What a sensor actually reports
A sensor converts a physical condition into an electrical quantity a module can measure. The same condition can be reported in different ways depending on design, so the wiring and the expected signal must be read together. The tabs compare the three broad families.
Analog voltage and resistance
A coolant temperature sensor is often an NTC thermistor: its resistance falls as temperature rises, and the module reads the changing voltage across it. Throttle pedal position sensors commonly use a 5 V reference and return a proportional voltage, often as two independent signals so the module can cross-check them. Manifold pressure sensors frequently work the same way.
Frequency and pulse signals
Crankshaft, camshaft and wheel speed sensors detect a toothed target. Inductive sensors generate their own alternating voltage that grows with speed; Hall-effect and magnetoresistive sensors need a supply and produce clean square pulses, even at very low speed. The module counts pulses and their timing rather than reading a level.
Digital and smart sensors
Some sensors contain their own electronics and send encoded data. SENT, standardised as SAE J2716, transmits values as timed pulses on a single wire. Others sit directly on a LIN or CAN network. These sensors can report their own faults, but a basic voltmeter can no longer show the reading.
The path between sensor and module
Most sensor circuits have three conductors: a reference or supply, a signal and a sensor ground. The module watches the signal for values that make physical sense. When a voltage sits outside the expected window, such as near 0 V or near the full reference, many modules set a “circuit low” or “circuit high” code. These codes describe the electrical evidence at the module pin, which includes the wiring and connectors as well as the sensor.
A value that is in range but implausible, such as an engine temperature that never rises, raises a different kind of code. Here the circuit may be fine and the sensor, or the condition it measures, deserves attention.
Vehicle networks: sharing what is known
Bosch introduced the Controller Area Network in 1986 to cut wiring by letting modules share data on a two-wire bus. CAN is now defined in ISO 11898. Messages carry an identifier rather than an address; every module hears every frame and keeps the ones it needs. The two wires, CAN High and CAN Low, are twisted together and carry opposite voltage changes, which helps reject electrical noise. A 120-ohm resistor terminates each end of a typical high-speed bus, so with power off many technicians expect around 60 ohms between the two lines.
Not every function needs that speed. LIN (ISO 17987) is a single-wire, lower-cost network often used for window switches, mirrors and climate flaps, usually controlled by one master module. FlexRay (ISO 17458) offers deterministic timing for chassis systems on some vehicles. Automotive Ethernet, such as IEEE 802.3bw 100BASE-T1, carries camera and infotainment data. A gateway module typically joins these networks and controls which messages may cross between them.
| Network | Physical layer | Typical uses | Standard |
|---|---|---|---|
| CAN / CAN FD | Twisted pair, differential | Engine, transmission, braking, body | ISO 11898 |
| LIN | Single wire | Switches, mirrors, seats, sensors | ISO 17987 |
| FlexRay | Twisted pair, time-triggered | Chassis and active suspension on some models | ISO 17458 |
| Automotive Ethernet | Single twisted pair | Cameras, displays, driver assistance | IEEE 802.3bw |
One signal, many destinations
- A wheel speed sensor produces pulses as the tone ring turns.
- The ABS module counts the pulses and calculates each wheel’s speed.
- It broadcasts the values on CAN for any module that needs them.
- The instrument cluster, transmission and stability systems each use the same message.
- If the message stops, several systems may warn at once, even though one sensor started it.