Symptom: a Mercedes Benz s320 luxury sedan, the engine overhaul after all normal factory, but when the car a month after normal driving, but there has been idle instability and less than 600 rpm, open air, steering has flameout phenomenon, and the good times and bad, sometimes not idle.
Maintenance process: First, the transfer of the fault code, because it is an intermittent fault so no fault code appears, judging by past experience to idle speed stepper motor has a problem, but Benz stepper motor and throttle position sensor as a whole, is more advanced under normal circumstances there will be no problem. The car was out of the way to flush the engine when the car wash for a moment then now this situation, thus it can be concluded from the computer to the line of contact between the stepper motor idling bad actions. First with ordinary odometer correction tool push-pull method, that is, the computer idling motor to the line between pushing and pulling performed, then found the engine speed is high and sometimes low, the line break open the jacket and found the line has been aging, loss, causing the input signal confusion conduct line dressing, then test, everything is normal, troubleshooting.
Failure Analysis: When the steering or air conditioning has played an output signal input into the computer, the computer and then send the signal to the acceleration signal according to the idling motor, which was an increase of 100 rpm the engine would not turn off, but when the aging once the water has caused even the electricity, that the computer output signal, transmitted to the line when it hit the aging of local capacity together several signals, causing signal confusion, not receiving proper idling motor speed signal, and not speed, which what caused the engine to increase the negative, so the engine will stall.
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