Tips for Checking CAN, Power, Ground, and Feedback Signals on a Motor Controller

When I troubleshoot a motor controller, I check the signals in a defined order: power and ground first, CAN communication second, and feedback signals only after the controller has a stable electrical foundation. I use a wiring diagram, the controller pinout, a suitable multimeter, and—when communication or intermittent faults are involved—an oscilloscope or CAN analyzer. This sequence helps separate supply faults, grounding problems, network faults, sensor issues, and controller faults without replacing parts unnecessarily.

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For safety, I isolate the machine according to the equipment manufacturer’s procedure before probing high-current circuits. I also confirm whether the system uses 12 V, 24 V, or another nominal voltage, because the correct test limits depend on the application. The values below are practical reference points, not substitutes for the motor controller’s approved specifications.

What I Check Before Measuring Any Signal

I begin with the electrical documentation rather than the test meter. The wiring diagram should identify battery or DC input terminals, control grounds, chassis ground, CAN-H, CAN-L, sensor supplies, feedback inputs, shielding, fuses, and service disconnects. I also record the fault code, operating condition, motor state, and whether the problem appears during startup, load changes, vibration, or temperature changes.

I then inspect connectors and harnesses for corrosion, loose terminals, pushed-back pins, damaged insulation, water ingress, and incorrect repairs. A signal can appear normal at the controller connector while failing several centimeters away because of a broken conductor or high-resistance crimp. I prefer to test both at the controller and at the remote device when the circuit length or environment makes voltage drop possible.

Tools and Test Conditions

• Use a calibrated digital multimeter for voltage, resistance, and continuity checks.
• Use a two-channel oscilloscope or CAN analyzer for communication timing, noise, and intermittent faults.
• Use back-probing methods that avoid spreading terminals or damaging insulation.
• Measure under the same operating condition in which the fault occurs.
• Never measure resistance on an energized circuit.

Step-by-Step Tips for Checking Power and Ground

1. Confirm the Supply at the Controller

I measure voltage directly across the controller’s positive and negative supply terminals, not only at the battery. The reading should remain within the controller’s specified operating range during key-on, startup, motor operation, and load changes. A nominal 24 V system, for example, should not be judged by its label alone; I check the actual voltage during the event that produces the fault.

Next, I perform a voltage-drop test while the circuit is energized and carrying its normal load. A cable can show acceptable voltage with no load but lose significant voltage through a damaged fuse holder, loose terminal, undersized conductor, or oxidized connector. I compare the voltage at the source with the voltage at the controller and document the difference in volts under the same test condition.

2. Check Fuses, Switching, and Protection Components

I inspect the main fuse, pre-charge circuit if present, contactor, ignition or enable line, and any auxiliary fuse feeding sensors or communication devices. I do not assume that a visually intact fuse is electrically reliable, because the fuse holder and crimped terminals can also create resistance. I verify that the controller receives both its main supply and any required wake-up, enable, or ignition signal.

3. Verify Ground Integrity

I test the ground path with a voltage-drop measurement while the controller is powered and operating. The test points may include controller ground to battery negative, controller ground to sensor ground, and controller ground to chassis ground, depending on the design. A stable ground reference is essential because a ground offset can make a good sensor appear faulty and can also disturb CAN communication.

I avoid using continuity alone as proof of a good ground. A continuity buzzer may indicate a connection through a very small test current, while the same path may fail when the controller draws current. For reliable evaluation, I inspect the terminal mechanically, check the voltage drop under load, and compare the result with the equipment maker’s limit.

Tips for Checking CAN-H and CAN-L

4. Identify the Correct Network Architecture

Before testing CAN, I confirm the bus topology, node list, baud rate, termination arrangement, and whether another gateway or diagnostic device is connected. A CAN bus commonly uses a differential pair called CAN-H and CAN-L, but the connector location and pin assignment vary by controller and vehicle. I never apply a generic pinout to an unknown harness.

With power removed and the network isolated according to the service procedure, a healthy high-speed CAN network is often designed with two 120 ohm termination resistors in parallel, producing an approximate 60 ohm reading across CAN-H and CAN-L. This is a reference value, not a universal acceptance criterion; additional nodes, service plugs, or a different CAN physical layer can change the expected result. Resistance testing must be performed only when the circuit is de-energized.

5. Measure CAN Voltage and Waveform

With the network powered, I observe CAN-H and CAN-L relative to the approved signal ground, then examine the differential waveform when traffic is present. In many high-speed CAN systems, both lines sit near a recessive common-mode level around 2.5 V, while a dominant bit drives CAN-H higher and CAN-L lower. The exact voltage depends on the transceiver, termination, supply, wiring, and measurement method.

A multimeter can help identify an open line, short to ground, or abnormal DC condition, but it cannot reliably show bit timing, ringing, intermittent noise, or bus-off behavior. For those faults, I use an oscilloscope or CAN analyzer to check message activity, error frames, missing acknowledgements, and repeated retransmissions. I compare the observed traffic with the controller’s required message identifiers, update rate, and timeout settings.

