The vehicle's cruise control system (abbreviated as CCS) is also known as the constant speed travel system. It is an electronic control device that uses advanced electronic technology to automatically adjust the speed of the car to achieve a preset speed. When driving on the highway for a long time, after opening the automatic control switch of the system, the cruise control system will automatically increase or decrease the throttle opening according to the driving resistance. This eliminates the need for the driver to frequently step on the accelerator pedal to ensure that the car travels at a preset speed. In the process of driving the car, the driver only needs to hold the steering wheel, which greatly reduces the fatigue strength of the driver and reduces the occurrence of traffic accidents. At the same time, since the cruise control system can automatically maintain the speed of the vehicle, unnecessary man-made changes in the accelerator pedal are avoided, thereby improving the fuel economy of the vehicle and the engine emissions.
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This kind of system is widely used in foreign cars. In the United States, the installation rate has reached more than 60%. However, in China due to road conditions, the use rate is not high. Even if the system fails, the driver does not care. In addition, because the maintenance technology can not develop rapidly with the automobile technology, many maintenance personnel still have insufficient understanding of the system, which causes unintentional interference or damage to the system during the maintenance process, and cannot scientifically analyze the fault phenomenon and accurately determine the root cause of the fault. Here, talk about two examples of fault diagnosis for the cruise control system.
Example 1: venting hole blocked, fixed speed cruise system dysfunction
A Dodge CARAVAN car was disassembled and overhauled due to mechanical failure of the engine. After the restoration, the cruise control function is lost.
On the expressway test, it was found that when the fixed speed switch was started, the servo device pulled the accelerator pedal to the limit position. Unable to set the driving speed.
Using the "OTC" decoder test system, the test found that all the switch states are normal. Then the solenoid valve action test is performed, and the solenoid valve of the fixed speed servo device has obvious response. Analysis of three solenoid valves of the servo device: When the cruise control system is working, the control module PCM passes the brake switch (ON) and provides 12V working voltage to the three solenoid valves. The unloading solenoid valve is in the "ON" state and the vacuum relief passage is closed. The vacuum solenoid valve is in the "ON" state, opening the vacuum channel of the vacuum storage box to the servo bellows. The exhaust solenoid valve is controlled by the PCM to be in the "ON-OFF" intermittent opening and closing state, and the vacuum degree of the servo bellows is adjusted, thereby adjusting the vehicle speed.
When the brake pedal is depressed, the action of unloading the solenoid valve and the vacuum solenoid valve is reversed, thereby releasing the constant speed function.
Through the above analysis and detection, it can be known that the state of each switch is normal, and the solenoid valve works normally, and only the vacuum passage may be faulty. If the venting hole controlled by the exhaust solenoid valve is blocked, it will definitely cause the pacing of the cruise control system.
The servo device was disassembled and it was found that the venting hole was blocked by the sealant. It turned out that the car accidentally broke the servo harness harness plug during the process of disassembling the engine, so the repairer glued it with the sealant, and the excess glue blocked the vent hole. Therefore, after the cruise control system is started, the cruise control unit cannot adjust the vacuum of the servo capsule, and the vehicle speed cannot be adjusted.
The sealant that blocks the vent hole is removed, the vehicle is re-commissioned, and the cruise control system function is restored.
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