Analysis of Principles of Waveform Faults in Automotive Engine Control Valves

1. Idle speed control (IAC) Solenoid valve idle speed controller is designed in many mechanical and electromechanical devices. In the electronically controlled engine management system, the electronic control replaces the old-fashioned thermostat plug, paraffin wax, vacuum tank type and similar devices. s position. The electronically controlled idle solenoid valve can prevent idle stall and increase the stability of the idle speed. When decelerating, the use of step-down deceleration means reduces the emission of hydrocarbon HC. The idle solenoid valve can keep the idle speed as low as possible without extinguishing it. Even when the auxiliary equipment such as the air conditioner, generator, and power steering is turned on to increase the load on the engine, it can maintain the smoothness.

Some idle controllers are solenoid valves (most Ford cars) and some are rotary motors (Bosch BOSCH in Europe), others are gear-reduction DC stepper motors (most suitable, Chrysler cars), but all examples are controlled by The computer changes the signal amplitude or pulse width to ultimately control the idling operation.

The idle speed is detected by the engine speed sensor, and the idle speed is adjusted within a predetermined program-set hold period. The control computer receives digital switch input signals from accessories such as air conditioner compressor switches, battery charge indications, power steering switches, and neutral/parking switches. Each switch triggers a predetermined set of idle speed compensation commands and transmits them to In the idle controller, the idle speed control system usually starts to work when the throttle is closed to the engine speed below the minimum value or the vehicle speed is stable. The electronically controlled idle valve allows air to bypass the throttle to produce an effect similar to opening the throttle.

experiment method:

Allow the engine to idle and turn the accessory (air conditioner, fan, wiper) on or off. If the vehicle is equipped with an automatic transmission, switch between parking and forward gear. This will change the engine load and cause the engine control computer to change the output command signal to the idle valve.

Waveform result:

Verify that the decisive scales such as amplitude, frequency, shape, and pulse width of various idle compensation mode waveforms are within the correct range, with repeatability and consistency.

Confirm that when the command signal of the control computer changes, the solenoid valve reacts, and the engine speed also changes, observe the following conditions:

When the auxiliary electrical equipment switch is opened or closed or the transmission is in gear, the control computer's idle speed control output command will be changed. When the idle speed changes, the idle speed control valve shall open and close the throttle bypass hole. If the idle speed does not change, it shall be suspected that the idle speed valve is damaged or the bypass channel is blocked.

Before diagnosing the idle speed control valve and control circuit, make sure to check the vacuum leaks or unsuitable air leaks that will cause problems with the idle speed control system.

2. Mixed gas control valve General Electric's gas mixture control valve is widely used. Other feedback carburetor gas mixture control waveforms are slightly different in appearance, but their signals show the same criteria and appear in a very similar pattern. In the carburetor fuel feedback control system, the mixture control signal is the most important output signal. At General Motors, this circuit has about 10 pulses per second and changes in each individual pulse (pulse width or open time). It depends on the needs of the fuel mixture.

In General Motors, this circuit controls the time for each pulse of the main jet needle valve in the low position (lean position) of the carburetor. In other systems, the air-fuel mixture control circuit controls the air solenoid valve, and when the moving air enters the main orifice passage or enters the conduit, the air-fuel mixture is diluted. Most feedback carburetor systems work in the same way, and longer mixed gas control on-time means issued diluted mixture commands.

In general, control commands from the engine control computer fluctuate around a duty cycle greater than 50%.

This means that the system is controlled to compensate for long-term rich conditions (eg, carburetor rich adjustment) in the lean mixture state, whereas the control commands issued from the engine control computer are centered around the duty cycle of less than 50%. Fluctuations mean that the system is controlled in a rich mixture to compensate for long-term lean conditions (such as vacuum leaks).

