Symptom: 2010 models Audi A4L FSI 2.0T sedan, mileage 1 500 km. Users reflect the car’s sunroof suddenly open, the sunroof control button malfunction.
Check Analysis: firstly maintenance personnel test, control button repeatedly sunroof, skylight no reaction to the faults as the user; VAS5052 connection specific fault diagnostic testing and found stored in the meaning of the comfort system It is ldquo; LIN bus is shorted to ground occasional rdquo; fault code.
FIG 1
Normal car is equipped with the sunroof (non-solar), by independent and sunroof control unit J245 switches louvre regulator E139 (FIG. 1) control. No maintenance personnel check the skylight external signs of damage; roof control unit and plug inspection, and found no abnormalities; check the relevant fuse and a circuit diagram of the line, found no phenomenon of short circuit or open circuit.
no fault codes, each of the constituent elements is also no exception, so the maintenance personnel has to replace the new roof control unit J245 E139 and sunroof control switch, but the fault is not ruled out. So far, the repair work in trouble.
FIG. 2
After I took the car, first careful analysis circuit diagram (FIG. 2), the control unit J245 found sunroof and comfort system control unit J393 communicate via a LIN bus, which means that the sunroof control unit needs to receive the amenity system signal processing control unit, and executing instructions, operating opening and closing the sunroof. Accordingly, I try to comfort after replacing the system control unit test, the fault remains.
Figure 3
Analysis of the maintenance manual works signal skylights, amenity system control unit to be received in accordance with a plurality of sensors, these signals via the LIN bus transmission. The author related inspection sensors one by one, upon detection of the air humidity sensor G355, traces of water plugs are found, after decomposing the sensor, the sensor was found signs of water inside the circuit board (FIG. 3). It seems likely to have air inlet humidity sensorDamaged, so I try to replace a new air humidity sensor, where operation control button sunroof, skylight discovery function returned to normal. This result was unexpected because the analysis from the function, the air humidity sensor is to prevent the windshield frost set in, it did not seem any contact with skylights. But why in this case the fault, the air humidity sensor water damage, it will lead to malfunction skylight?
Figure 4
With questions, I looked at the car SSP, but no relationship between the manual and the sunroof between the air humidity sensor introduction. Thus, the author of the relevant circuit diagram of a network topology and a careful analysis, found that the air humidity sensor G355, sunroof control unit J245, burglar alarms, and garage door opener sensor G578 manipulation unit E284 LIN bus in the same network (FIG. 4). Accordingly, my analysis, the reason why air humidity sensor failure sunroof cause damage, because due to a communication failure LIN bus.
we know, the LIN bus is mainly used as an adjunct network or sub-network CAN bus, CAN bus is able to provide more comprehensive not need to use network functions. (E.g., the electrical control of the vehicle body), the LIN-bus network can effectively simplify the wire harness in a simple bandwidth less demanding function system (single-wire transmission), reducing the cost. However, compared with CAN bus, LIN bus, there are some drawbacks, the most significant point is that, LIN bus is not fault tolerant.
Since the wire transmission using the CAN bus (CAN-H, CAN-L), wherein when the occurrence of a line fault or short to power, all the transceivers are switched to the single wire CAN mode, to ensure that the transmission signal. And LIN bus uses a single-wire transmission, is also not fault tolerant. To this end, LIN bus you need to set short-circuit fail-safe mechanism when the bus line, that is, each node must have the ability to distinguish between short-circuited bus lines, and make the appropriate protective measures for different systems. For example identifying when a node fails short, LIN bus, the electronic controller can turn off themselves.
For this fault case, my analysis, air humidity sensor and the control unit of the sliding roofAt the same node, when the inlet air humidity sensor short circuit protection LIN bus started, close the communication LIN bus at the node, causing the sliding roof control unit can not receive the control unit for the sunroof system comfortable operation permission signal, resulting in skylight malfunction.
troubleshooting: After replacing air humidity sensor G355 test, sunroof function returned to normal, troubleshooting.
Figure 5
reviewed: asking the user to know the car window film done in non-professional beauty and decorating shops, the air humidity sensor mounted on the windshield (FIG. 5), the construction process is likely to result in water damage to the sensor. After I encountered a few cases of the same case, because the film is all the process leading to the water due to air humidity sensor, therefore, to film the construction of the car, should pay particular attention to air humidity sensor, avoid water damage.
for new and more complex system failure, maintenance personnel must carefully analyze their works, from simple to complex to be checked. Maintenance ideas should be clear and should not rush to change parts operations.
(Zhouyu Fu)