open circuits caused by corrosion of electrical connectors.
- Loss of ground.
- Internal electrical failures in actuator motors/solenoids.
- Failures of clutch motion/travel sensors.
- TCM failures, but this is rare, and the fault must be sought elsewhere before any control module is replaced.
What are the symptoms of code P0900 ?
As a rule, the main symptom of code P0900 is the non-engagement of any gear, but depending on the application, there may be other symptoms present besides an illuminated CHECK ENGINE light (which may, or may not illuminate) and a stored trouble code. Other possible symptoms may include-
- Engine may stall when the control system fails during driving.
- In some applications, the engine will not crank as a safety precaution when code P0900 is present.
- In some cases, the transmission may enter into a “limp” mode.
- In some cases, the transmission may be stuck in one gear, without entering “limp” mode.
How do you troubleshoot code P0900 ?
NOTE #1: Diagnostic, repair, and replacement procedures are for the most part make and model specific, meaning that apart from a code scanner and a digital multimeter, a good repair manual is an essential item when attempting to diagnose and repair code P0900.
NOTE #2: The design of clutch actuators and their control circuits vary widely between manufacturers, as do the actual working of different actuator designs. For this reason, it is important to have a repair manual and wiring diagram for the application being worked on at hand, since the code-setting parameters, diagnostic/repair procedures, and replacement procedures are often unique to many applications and designs.
NOTE#3: Some systems require that the actuator has an adequate supply of brake fluid. Low brake fluid levels can mimic the symptoms of code P0900, so consult the manual to locate the brake fluid reservoir. Bear in mind that a low fluid level can set code P0900 on some applications, so make sure that on fluid operated systems the fluid level is up to the mark before starting an electrical diagnosis.
Once the presence of code P0900 is confirmed with a scan tool, and you have ascertained that the system is not fluid-operated, record all freeze frame data for future reference in the event that an intermittent fault is identified later on.
NOTE: It is entirely possible that one, or more of closely-related codes P0901, P0902, and P0903 may be present along with P0900.
Clear all codes, and consult the manual to determine the color-coding, location, and routing of the control circuit wiring. Perform a thorough visual inspection of all associated fuses, relays, and wiring to check for damaged, broken, pinched, or corroded wiring and connectors. Repair as required, but pay particular attention the selector switch on the gear selector, as well as the battery condition and voltage.
Clean and retighten battery terminals as required, and consult the manual on the correct test procedure(s) for the selector switch, or any other switches related to the clutch actuator. Replace any switches that do not conform to manufacturer’s specifications.
If the switch(es) appears to be fully functional, disconnect all relevant control modules, and perform ground, power supply, and continuity checks on all wiring. Also, be sure to perform an amperage draw test on the actuator motor/solenoid, as well as on the actuator position sensor. These components form part of the control circuit, and must be tested along with everything else. Repair, or replace components as required. Retest the system to see if the code returns, or if repairs were successful.
NOTE: The setting parameters of code P0900 are different for almost all applications, so be sure to consult the repair manual for the exact resistance, amperage, and reference/system voltages/values.
If all values fall within specification, consult the manual on the correct procedure to apply power directly to the clutch actuator mechanism, paying due regard to the required voltage, which will be stated in the manual. If the mechanism does not respond, check all connections and the ground. Also be sure to check the mechanism for continuity and resistance before applying power directly.
If the mechanism works, test it repeatedly to check for intermittent faults or signs of overheating. Normal operating temperatures will be stated in the manual. If the unit overheats, or works intermittently, replace it. Reconnect all wiring and control modules, clear all codes, and operate the vehicle with a scan tool connected to see if the code returns.
WARNING: On some applications, replacement of the actuator mechanism requires reprogramming, and/or integration with the TCM or PCM, or both. It is critically important to follow the correct procedures during removal and installation to prevent damage to clutch, or clutch actuator parts and components. Always consult the manual on the correct installation/reprogramming/integration procedure(s).
If the code returns after a test drive, there is likely to be an intermittent fault present, which you may have to allow to worsen before an accurate, and definitive repair can be made.