Using a Lab Scope for Network Diagnosis

9-07 Categorize network operations by using a lab scope.

Network communication between modules is in digital format. Even though the information present in the data packets can’t be decoded, the scope provides a means to verify the signal is present, consistent, and correct. Storing and saving known-good waveforms provides a diagnostic information library that may be hard to find. With a known-good waveform, the signals can be compared when looking for a fault.

Properly operating digital signals have certain characteristics to look for when viewing and testing:

• The waveform should be free from blatant distortion and noise spikes.
• EMI/RFI is extremely detrimental to network signals due to its low operational voltages.
• The upper and lower measured levels of the waveforms should be consistent and provide clean transitions while switching states.
• Rounded corners and dropouts are generally signs of a fault.
  • Having a known-good waveform for comparison will confirm whether the signal is normal or faulty.
  • For most square wave signals, the upper-level or high voltage (logic one) should be equal to or greater than 90% of the source voltage.
  • The levels shown in the following examples are KOEO. If the engine is running, the voltage levels may change slightly.
• For most square wave signals, the lower-level or low voltage (logic 0, or ground) should be less than 10% of battery voltage, under 1.0 volt maximum, with less than 0.5 to 0.1 volts being ideal.
  • Communication networks are low amperage circuits.
  • Intended low-current flow results in low-voltage-drop specifications.
  • On any digital circuit, voltage drops of over 0.5 volts on either the power or ground feed require additional testing to find the high resistance.
  • Again, a difference will exist based on battery state of charge and if the engine is running.
• For intermittent concerns, the signal can be monitored while wiggling, moving, or shaking wiring harnesses and connectors, tap testing components, and cycling the vehicle and its devices through temperature variations.
• Network faults may include all modules on the bus or may be unique to a module or its circuitry.
  • Before condemning any module, check its supply voltage and ground. If available, use a lab scope to check whether and verify that the signal is correct.
• Module faults can also include specific functions of a module, such as a faulty driver in the PCM that cannot pull source voltage to ground, to activate a canister vent or purge solenoid.
  • Before replacing the module, use a DMM to test the voltage drop on the module’s power and ground supply circuits and the power and control wire to the component.
  • Use a lap scope, if available, to check the signal output, which will be On/Off or duty cycled, from the module to the component.

Scope Setup and Preparation

Depending on the network being tested, adjust the voltage levels accordingly. On a CAN, for example, typically select 5 volts since the voltage range is 2.5 volts ± 1.0 volt. Adjust the time base to capture as stable a pattern as possible while still proving good detail. On a Pico Scope, begin with the scope at 5 µs/div (5 microseconds per division). For a Snap-on scope, set the screen time frame to 100 µs (microseconds). The fast time bases are needed due the speed of the CAN signal transmissions. Freeze the waveform on the scope, and either zoom in (computer-based scope) or zoom out a Snap-on style scope to analyze the waveform (FIGURE 9-17).

TECHNICIAN TIP

The data packet contents do not matter and are not able to be read (uninterpretable) by technicians in the field. What does matter is voltage signals. Are they reaching the required levels and the waveform? Are the edges square and consistent? Does the transition from high-to-low and low-to-high occur quickly, with minimal noise or “fuzziness” on the circuit?

FIGURE 9-17 Proper scope setup is crucial to seeing and validating the communication between modules. Improper setup will not yield a usable picture.

When using a lab scope for network diagnosis, do not use a trigger on a CANbus. Triggering on a network can cause the scope to miss an intermittent data glitch. Let the scope run continuously with the vehicle running to create heat until a fault is noticed. Network faults, like all electrical signals, are often affected by temperature extremes, with heat typically causing the most issues. Once the waveform shows a fault, unplug the suspected module, leaving the scope hooked up to the communication pins at the module’s connector. If the signal returns to normal with the module unplugged, check power and ground to the module. If the module’s power and ground circuits are OK, replace the module. If the signal is still corrupt, check and repair the communication wiring to the module.

Testing the Controller Area Network

The CAN system is a straight-line configuration. Each end contains two 120 Ω terminating resistors, wired in parallel, totaling 60 Ω for the network. The location of the resistors varies. They may be found inside a module, inside fuse box, or in-line of a wiring harness. The network uses a twisted pair of wires to reduce the effects of EMI. At rest (no communication/recessive state) both wires should show 2.5 volts. During communication, CAN HI is pulled up to 3.5 volts and CAN LO down to 1.5 volts (FIGURE 9-18). This information is beneficial for diagnosing a network or communication fault.

