Engine sensors and signal types

An engine control unit has no senses. Everything it knows about the engine arrives as a voltage, a frequency or a stream of pulses on a handful of wires, and every control decision it makes is only as good as that arriving signal. Understanding sensors is mostly understanding four ways of turning a physical quantity into something a microcontroller can read.

The reference supply and the return

Before any individual sensor, one piece of shared infrastructure. Analogue sensors are fed a regulated 5 V reference from the module, and they return through a dedicated sensor earth back to the same module rather than to the nearest piece of bodywork.

Both halves of that matter:

• The module measures the signal as a fraction of its own 5 V, so if the reference sags, every sensor on it reads wrong together. A shorted sensor that pulls the reference down takes a whole group of readings with it, which is why several unrelated codes can appear at once.
• The module measures the signal relative to its own zero. A sensor earthed locally sits at whatever potential that bracket happens to be, and that difference is added to the reading invisibly. See DC circuits, voltage drop and earthing for why two earth points are rarely at the same potential.

Four kinds of signal

1. Switched. A contact that is open or closed — a brake pedal switch, a clutch switch, an oil pressure switch. The module usually pulls the line up and the switch pulls it down, so an unplugged connector reads as one definite state and a shorted wire as the other. Simple, and almost always duplicated where it matters, which is why brake pedals have two switches wired oppositely.

2. Resistive divider. The sensor is a variable resistance in series with a fixed resistor inside the module, and the module reads the voltage at the junction. This covers thermistors and potentiometers, which between them are most of the analogue sensors on an engine.

• Thermistors — coolant temperature, intake air temperature, fuel temperature, oil temperature. These are negative temperature coefficient: resistance falls as temperature rises, steeply and non-linearly. A typical coolant sensor is around 5,000 Ω at 0 °C and 200 Ω at 90 °C. The module holds a lookup table. The useful consequence is that resolution is highest when cold, which is exactly where fuelling is most sensitive to getting it right.
• Potentiometers — throttle position, accelerator pedal position, EGR valve position. A wiper on a resistive track. These wear, and worn tracks produce dropouts that look like a momentary open circuit.

3. Generated or frequency. The signal is an alternating voltage or a pulse train, and the information is in its frequency rather than its level. Crankshaft and camshaft position, wheel speed, and some flow sensors work this way. Because the level does not carry the message, these signals shrug off voltage drop.

4. Digital. The sensor contains its own conditioning and transmits a coded stream. SENT (SAE J2716) sends the value as a series of pulses whose widths encode nibbles, with a status field and a checksum, one way, on a single wire. PSI5 does something similar on a two-wire current loop. Both are effectively immune to the offsets that corrupt an analogue reading, cost no extra wires, and — because of the checksum — let the module know when a reading is corrupt instead of believing it.

Measuring the air

Two sensors compete, and most engines carry both.

MAP — manifold absolute pressure. A silicon diaphragm with a sealed evacuated chamber behind it, so it reads pressure against a true zero rather than against the atmosphere. Absolute is the whole point: atmospheric pressure changes with altitude and weather, and the engine cares about the density of what it is actually breathing. A MAP-based system infers airflow from pressure, speed and a volumetric efficiency table — the so-called speed-density method.

MAF — mass air flow. A heated element in the intake tract is held at a fixed temperature above ambient, and the current required to do that is measured. Faster-moving air carries away more heat and needs more current. Because the cooling depends on the mass going past rather than the volume, the output is already the quantity the fuelling calculation wants, with no correction for temperature or pressure needed.

	MAP	MAF
Measures	Absolute manifold pressure	Air mass per unit time
Position	On or near the manifold	In the intake duct, upstream
Fails by	Blocked vacuum passage, drift	Contamination of the element, drift low
Unmetered air after it	Not detected as such	Leans the mixture and shows as positive fuel trim
Needs	A volumetric efficiency model	Nothing further

Position — the two sensors the engine cannot run without

Variable reluctance, also called inductive. A coil wound round a permanent magnet, mounted over a toothed wheel. A passing tooth changes the magnetic circuit and induces a voltage in the coil. Two wires, no supply needed. The catch is that induced voltage depends on rate of change, so the output is a couple of volts at cranking speed and tens of volts at high rpm, and at standstill there is nothing at all.

Hall effect. A powered semiconductor that produces an output when a magnetic field crosses it, with a switching circuit that squares it up. Three wires — supply, earth, signal. The output is a clean square wave of constant amplitude at any speed including zero, which is why camshaft sensors are nearly always Hall: the module wants to know where the camshaft is before the engine has turned at all.

The reluctor wheel. Evenly spaced teeth give speed but not position, because every tooth is identical. The standard trick is to remove two of sixty, leaving 60−2. The module finds that gap and counts from it, which gives absolute crankshaft angle.

That still leaves an ambiguity. A four-stroke engine turns twice per cycle, so crank angle alone cannot say whether cylinder one is on compression or exhaust. The camshaft sensor, running at half crank speed, resolves it. This is why an engine will often start and run on the crank sensor alone after a longer cranking period — it has had to guess and then confirm.

Knock

A knock sensor is a piezoelectric crystal bolted to the block that turns vibration into a small alternating voltage. The block is full of vibration, so the raw signal is useless on its own. What makes it work is the processing:

1. The module knows which cylinder has just fired, so it opens a window at the crank angle where knock would appear.
2. It filters for the frequency band in which that bore rings, typically 5–15 kHz depending on bore diameter.
3. It compares the level against a rolling background, so the threshold adapts as the engine ages and as mechanical noise changes.
4. If it hears knock, it retards ignition on that cylinder only, then walks the timing back towards optimum over the following cycles.

This is also why a knock sensor’s mounting torque is specified and matters. It is a vibration path, and changing the clamping changes what the sensor hears.

Range, rationality and why a sensor can be wrong and legal

Every input is checked twice.

• A range check asks whether the voltage is between plausible electrical limits. A signal at 0 V or at 5 V is almost certainly an open or a short, not a real reading.
• A rationality check asks whether the reading agrees with everything else that is known. A coolant sensor reading 40 °C is perfectly in range, but after twenty minutes of running, with a warm intake temperature and the vehicle moving, it is not plausible.

Most of the more interesting fault codes are rationality codes. They are also the ones that take longest to appear, because the module has to satisfy an enabling condition before it can judge.

What to take away

• Analogue sensors share a 5 V reference and a dedicated return, so one shorted sensor can corrupt a whole group of readings.
• Thermistors are non-linear NTC devices with their best resolution when cold.
• MAP measures absolute pressure; MAF measures mass directly by how much heat the air carries away.
• Variable reluctance generates its own signal but gives nothing at zero speed; Hall needs a supply and works down to standstill, which is why cam sensors are Hall.
• A 60−2 reluctor gives absolute crank angle; the cam sensor resolves which of the two revolutions the engine is on.
• Knock detection is a windowed, filtered, adaptive comparison — not a threshold on a raw voltage.
• SENT and PSI5 move the value into the timing between edges, so voltage drop stops mattering.

What the module does with all of this — and how it drives the other direction — is actuators and PWM control.