The engine management in a car consists of a control unit, sensors, and actuators. The sensors provide information to the control unit. The actuators are controlled by the control unit to perform actions. With a good diagnostic device, you can view the information coming into and going out of the control unit in real-time.
In OBD, we call this information live data. Brand-specific testers often have their own name for it, for example:
- Measurement value blocks (Volkswagen).
- Istwerte (Mercedes).
In both cases, it concerns the same data.
In OBD, we find this data under Mode 01. Also, for control units in the vehicle that do not fall under OBD, we can read live data with a good system tester.
Sensor data
The information coming into the control unit via the sensors is called sensor data or live data. If a fault occurs in a system, this sensor data can help us locate the fault.
| Fault in the ABS system (Rear left speed - Electrical fault in the power circuit) |
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We have a current fault code for a wheel sensor in our ABS computer, but we do not know which sensor it concerns. By selecting the four wheel sensors in the live data list and rolling the vehicle forward a short distance, we can see which sensor signal is not received by the control unit.
| Sensor data ABS sensors |
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| Sensor data ABS sensor with a fault |
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Actuator data
The live data list also contains actuator data. These are the data sent out by the computer.
For example, in an air conditioning system, you can see when the air conditioning pump is being controlled. If you see that the air conditioning pump is being controlled but there is no voltage at the output of the computer, then you know that the final stage in the computer is broken and you need to replace the computer.
| Control of air conditioning compressor |
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