Analog · digital · Hall · VR
Automotive sensor wiring: analog, digital, Hall and VR
Every sensor on an engine falls into one of four groups, and each group is wired differently. Knowing which one you have tells you how many wires to expect, what voltage to measure and which ECU input it needs.
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The four types
| Type | Wires | Signal | Examples |
|---|---|---|---|
| Analog, 3-wire | +5 V, signal, sensor ground | 0–5 V proportional to the value | TPS, MAP, pedal |
| Analog, 2-wire (thermistor) | Signal, sensor ground | Resistance; the ECU’s pull-up turns it into 0–5 V | Coolant and air temperature |
| Switch (digital) | Signal, ground (or 1 wire, case ground) | On or off | Oil pressure switch, neutral, clutch |
| Hall effect | Supply (5 or 12 V), signal, ground | Square wave 0 / 5 V or 0 / 12 V | Crank, cam, speed |
| Variable reluctance (VR) | Two signal wires + shield | AC sine wave, grows with speed | Crank, ABS, cam on older engines |
What the ECU input must match
- Analog sensors go to analog inputs; the ECU reads them against sensor ground.
- Many Hall sensors have an open-collector output: they only pull the signal to ground, and a pull-up resistor (in the ECU or external) makes the high level.
- VR sensors need a VR input that can read an AC signal; a Hall input set up for a VR sensor sees nothing reliable.
- Switches need either a pull-up (switch to ground) or a pull-down (switch to +), set in the ECU.
How to tell Hall from VR
Two wires plus a shield and a coil resistance of a few hundred to about a thousand ohms: VR. Three wires with power present on one of them at ignition on: Hall. When in doubt check the part number — the connector shape does not tell you.
Try them in WireForge
- Open the “ECU fuel injection” demo or place an ECU module.
- Add a Throttle Position Sensor (3-wire analog), a Coolant Temp Sensor (2-wire) and a Crankshaft Position Sensor (signal, ground and shield).
- Wire each to the matching ECU pins: +5 V, sensor ground and inputs.
- Simulate and move the sliders: click the signal wires to read their voltage; set Engine speed to see the ECU react to the crank signal.
Questions
Can I replace a VR crank sensor with a Hall one?
Only if the ECU input can be switched to Hall and the trigger wheel suits the sensor. It also needs a supply wire the VR sensor did not have.
Why does a temperature sensor have no 5 V wire?
The 5 V is inside the ECU: a pull-up resistor feeds the sensor through the signal wire, and the voltage left on it shows the temperature.
Do digital sensors need a shield?
Hall sensors usually do not; VR sensors do, because their small AC signal picks up ignition noise.