Case 1
Failure phenomenon: a 2009 BMW E71 X6 SUV, configure the 3.0 L N54 engine, 6 300 km mileage. Say users, normal driving of the vehicle suddenly weak acceleration phenomenon, suggesting engine power meter to be lowered, the red coolant temperature warning lamp. Moreover, the vehicle breaks ago, did maintenance at the repair shop.
Check Analysis: towed vehicle repair shop, check the coolant does not lack. Connection failure diagnostic fault records in vehicle testing, DME fan motor storage system, then maintenance personnel radiator fan motor is determined faulty, causing the engine temperature is too high, so that the engine power is lowered. That the fan motor from the maintenance information is 600 W high-power motor, rotating blades of the fan motor is not catching phenomenon, the current temperature is not very high, fan cooling load is not large, but very little mileage car the fan motor should not faulty. Replace the same type of fan motor, direct-drive fan motor diagnostic use, the results of the motor is not running, it seems not a fan motor problem.
FIG 1
Electrical Maintenance BMW cars higher degree of dependence on the diagnostic apparatus, so in accordance with BMW maintenance process diagnostic, the diagnostic use the maintenance and inspection plan for this failure was progressive analysis and judgment, the result was considered a fan motor failure or line failure. The fan control circuit diagram (FIG. 1), the fan can be seen that if the motor does not turn, in addition to motor failure itself, also need to check fan relay, fuse fan passenger side of the fuse box (60 A), the rear portion of the fuse box fan relay control signal in the fuse (5 a) and the ground line control and other related engine control unit. Check the rear portion of the fuse box to 5 A and fuse the normal, measured with a No. 1 terminal of the fan relay 12 V power supply, but did not relay X17278 12 V power pin, and X17278 at 30 is normally provided by the regular power source 12 V supply. Then check the passenger side of the fuse box 60 A fuse, but not found in the 60 A fuse in the fuse box. This is strange! X16817 circuit diagram can be seen from the passenger side of the fuse box foot is indeed the fan power supply line, seeTo the problem is inside the fuse box. Removing the fuse box and disintegration, found inside the fuse box has four high-power fuses are connected in parallel (FIG. 2), wherein there is looking for 60 A fuse. Measuring the other three fuses are normally turned on, and both ends of the disconnection of the fuse 60 A seems to find the point of failure.
FIG. 2
Troubleshooting: power since the fuse can not be replaced individually, so the replacement of the passenger side of the fuse box assembly, commissioning fan is running .
Review Summary: Although the fault has been ruled out, but there are some questions about the author. Since the design of this high-power fuses and fuse boxes together, its reliability should be high, so what is the reason the fuse open is it? After communication with service personnel to know, before to help troubleshoot electronic fan failure of another car, another car once the fan is installed in this car, and then use the diagnostic test repeatedly drive through the fan, and finally found internal fan of another vehicle is indeed short while after the car properly maintained vehicles shipped soon appeared high temperature fault. Thus, the car 60 A high power fuse should an internal short circuit due to the fan and another vehicle fuse.
Case 2
Failure phenomenon: a 2006 annual BMW E60 520i sedan, equipped with M54-type 6-cylinder engine, mileage 166 000 km. According to reflect the user, the normal running of the vehicle on the coolant temperature warning lamp suddenly lit meter, coolant temperature information presentation center of the display.
Figure 3
Check Analysis: First connection failure diagnostic apparatus for vehicle inspection, and found no high temperature engine cooling system failure. Check the coolant level within the normal range, the fan is operated by the air conditioning or a diagnostic test function. Situ idle operation of the vehicle about 1 h, and does not reflect the user’s fault phenomenon observed coolant temperature data is normal (FIG. 3), engine coolant temperature of 105 ℃, coolant temperature at the radiator outlet is 63 ℃, open the engine hatch can see the occasional fan motor at low speed. Road test, the high speed of several tens of kilometers and no coolant temperature alarm occurs. Given the vehicle has traveled 160,000km, so the maintenance staff based on experience are recommended to replace the thermostat, water pump and coolant, but found a few months ago replaced the thermostat characteristic curve type queries car maintenance records, so maintenance personnel will replace the water pump and Coolant.
