When you open the aluminum alloy casing of the ECU, you will see a PCB (multi-layered epoxy resin board) that is soldered with various ICs (integrated circuits or commonly known as chips), resistors and capacitors the size of sesame seeds, power MOSFETs (field-effect transistors) and power diodes and transistors closely attached to the aluminum casing, as well as connectors for the wiring harness, and more. The ECU computer box is a purely electronic product, and compared to household electronic products, it operates in a much harsher environment (high temperature – vibration – weather); however, compared to industrial control electronic products, it does not have the interference problems caused by the power grid.
Besides resistors and capacitors, the chips, diodes, and transistors are semiconductor devices, and the possible causes of damage include:
1. Component issues from the factory that may manifest within a short usage time, but given the current manufacturing standards for semiconductor components, this probability is quite small, and we can consider it negligible;
2. Component lifespan expiration. It is a common understanding that even within the normal operating range, increased temperature significantly reduces the lifespan of electronic components. When exceeding the normal operating temperature range, some components, such as MOSFETs, may experience increased internal resistance, which in turn raises the temperature further, creating a vicious cycle.
Thermal damage is fatal, and electronic engineers put a lot of effort into this aspect during design. For us repairers, we can trace circuit board faults through components that are abnormally hot. Abnormal means different from normal, so we need to pay attention to measuring the operating temperature of the circuit board as a whole or of specific components to determine what is normal and what is abnormal when problems arise, such as the temperature of the ECU casing, especially considering the impact of oil cooling for oil-cooled ECUs.
3. Voltage breakdown. Any electronic component has a specific operating voltage range and withstand voltage. When the voltage reaches a certain level, previously non-conductive parts of the component can become conductive. High voltage in the ECU circuits typically comes from external sources, via the wiring harness connectors. A well-designed ECU will incorporate safeguards against these potential hazards, including over-voltage protection circuits, while circuits without such designs are almost impractical. However, product design usually considers cost, so the voltage withstand specifications cover the vast majority of situations but are not foolproof.
Potential causes of ECU voltage breakdown include:
① Surge and spike voltages caused by welding;
② Excessive battery voltage due to series connection of multiple batteries during difficult starts;
③ Static electricity, especially human static during repairs, which comes from us repairers. In dry weather, friction between synthetic clothing and the body generates static electricity, and shoes are dry and insulated from the ground, causing charges to accumulate on our bodies. As the charge increases, the voltage between our body and the ground also rises. When testing circuits, if the ECU ground is not insulated from the ground, touching the wiring harness or pins directly with our hands or through measuring tools can apply several thousand volts of high voltage to the internal components of the ECU.
To avoid such hazards, it is advisable to develop the habit of discharging static electricity by touching a metal tool to the chassis or ECU ground before handling the ECU circuit.
④ Short circuits between wiring harness wires can cause pins with lower working voltages to become abnormally high. The maximum operating voltage of the entire circuit is not the battery voltage but the pulse voltage from injector drive outputs and metering valves. The observed and theoretical waveform voltages may not be very high, but during the switching on and off moments, spike voltages caused by distributed inductance can exceed the withstand voltage of some pins.
4. Current burn-out. We know that current and resistance will generate heat, and power is the square of the current multiplied by the resistance. If the current becomes abnormally high for some reason, it can cause certain components or circuits to overheat severely, leading to burnout, including the copper layer of the PCB and conductive pin connection pieces. The result of a high current burn-out is often that previously conductive parts are burned open, which can help distinguish whether the circuit has suffered a voltage breakdown or a current burn-out; this is not strict but has a higher probability. The main causes of current burn-out are usually short circuits between wiring harness wires or incorrect wiring by humans.
Another component is the electrolytic capacitor, which is slightly larger and has a larger capacitance. Currently, there are no other components to replace it, and it is mainly used for power filtering. This type of component contains electrolyte, which can dry out over time, leading to decreased capacitance and easier damage, especially when operating in high-temperature environments. The probability is high for circuits that have been in use for a long time, and repair technicians for household appliances should be aware of this.