What Does Positive Ssa And Negative Ssb Mean

By Bella Sungkawa | Published: | Updated: | Automotive | 2 min read

Short Answer

Positive Ssa and negative Ssb refer to the complementary Signal A and Signal B outputs of many automotive sensors. The pair provides a differential signal that allows vehicle control modules to calculate speed, direction, and filter out electrical noise.

Complete Explanation

In automotive electronic systems, “Ssa” and “Ssb” are abbreviations for Signal A and Signal B, the two complementary outputs that many speed, position, and rotation sensors provide. A “positive Ssa” refers to the high‑level (logic 1) portion of the differential pair, while a “negative Ssb” refers to the low‑level (logic 0) portion that is 180° out of phase. By comparing the timing and polarity of the two signals, the vehicle’s control modules can determine direction of rotation, calculate speed, and filter out electrical noise.

  • Signal A (Ssa):
    Typically the reference or “positive” pulse of a two‑channel sensor, used as the leading edge for timing calculations.
  • Signal B (Ssb):
    Usually the complementary “negative” pulse that follows Ssa by a fixed interval, allowing the module to confirm the pulse and reject spurious glitches.
  • Differential Measurement:
    Because the two signals are opposite in polarity, the controller can subtract one from the other, improving immunity to electromagnetic interference.
  • Direction Determination:
    When Ssa leads Ssb, the sensor is rotating in the forward direction; when Ssb leads Ssa, the rotation is reverse.
  • Common Applications:
    Vehicle speed sensors (VSS), crankshaft position sensors (CKP), camshaft position sensors (CMP), and steering angle sensors.

Common Misconceptions

Myth

Positive Ssa and negative Ssb are separate sensors.

Fact

They are two outputs of the same sensor, forming a complementary pair.

Myth

Only the “positive” signal carries useful data.

Fact

Both signals are required for accurate speed, direction, and noise‑cancelling calculations.

FAQ

Why do sensors use two signals instead of one?

Using a complementary pair (Ssa and Ssb) creates a differential signal that cancels common‑mode noise, improves signal integrity, and allows the controller to determine rotation direction.

Can a sensor work if only Ssa is present?

In most OEM designs the sensor will not function correctly without both signals, as the control module relies on the phase relationship to calculate speed and direction.

How can I test Ssa and Ssb signals with a multimeter?

Set the multimeter to measure voltage while the wheel or engine rotates; you should see alternating high and low voltages on the two pins, with Ssb lagging Ssa by a fixed interval (typically 90‑180 degrees).

References

  1. SAE International, ‘Sensor Signal Standards for Vehicle Networks’, 2020.
  2. Bosch Automotive Handbook, 12th Edition, Chapter 6: Speed and Position Sensors.
  3. ISO 11898‑1:2015, ‘Road vehicles – Controller area network (CAN) – Part 1: Data link layer and physical signalling’.
  4. Freescale Semiconductor Application Note AN4475: ‘Understanding Dual‑Channel Sensor Outputs’.
  5. National Highway Traffic Safety Administration (NHTSA), OBD‑II Diagnostic Trouble Code Guide, 2021.

Related Terms

Hall Effect Sensor
A magnetic sensor that generates voltage proportional to magnetic field strength, commonly used for Ssa/Ssb outputs.
Vehicle Speed Sensor (VSS)
A sensor that provides speed data to the vehicle’s control modules, typically using Ssa and Ssb signals.
Crankshaft Position Sensor
A sensor that monitors the rotational position of the crankshaft, often delivering complementary signals for precise timing.

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