The following figures are examples of common scope patterns:

• Waveform shows data exchange on CANbus.
• Verify that voltage levels are correct on both CAN lines (FIGURE 9-19).
• In a recessive state, network at rest, both CAN HI and CAN LO should read 2.5 volts (FIGURE 9-20).
• CAN uses differential signaling, meaning the signal on one line should be a mirror image of the data on the other line.
• CAN HI = 2.5 volts at idle state, pulled high to 3.5 volts during communication.
• CAN LO = 2.5 volts at idle state, pulled low to 1.5 volts during communication.
• The delta difference between the CAN HI and CAN LO is 2.0 volts, and the voltage differential allows communication.

FIGURE 9-18 An example of a known-good CAN waveform zoomed to verify condition of square wave, sharp consistent corners, and voltages in specification. The data packet transmission and proper voltages are proof that the there are no circuit or module issues currently. The information contained in the data packets is not able to be interpreted, but it does show that the network is active.

FIGURE 9-19 Known-good HS-CAN waveform.

FIGURE 9-20 During normal communication, both the CAN HI (+) and CAN LO (–) are regulated to approximately 2.5 volts at rest, when there is no communication between modules. When a message is sent over the network, CAN HI voltage increases by 1 volt to about 3.5 volts. Inversely, CAN LO voltage decreases by 1 volt to about 1.5 volts.

The scope settings are 5 volts for both channels, and the time sweep is 50 microseconds per division. There is no trigger selected, allowing all the data to be captured. When diagnosing an intermittent concern, a dropout may be missed by setting the trigger incorrectly.

During a successful data transmission, a spike may be present on the waveform. The peak signifies a successful message transmission. The contents of the waveform are of little importance since technicians are unable to interpret them. What does matter is the pattern showing that communication is occurring. Network faults will significantly impact the signal waveform. Signals that are different can cause network DTCs (U-codes), limited communication, or a complete network failure, resulting in no communication on the bus.

The following images show the different situations if there is a fault within the data lines (FIGURES 9-21–24).

FIGURE 9-21 CAN HI and CAN LO shorted together. If the CAN HI (+) and CAN LO (–) data circuits short together, the signal will stay at the base voltage of 2.5 volts continuously and all communication will be lost.

FIGURE 9-22 CAN HI shorted to 12 volts. If the CAN HI (+) circuits short to battery voltage, the data line will be pulled high. The CAN LO (–) line also shows abnormally high voltage during communication. The CAN waveform may display battery/system voltage or be clipped, as it is here, limiting the peak voltage to approximately 7 volts. Communication may stop altogether or continue but at a severely degraded level.

FIGURE 9-23 CAN LO shorted to 12 volts. A short to battery voltage in the CAN LO (–) circuit pulls both the CAN HI (+) and CAN LO circuits high, and all communication stops. The voltage displayed on the scope may be battery/system voltage or be limited, as seen here, depending on the manufacturer.

FIGURE 9-24 If the CAN LO (–) circuit shorts to ground, the signal will be pulled low and stay at 0 volt. At rest, the CAN HI (+) circuit is pulled low, 0 volt, instead of the standard 2.5 volts. During communication, CAN HI reaches near normal voltage: 3.5 volts. The blips of voltage on the CAN LO are a failed attempt at communication. The short to ground typically prevents communication. However, on some systems, it may continue but at a severely degraded level.

Scope Testing Other Networks

• Like all communication networks, the K-line bus waveform is a square wave (FIGURE 9-25).
• Diagnosis consists of checking that the peak-to-peak voltage levels are correct.
• The signal is present only when a request for information is made by the scan tool to the PCM.
• Data are transmitted only as a pulsed voltage signal for diagnostic purposes during a request.
• The viewable waveform should be free of noticeable noise/voltage spikes and distortion.
• Voltage high (logic 1) should be more than 80% of battery or system voltage. With a fully charged battery (12.6 volts), the upper-level voltage should be greater than 10.1 volts.
• Voltage low (logic 0) should be less than 10% of battery voltage, under 1.0 volt.

FIGURE 9-25 Known-good K-line waveform.

Local Interconnect Network

• The LINbus is a sub-bus of a CAN (FIGURE 9-26).
• A LIN is a low-speed, single-wire serial data bus typically used to control low-speed, non-safety body control functions.
• A LIN is a low-cost network designed to reduce the bus load of a CAN.
• A LIN operates as a link between intelligent control modules and the remote actuators/sensors.
• LINs are currently not used in OBD II protocols.
  • Its use is currently limited to body functions.
  • Predictions for future growth exceeds that of any other automotive communication network.

