Four Major Causes of ECU Damage

When you open the aluminum alloy case of the ECU, you will see a PCB (a multilayer epoxy resin board) 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, as well as connectors for wiring harnesses, etc. 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); compared to industrial control electronic products, it does not have the interference issues caused by the power grid.

The chips, diodes, transistors, and field-effect transistors other than resistors and capacitors are semiconductor devices, and the possible reasons for their failure include:

1. Components may have issues from the factory, which could manifest in a short usage time. However, given the current level of semiconductor manufacturing, this probability is quite low, and we can consider it negligible.

2. Components reach the end of their lifespan. It is common knowledge that electronic components, even within their normal operating range, have their lifespan significantly reduced with increased temperature. When the operating temperature exceeds the normal range, some devices, such as MOSFETs, may have increased internal resistance, which in turn causes further temperature increases, creating a vicious cycle.

Thermal damage is fatal, and electronic engineers put a lot of effort into addressing this in their designs. For us maintenance personnel, we can track circuit board faults through abnormally hot components. Abnormality means it differs from the norm, so we need to pay attention to the working temperature of the entire circuit board or a specific component to determine what is normal and what is abnormal when issues arise, such as the temperature of the ECU casing, which must consider the effects of oil cooling for oil-cooled ECUs.

3. Voltage breakdown. Every electronic component has a specific operating voltage range and withstands breakdown voltage. When the voltage exceeds a certain threshold, previously non-conductive areas of the component become conductive. High voltage in the ECU’s internal circuit usually comes from external sources via wiring harness connectors. A well-designed ECU will have protective circuits against these potential hazards, but circuits without such designs are almost impractical. However, product design typically considers cost, so the voltage withstand rating is designed to cover most situations rather than being foolproof.

Possible causes of ECU voltage breakdown include:

① Surge and spike voltages caused by electric welding;

② Excessive battery voltage caused by connecting too many batteries during difficult starts;

③ Static electricity, especially human static during maintenance, comes from us, the maintenance personnel. In dry weather, friction between synthetic fiber clothing and the body generates static electricity; shoes are dry and insulated from the ground, causing charges to accumulate on us, creating a large capacitor between our body and the ground. As the charge increases, the voltage between our body and the ground also rises. When we directly touch or use measuring tools on the wiring harness or pins, these lead wires can apply several thousand volts of high voltage to the internal components of the ECU.

To avoid this harm, it is advisable to develop the habit of touching a metal tool to the chassis or ECU ground wire before contacting the ECU circuit to discharge static electricity.

④ Short circuits between wiring harness wires can cause pins with lower working voltage to become abnormally high. The maximum operating voltage of the entire circuit is not the battery voltage but rather the pulse voltage from injector drive outputs and metering valves. The observed and theoretical waveform voltage may not be very high, but during the switching on and off moments, the spike voltage caused by distributed inductance may exceed the withstand voltage of some pins.

4. Current burnout. We know that there is current and resistance, which generates heat; power is the square of current multiplied by resistance. If, for some reason, the current becomes abnormally high, it may cause certain components or circuits to overheat severely and ultimately burn out, including the copper-clad layers of the PCB and the contact conductive pin connections. The result of high current burnout is often that previously conductive areas are melted and broken, allowing us to distinguish whether the circuit has experienced voltage breakdown or current burnout; this is not strict, just more probable. The main causes of current burnout are usually short circuits between wiring harness wires or incorrect wiring by humans.

Another type of component is the electrolytic capacitor, which is slightly larger in size 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, reducing capacitance and making it prone to damage, especially when operating in high-temperature environments for extended periods. The probability of this happening is quite high for electrical circuits that have been in use for a long time; technicians repairing household appliances should be familiar with this.