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If CAN-H and CAN-L are shorted together, shorted to ground, or shorted to a supply rail, communication may stop or become unstable. I also check shield termination and cable routing where the installation is exposed to inverter switching, high-current motor cables, relays, or contactors. I do not add a termination resistor simply to improve a measurement unless the network design explicitly requires it.

Tips for Checking Feedback Signals

6. Start with the Sensor Supply and Ground

Feedback signals include position, speed, current, temperature, pressure, or other signals used by the motor controller. I first verify the sensor supply and sensor ground at the sensor connector, because a missing reference can affect several feedback channels at the same time. If multiple sensors show unusual values simultaneously, I investigate their shared supply, shared ground, connector, and controller reference circuit before replacing individual sensors.

7. Check Signal Range and Response

I measure the feedback output at rest and while the relevant mechanical input changes. Some analog sensors may use a ratiometric output, while others use a digital pulse, frequency, resolver, Hall, encoder, or serial interface. A 0.5–4.5 V range is commonly used by some analog sensor systems, but I treat it only as an example and follow the sensor and controller specification for the actual acceptable range.

For a position or speed sensor, I look for a smooth, plausible response rather than one isolated voltage reading. An oscilloscope can reveal dropouts, flat spots, excessive noise, missing pulses, or a duty-cycle error that a multimeter averages out. I also rotate or move the mechanism only within its approved range and keep hands clear of moving parts.

8. Compare Feedback with the Controller’s Interpretation

A physically valid sensor output can still be rejected if the controller expects the wrong polarity, scaling, offset, pulse count, direction, or calibration. I compare the raw signal with the value shown in diagnostic software and confirm that the communication data uses the correct units. If the raw signal is correct but the reported value is wrong, I investigate configuration, calibration, signal processing, and software compatibility.

Key Decision Points During Diagnosis

Common Mistakes I Avoid

One common mistake is replacing the motor controller before proving that its supply, ground, communication, and feedback inputs are correct. Another is testing only at rest, even though the fault appears during acceleration, braking, vibration, or thermal expansion. I also avoid piercing sealed automotive connectors with oversized probes, because the damage can create a later intermittent fault.

I do not interpret a single CAN resistance reading without considering the complete network. I also avoid forcing a sensor signal to a guessed voltage, bypassing a safety input, or connecting a test supply without confirming the input circuit’s permitted range. These actions can damage the controller and may create an unsafe operating condition.

How to Improve the Troubleshooting Process

I create a measurement record with the test point, operating condition, measured value, instrument, connector status, and time. For intermittent issues, I capture minimum and maximum voltage, waveform changes, temperature, vibration, and the exact event that causes the fault. This evidence makes it easier for an engineering team or supplier to distinguish a harness problem from a controller design or configuration issue.

I also separate symptoms from root causes. A CAN timeout may result from low controller voltage, a poor ground, a failed node, incorrect baud rate, or actual CAN wiring damage. By checking power and ground before communication, then validating feedback after the network is stable, I reduce the risk of confusing a secondary fault with the primary failure.

How QEXPAND Can Support Motor Controller Projects

At QEXPAND, we support B2B customers evaluating motor controllers and EPS controller applications by reviewing the electrical interface before quotation or production planning. I can help organize the required supply range, ground structure, CAN interface, feedback type, connector definition, mounting conditions, and operating environment into a technical requirement sheet. This early review helps identify missing information before samples or tooling decisions are made.

Our support approach is based on documented requirements rather than unsupported universal specifications. Depending on the project, the discussion may include pinout confirmation, harness compatibility, signal definitions, controller configuration, sample evaluation, and production coordination. Customers should provide the motor data, vehicle or machine voltage, communication requirements, feedback devices, expected operating conditions, and applicable validation criteria.

Key Takeaways and Next Steps

• Check power and ground under load before diagnosing CAN or feedback faults.
• Use the wiring diagram and controller specification instead of assuming generic pin assignments or voltage limits.
• Use resistance testing only on a de-energized CAN network, and treat approximately 60 ohms as a common reference for a properly terminated high-speed bus.
• Use an oscilloscope or CAN analyzer when a multimeter cannot reveal timing, noise, dropouts, or intermittent errors.
• Validate feedback supply, ground, raw signal, scaling, direction, and controller interpretation.

The direct answer is simple: I check CAN, power, ground, and feedback signals in sequence, beginning with the electrical foundation and confirming each measurement against the controller’s documented requirements. My next step would be to collect the pinout, fault conditions, measured values, and waveform evidence, then review them with the controller supplier. If you are sourcing a motor controller or EPS controller for a defined application, contact QEXPAND with your technical requirements so we can assess interface compatibility and the appropriate support path.

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