In 2004, Prima, 1.8L displacement, automatic transmission, Mazda engine management system. The car owner shows that the car has no overspeed gear (at ordinary times, the accelerator pedal is fixed on the back of the O/D button, the speed will rise more than 3000r/min), and the left front glass can not be lifted up and down. The most obvious is that the driver's side door has no lifting function when any door opens, and sometimes there is no reaction to the firing machine, but it is released. After a period of time, it can fire normally, and everything is normal after the fire. The air conditioning panel LCD is not displayed (but the air conditioner works normally). This fault has been repaired at a famous import maintenance plant in our county. It is diagnosed as a transmission failure and needs to change the transmission assembly. However, after the replacement of the transmission, the fault remains the same. The owner of the gearbox returns to the factory to hear the introduction of the Guangzhou transmission maintenance manufacturer to repair our factory. These faults do not seem to be directly related, so we deal with one fault and one fault. As our factory rarely repairs this Mazda's car, it is transferred out of the circuit diagram of the overspeed switch (as shown in Figure 1, the circuit Icon: keep the switch) in the circuit diagram (in the red circle). According to the circuit diagram, the engine / transmission ECM is found at the front and bottom of the cab. According to the maintenance manual, it is confirmed that the switch plug has no circuit breakage on the ground side line to the ground (the resistance to the ground resistance is 0), and the plug control unit side line has no short circuit and circuit breakage on the other parts of the body. There is a 12V voltage to the ground when the next switch is switched. Open the key to the ON and press the switch, the instrument shows the hint of the connection, and then the sign of the instrument disappears, indicating that the control unit receives the signal and passes the signal to the instrument. Turn off the key to measure the resistance on both sides of the switch. When the button is pressed, the meter will stop and stop. When the road test is pressed, the speed rises and falls. Is it a control panel problem? Just the guests have another same car, the two car control board exchange, the fault car symptoms did not improve, as if the control unit board has no problem, the switch line has no problem, the switch itself seems to have no problem. At this point, the maintenance without a train of thought, suddenly, the usual popular name called overspeed switch, maintenance manual called "keep the switch" name caused my attention, since it is to keep the switch, it should be pressed to maintain a certain position, until again pressed to loose! Pull down the key, adjust the multimeter to 200 Omega, test the resistance of the plug at both ends of the switch, and change the resistance from the original infinity to the 0 Omega, then it becomes infinity again. The switch can not be kept! Driving the road test, it is found that as long as the "overspeed switch" has been pressed, the engine will roar at the high speed of the super 3000r/min. It turns out that it is difficult to overspeed the speed. Replace a good keep switch, this problem is solved. The left front glass often can not lift the problem, check the left front engine compartment inside the safety box has burned marks, after the circuit diagram is re connected to the line, the left front glass can be normal rise and fall, turn in the car. The magic is that after a few days, the owner said that the phenomenon of air conditioning LCD was not correctly displayed. For air conditioning LCD display is not correct, we have no maintenance, do not know why it will be good, need to specialize in Mazda's colleagues pointing! After more than one month's tracking, all the faults will not be reproduced and the problem can be solved.