If you want to understand automotive circuits, you need to understand these 3 parameters!

When detecting automobile circuit-related faults, it is an indispensable step to make necessary measurements.

Of course, if the circuit diagram can be used as an aid throughout the process, it will be even more helpful!

Whether it is a sensor or an actuator, whether it is a control unit or an integrated module, as long as it involves the circuit part, when measuring, it is always necessary to use the three parameters of "voltage", "current" and "resistance".

It can be said that these three parameters are the root of analyzing circuit faults.

Therefore, only when we understand these parameters can we better use them to troubleshoot and repair circuit faults.



Note: In the following content, the specific values ​​mentioned are all based on Buick as an example. Because the value ranges of different brands and models are different, they need to be treated differently.

Voltage

What is voltage?

Voltage refers to the potential difference between two points in a circuit loop. The direction of the voltage is from high potential to low potential, that is, the direction of potential drop.

It should be noted that the voltage in the circuit is caused by the power supply.

In order to facilitate the distinction, the voltage reference direction is introduced in the actual circuit.

The reference direction of the voltage can be selected arbitrarily. The voltage in the circuit with the same reference direction is positive, and the opposite is negative.

The potential of a point in the circuit is relative to the reference point, and the reference point potential is specified as 0.

When the potential of a point in the circuit is greater than 0, the potential of the point is higher than the reference point; when the unit of a point in the circuit is less than 0, the potential of the point is lower than the reference point.

Application By measuring the voltage in the circuit, we can infer whether the power supply, grounding and signal in the circuit are in normal working state, so as to determine the location of the circuit fault.

Of course, there are far more voltages in the automotive circuit than those listed below.

How to judge the normal voltage value in each brand and model requires continuous accumulation in the normal maintenance process. With more measurements, it will not be so mysterious!



What voltages are there in automotive circuits?

Battery rated voltage: 12V

Engine starting voltage: not less than about 10V

Generator voltage: 12.5V~15.5V



Control unit working voltage:

Example: The engine control unit controls the output circuit by controlling the ground or controls the power circuit through the transistor/output driver module device. Provide 5V or 12V voltage to different sensors or switches.

Sensor operating voltage:

example

Crankshaft position sensor/camshaft position sensor: a 5V reference voltage, a low-level reference voltage, a signal voltage

Fuel pressure sensor: a 5V voltage, a ground, a fuel pressure signal

Heated oxygen sensor: When the engine is running, it generates a 0~1V voltage (non-fixed value)

air conditioning pressure sensor: a 5V reference voltage, a low-level reference voltage, a pressure signal voltage that varies between 0.2~4.8V



Actuator operating voltage:

example

Ignition coil module: an ignition voltage, a ground, a low-level reference voltage, an ignition coil control voltage

Injector: The engine control unit uses a 65V voltage (controlled by a boost capacitor) to achieve the initial opening of the injector, and then maintains 12V.

High-pressure fuel pump: The engine control unit provides a 12V pulse width modulation signal to close/open the control valve at a specific time to adjust the fuel pressure.



Current

What is current?

Electric charges move in a directional manner under the action of electric field forces to form current. It is customary to define the direction of movement of positive charges as the direction of current.

In actual circuits, the reference direction is often used to specify the current direction. That is, the direction consistent with the reference direction is positive, and the opposite direction is negative. The reference direction can be selected arbitrarily, and once selected, it cannot be changed in the entire circuit.

It should be noted that when there is no voltage in the circuit, there is no current in the circuit.

Application In actual maintenance, the most widely used is to infer whether the vehicle has leakage by measuring the current in the circuit. And further determine the fault location based on the change in the current value.

What currents are there in the car circuit?

Battery current: The battery current sensor is used to monitor the current state of the battery, and then judge the battery life and charging state.

Static current: Generally refers to the current when there is no signal input, that is, the current after the vehicle turns off the key switch and the electrical equipment is turned off. Generally speaking, the static current of the vehicle is about 50mA, or 0.05A. When it is greater than this value, it may cause the vehicle to leak when it is placed for a long time.



Sleeping current: The current when the vehicle is not operated for a period of time after the engine is turned off and the car key is turned off (i.e. entering sleep mode). The existence of this current is to maintain the memory function of the data in the control unit, so that it can enter the working state at any time when it is activated next time.

Working current: refers to the current of the vehicle under normal working conditions, and the current must not exceed the fuse current of the fuse in the wiring harness of the circuit. For example, the maximum working current of the fuel pump is 15A.

Resistance

What is resistance?

Resistance is a physical quantity used to describe the conductivity of an object. It has the function of hindering the flow of current. Any object has resistance. The resistance mentioned in daily life is mostly conductor resistance, that is, there is resistance in the wire and resistance elements.

Of course, the smaller the resistance in the wire, the better, which can reduce unnecessary energy loss. Because the greater the resistance, the greater the energy generated (manifested as heating of the wire). This is also the root cause of the thermal effect of current.

Application The measurement of resistance can not only be used to determine the on-off of the wiring harness, but also to measure whether there is an abnormality in the coil of the electronic component. The specific location of the circuit fault is determined according to the value of the resistance.

What are the resistors in the automotive circuit?

Wire resistance, resistance between module terminals, sensor resistance, actuator resistance, etc.

The measured value of resistance generally has three states: infinity (indicated by 1 or OL), specific numbers, and 0.

If the resistance value is infinite (1), it means that the measured parts are in an open circuit state, that is, they are not connected to each other. If

the resistance value is a specific number, it indicates the resistance between the measured parts. It should be noted that if the value exceeds the normal range, it means that there is a circuit fault in the measured part.

If the resistance value is 0, it means that there is a short circuit between the measured parts or the selected measurement range is small, or it may be caused by the needle not making good contact.