FIGURE 9-26 The LINbus is a sub-bus of a CAN.

Using a Digital Volt-Ohmmeter to Test the Controller Area Network

Before using a scope to test the CAN, using a digital volt-ohmmeter (DVOM) can quickly determine whether there is short, break in the network, or high resistance. A DLC BOB is a useful tool that plugs into the DLC and gives the technician 16 pins to probe with their DVOM without disturbing the pins in the actual DLC. Probing in the BOB is a quick way to identify areas of concern and direct the diagnostic routine toward the area that is showing the issue. The following images are the different checks that can be performed with a DVOM to help guide a repair.

Voltage Checks

The following photo sequences will illustrate the different ways to check the communication lines with a voltmeter (FIGURES 9-27 through 9-30).

FIGURE 9-27 Using a DVOM to check a CAN. Turn the key on, turn the engine off, or leave the engine running. Set the meter to Peak Min/Max. Check both Pins. CAN HI (+): Pin 6 to ground and CAN LO (–) Pin 14 to ground. Pin 4 or 5 can be used for the ground point. At rest (no communication), the network should show 2.5 volts, as shown here.

FIGURE 9-28 Measuring voltage incorrectly across both CAN HI (+) Pin 6 and CAN LO (–) Pin 14 instead of measuring each pin to ground. The voltage displayed is the average difference between the two CAN circuits. To perform this test properly, measure both CAN HI and CAN LO to ground in KOEO or with the engine running.

FIGURE 9-29 Checking the voltage level on the CAN HI (+) line. Place the positive meter lead on Pin 6 and the negative meter lead to ground. Here, Pin 4 is being used for the ground. Turn the key on, and leave the engine off. With the DVOM set to Peak Min/Max during communication, the CAN HI signal should increase by 1 volt to about 3.5 volts. Due to the transmission speed of the signals, Peak Min/Max must be used to capture the voltage. If peak is not used, the voltage level will be incorrect since the meter will display the average of the signals’ time, between 2.5 volts and 3.5 volts.

FIGURE 9-30 Checking the voltage level on the CAN LO (–) circuit. Install the DVOM positive lead to Pin 14 and the black lead to ground (Pin 4 here). Turn the key on, and leave the engine off. Place the meter in the Peak Min/Max mode. During communication, the voltage should decrease 1 volt to approximately 1.5 volts. If the voltage reading is incorrect, check the network for a wiring or module issue.

Resistance Checks

The following sections will show how to use a DVOM to check CAN data lines for failures (FIGURES 9-31 through 9-36).

FIGURE 9-31 Another method to test a CAN: using an ohmmeter. With the key off, check the resistance across CAN HI Pin 6 and CAN LO Pin 14. Note: Some manufacturers may state to disconnect the negative battery cable during this test to prevent unwanted current flowing back to the battery from corrupting the measurement.

FIGURE 9-32 A resistance test taken with the negative battery cable still connected. Notice the LED lights on the DLC BOB are illuminated for the CAN. These are powered Pin 16 and ground, which indicates that there is power available at the DLC. The CAN does not power down as soon as the key is shut off. Checking the resistance while the network is communicating can lead down the wrong diagnostic path.

FIGURE 9-33 Resistance being checked across CAN HI (+) Pin 6, and CAN LO (–) Pin 14 with a circuit or module fault. An open in the network due to a wiring or module failure that eliminates one of the two terminating resistors will result in a resistance reading near 120 Ω. Limited communication on the network may or may not be available. Additional diagnosis will be required to find the fault.

FIGURE 9-34 Resistance check between CAN HI (+) Pin 6 and CAN LO (–) Pin 14 with a short together in the network. A short can occur in either of the two CAN wires or a module. A short in the network will prevent communication.

FIGURE 9-35 Resistance test showing CAN LO (–) Pin 14 shorted to battery/system voltage. No communication will occur.

FIGURE 9-36 Resistance test showing the results of a CAN HI (+) Pin 6 and CAN LO (–) Pin 14 when shorted to ground. No communication.

Each CAN uses two terminating resistors. Each resistor is 120 Ω. The resistors bridge across the CAN HI and CAN LO circuits and are connected in parallel. Since the two resistors are connected in parallel, the total circuit resistance is 60 Ω (Figure 9-31). The terminating resistors may be found in a module, in a fuse box or in-line of a wiring harness.

The primary purpose of the terminating resistors is to improve the transfer of data across the network by stabilizing the bus voltages and eliminating electrical interference. The resistors also provide a technician with a diagnostic tool. The following DVOM images (Figures 9-32 through 9-36) show what to expect in different situations on a communication system.