Figure 4
The next day, user complaints stop coolant temperature warning is not resolved . Communication with users again, maintenance personnel found some failure of the law: the number of occurrences of the higher frequency of failure in city traffic, the general running for some distance after stopping (eg lights, etc.) are more often occurring in normal driving very little. Thus maintenance personnel connected fault diagnostic road test, in order to capture some of the key data when a fault occurs. For stop and go to 40 ~ 60 km h speed / road test, and deliberately raising the engine speed, about half an hour after the road test, diagnostic data is observed to some abnormality occurs (FIG. 4), a radiator outlet coolant temperature quickly drops suddenly, the engine coolant temperature has rapidly increased, reaches a critical point of the alarm (the engine coolant temperature reaches or exceeds 115 deg.] C, the engine control unit performs alarm). Then immediately stopped, the vehicle has not stopped completely meter coolant temperature warning light will be illuminated, the display shows the center-temperature information presentation. Here, observation of the coolant temperature continue to change rapidly, the engine coolant temperature reaches 117 deg.] C, the radiator outlet coolant temperature quickly rose to 76.5 deg.] C, but the meter display and alarm soon disappear naturally, coolant temperature the data also immediately returned to normal.
5
This phenomenon shows what? Description On the thermostat time lag. The thermostat is not completely not open, otherwise it will continue to high temperature, it will store records related failures. But the car has just replaced the thermostat, and it is prone to failure only in certain conditions, which is worth pondering. Conventional wax-type thermostat controlled coolant temperature, coolant realize small loops when the thermostat closes, the cooling fluid to achieve a cycle time of the thermostat opens, the BMW 520i is used in subsection characteristic type thermostat (FIG. 5), the thermostat is mounted in the cooling of the engineFluid entrance to the outlet of the radiator, engine coolant temperature sensor is installed at the outlet of the engine cooling liquid, i.e., the inlet of the radiator. Internal thermostat mounted heating element, the engine control unit controls the heating element in accordance with the stored characteristic curve and the actual operating conditions, characteristic curve parameters include engine speed, intake air temperature, and coolant temperature. In this intelligent control, the engine can remain at the optimum temperature. Further, when the engine control unit with reference to other parameters for control intervention, will be able to open the thermostat (80 ~ 103 ℃) at lower coolant temperature; when the coolant temperature reaches 110 deg.] C, without thermostat other control intervention will begin to open; If the coolant temperature exceeds 115 deg.] C, the display shows the engine control unit activates the heating element directly to the thermostat, and the center temperature warning instrument. Whereby the data in the diagnostic analysis, engine coolant temperature is 114.75 deg.] C, the coolant temperature at the radiator outlet 51 ℃, the thermostat should be opened at this time but does not actually open. When the engine coolant temperature was raised to 117 deg.] C, until the thermostat begins to open, but this time has a high temperature alarm limits. Once the thermostat starts to open, the coolant begins a cycle, the radiator outlet coolant temperature is increased to reach 76.5 deg.] C, and the engine coolant temperature falls within a lower temperature alarm limits, the high temperature warning release.
Solution: The above analysis, replace the thermostat again after the road test, the failure does not appear again.
reviewed: if the cooling system than other parts faulty thermostat, it is not only the instantaneous high temperature, and there will be a specific fault record store, will not show more general engine temperature .
Summary: For fault engine cooling system, the pump comprising a common point of failure, the thermostat and a cooling fan, of course, routine examination basis is essential. After determining the possibility for Case 1, although the fault diagnostic record is clear indication of failure of the electric fan, but took into account various conditions of a fan failure is not large, but the end result also proved that analysis is correct . So whatever the model, if we can understand the characteristics of the cooling system and the failure of the working process, and then use diagnostic equipment to read the data stream analysis to help determine, you can quickly solve the problem.For Case 2, because the initial examination did not understand the characteristics of failure, coupled with the maintenance file recorded in the car once the holidays to replace the thermostat, so without careful analysis directly replace the water pump and coolant, can not solve the problem, of course, finally clearly the failure of the law to find exactly the point of failure.
(Zhougui Ming)