[Repair Case] Audi Q5 cruise control system is unavailable

Posted by Tony Brown on

Car: Audi

Model: Q5

System: Engine

Fault phenomenon: The cruise control function can be activated when idling, and the speed of the cruise control can also be increased or decreased (as shown in Figure 1), but the cruise control function cannot be activated when the vehicle is driving.


Figure 1


The test drive finds that the cruise control function and the engine start-stop function is not available. Check the relevant system with a diagnostic tool, we didn't find any trouble codes. Looking at the reason preventing the engine from stopping is the "diagnostic interface", the diagnostics have no further explanation. First, read whether the cruise control switch is normal in the driving state. The result of reading with the diagnostic instrument is that each switch state can be displayed correctly.

From the inspection results, the cruise control switch information is normal, which is also confirmed by the fact that the cruise control can be activated and the vehicle speed can be adjusted at idle speed. Then why can't it be activated while driving and the control unit does not generate a fault code?


After sorting out the ideas, it is believed that the cruise activation function may be limited for some specific reasons, such as control unit coding, basic settings, or other unknown reasons. First, we conduct an online code comparison for the car, and the result code appears normal. Then we try to check the basic settings of the relevant control unit and find no option for cruise off or on.

At this time, the diagnosis is confusing, and we don't know where to start. The working principle of the cruise control function is sorted out again. The cruise control switch transmits the relevant information to the steering column control unit J527; then J527 sends the relevant cruise information to the cruise control master control unit J623 (engine control unit). The engine control unit decides whether to start the cruise according to the cruise control activation conditions, such as whether the vehicle speed is lower than 30km/h. Since there is no fault code, first check whether there is any data flow related to limiting the cruise work in the cruise control unit. After a detailed inspection, it is found that there is indeed a data flow of "the reason for the shutdown of the cruise control of the transmission coordinator and the ACC control".


In the data stream, the reason for cruise shutdown is 32678, and there is no relevant information on the meaning of this digital code. However, in the text below, there is an option of "brake adjustment not available", which means that all current reasons for limiting cruise activation are only brake adjustment not available. However, the ABS system has no fault code and the ABS system works normally during the test drive.


Compared with other vehicles with the same configuration, it is found that the data flow of "transmission coordinator cruise control and ACC control shutdown reason" is normal when cruise control is available. It means that the fault is caused by the unavailability of brake adjustment. Could it be that the ABS control unit is faulty? However, the cost of replacing ABS is high and labor-intensive, so replacement is not directly considered. Recall that the engine start-stop function is also unavailable during the test run, and the factor affecting the start-stop function is the diagnostic interface (gateway); the gateway is the management and control unit of the battery. Could it be that the above two faults are caused by insufficient battery power?

Troubleshooting: Since the current customer has not reported difficulty in starting, and the generator fault light is not lit, check the generator voltage and the voltage after the battery is turned off. Use a battery check analyzer to check whether the battery is in good condition, and the result shows that the battery is damaged and needs to be replaced (as shown in Figure 2). After trying to replace the battery, the test drive finds that the cruise control function can be used normally and the start/stop function has returned to normal.


Fault Summary: In this example, the cruise control function is not available due to the deterioration of battery performance, which is rarely encountered in previous maintenance. There is no fault code in the related system and the basic function of the battery to start the engine is still normal, thus it's not a small challenge. Fortunately, we discover the reason for the cruise restriction through the data stream, but it takes a lot of effort to analyze the reason. In the process of constant speed cruise, the vehicle sometimes needs to control the vehicle speed through the brake adjustment of the ABS system, and the brake adjustment process needs to start the ABS hydraulic pump motor. The activation of cruise control is limited when the battery level deviates from the normal range. For some hidden faults, if it is suspected that the battery is not working properly, a special battery should be used for inspection and processing.



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