A mileage 2.5L car with a mileage of about 130 thousand km carrying the LB8 engine in 2004, Buick. User reflection: the fuse fan fuse frequently, at the same time the engine trouble light light, in other maintenance plants have carried on multiple maintenance, the maintenance personnel checked the related lines, no short circuit phenomenon, and finally replaced the cooling fan, but still failed to remove the fault. Fault diagnosis: after the test, the main cooling fan on the left and the auxiliary fan on the right are running at low speed, but can not run at high speed; connect the fault detector and read the fault code P0481, which means the cooling fan relay 2 and the cooling fan relay 3 control circuit barrier; check the fuse box in the engine room and find 1 5A heat dissipation fan fuse 2 fuse, 40 A radiator fan fuse 1 normal, 3 heat sink fan relay is normal. After replacing the fuse fuse and removing the fault code, the main cooling fan and the auxiliary cooling fan operate at low speed. When the fan meets the high speed conditions, the auxiliary heat fan starts to run at high speed, but the main cooling fan does not work, then the fuse 2 fuse again, the main cooling fan and the auxiliary heat dissipation wind The fan works at low speed. The cooling fan control circuit of the car consists of 2 cooling fan fuse, 3 heat dissipation fan relay, 2 cooling fan and power system control module (PCM). When the fan is running at low speed, PCM controls its terminal C1-6 iron lap, heat dissipating fan relay 1, cooling fan relay 2 and cooling fan relay 3 not working. At this time the main cooling fan and auxiliary cooling fan are composed of series circuit, together with low speed operation, the specific loop is cooling fan fuse 1 to cooling fan relay 1 Terminal 30 and terminal 87 - main cooling fan terminal B and terminal A - 2 terminal 30 and terminal 87A - auxiliary cooling fan terminal B and terminal A - G117. When the fan is running at high speed, the PCM control its terminal C1-6 and terminal C1-5 all overlap, the cooling fan relay 1, the cooling fan relay 2 and the cooling fan relay 3 are all working. At this time the main cooling fan and the auxiliary cooling fan make up the parallel circuit, which can run at high speed together. The concrete return way is the cooling fan 1 to the heat dissipation fan. Relay 1 terminal 30 and terminal 87 - main cooling fan terminal B and terminal A - 2 terminal 30 and terminal 87 - iron lap G117; heat dissipation fan fuse 2 - cooling fan relay 3 terminal 30 and terminal 87 - auxiliary cooling fan terminal B and terminals, A - tie - point G117. According to the above analysis, the power consumption parts controlled by the radiator fan fuse 2 are cooling fan relay 2, radiator fan relay 3 and auxiliary cooling fan. It is concluded that the fault of the vehicle is not the internal short circuit of the electrical parts, or the short circuit of the relevant peripheral lines. The related peripheral lines are carefully checked and the wire has not been found short circuited by the wire; the replacement of the cooling fan relay 2 and the cooling fan relay 3 after the test, the failure is still. The fuse of the fan, the fan relay and the cooling fan are all replaced, and the related peripheral lines are all normal. Is it short circuited by the inner line of the fuse box in the engine room? Replace the engine compartment fuse box after trial run, radiator fan fuse 2 will still fuse. At this point of the diagnosis, the fault diagnosis has reached a deadlock. After repeated tests, 1 abnormal conditions were found: pulling off the fan relay 3, when the terminal 87 was supplied directly to the relay seat, the main cooling fan began to run at high speed, and the auxiliary cooling fan did not work. Because the cooling fan relay 3 is supplied to the auxiliary cooling fan, the conductor connector that infers the main cooling fan and the auxiliary cooling fan is reversed. Failure to remove the main cooling fan and auxiliary cooling fan wire connector after the installation of the installation of the test, with the engine cooling liquid temperature rises, the main cooling fan and auxiliary cooling fan can run at the same time at the same time, and can also run at the same time, and heat fan fuse 2 no longer fuse, troubleshooting. Failure analysis: How did the conductor connector of the main radiator fan and the auxiliary cooling fan reverse the cause of the vehicle failure? The main heat fan is more powerful than the auxiliary cooling fan. Under normal circumstances, when the fan is running at high speed, the 40 A cooling fan fuse 1 is the main cooling fan, and the 15A fan fuse 2 is supplied to the auxiliary cooling fan; after loading, the 40 A cooling fan fuse 1 is supplied to the auxiliary heat fan fan, and the 15A radiator fan is the main fuse 2. The fan is powered by the cooling fan, because the 15A heat fan fuse 2 cannot withstand the starting current of the main cooling fan, so when the fan is switched from low speed to high speed, the fuse 2 of the 15A fan is fused. It is worth mentioning that the conductor connector of the main fan and the auxiliary cooling fan is reversed in the previous maintenance or maintenance, and then the maintenance personnel connect the fan wire connector according to the idea of "where to disassemble and where" to replace the cooling fan, so the failure can not be excluded. . Through this case, the maintenance staff should note that the fuse will not only fuse because of the short circuit of the wire, but also fuse because it does not match the load.