Before performing this carburetor mixture adjustment procedure, first make sure that the oxygen sensor is working properly.

experiment method:

Start the engine and idle at about 2500 rpm for about 2-3 minutes until the engine is fully warmed up. The fuel feedback control system goes into closed loop. The above process can confirm that the car is in park or neutral according to the electrical equipment. The recommended maintenance procedure is to perform lean stop on the carburetor being serviced, and adjust the air leakage and idle mixing ratio.

Waveform result:

When the main throat volume adjustment or carbon monoxide adjustment is appropriate, the mixed gas control signal duty cycle will fluctuate about 50%, the duty cycle can be checked with the cursor on the oscilloscope or according to the calibration of the waveform display, car oscilloscope can be The value of the duty cycle and the waveform are displayed on the oscilloscope at the same time.

It is not difficult to use the screen to calibrate the waveform. If the carburetor mixing ratio adjustment is set correctly, the peak of the waveform will be concentrated between two falling edges. This spike is caused by the grounding circuit of the engine control computer. The waveform description box in the example observes where the control computer turns on and off the circuit. When the main orifice and idling mix ratio adjustments are set correctly to the spikes slightly from right to left and then fluctuate in return, but stay very close to the middle of the two falling edges in the waveform, the computer is controlled from the left by the oxygen sensor's input signal The waveform signal is displayed to the right.

When the oxygen sensor signal is rich, the control computer will extend the cycle time of each cycle of the mixing ratio control computer (closed angle increase) to compensate and correct. When the oxygen sensor signal is lean, the control computer will control the mixing ratio control circuit for each cycle time. Shorten (closing angle decreases) to perform compensation correction. When the mixing ratio control waveform duty cycle fluctuates around 50% and the oxygen sensor works normally, the system mixture ratio control is normal, and the catalytic converter works best. At idle or at 2500 rpm, or under normal driving (excluding heavy load and acceleration/deceleration), the waveform display should fluctuate around 50%. At this time, the fuel feedback control system has the best performance and the lowest possible exhaust emissions.

If it is not normal under a working condition (for example, idling), the waveform duty cycle fluctuates around 50%, but under other conditions, the waveform shows that the duty cycle often fluctuates around 50%. The maximum mixing ratio compensation (from the lean side to the rich side) can be obtained when needed. When the peak of the waveform moves to the right side, the control computer is instructed to release the lean gas mixture command, which is based on the output of the oxygen sensor. High voltage.

3. Bosch (BOSCH) Frequency Mixture Ratio Regulator The Bosch (BOSCH) CIS system was in the late 1970s. Until now, it is still possible to see a mechanical multi-port fuel injection system installed on a European sedan. The CIS fuel injection system uses frequency valves to maintain proper fuel control under varying driving conditions. All Bosch CIS frequency valve waveforms (except for Mercedes-Benz frequency valves) are similar in shape and operation to those in the example. Mercedes-Benz frequency valves have different drivers and waveforms, but operate in the same way. The frequency valve signal is the most important output signal of the Bosch CIS fuel feedback control system. The pulse frequency of this circuit is as large as 70Hz. This frequency is constant, and its frequency is kept at 70Hz regardless of changes in engine speed and load. However, the on-off time (the closing angle of the pulse) changes with the oxygen sensor's signal to the control computer.

In the CIS system, this pulse circuit changes the oil pressure in the lower pressure chamber of the fuel distributor by controlling the length of time. When the pressure in the lower chamber of the differential pressure valve decreases, more fuel flows through the upper chamber and the fuel is enriched. mixing ratio.

When the duty ratio of the command signal from the control computer to the frequency valve is greater than 50%, it means that the system enters the rich-mix ratio control, which compensates for the long-lean state (eg, dilute carbon monoxide adjustment or vacuum leak). Conversely, when the duty ratio of the command signal from the control computer to the frequency valve is less than 50%, it means that the system enters the lean mixture control, which compensates for the long-term rich condition (eg, rich carbon monoxide adjustment or excessive fuel pressure. for). Before doing this adjustment of the fuel mixture ratio (carbon monoxide), make sure that the oxygen sensor is working properly.