Other Articles - April - 2018

Can We Talk? Diagnosing a Lack of Scan Tool Communication

There are many different types of communication protocols in today’s vehicles, depending on the manufacturer and system. These include CAN, Flex CAN, UART, Keyword 2000, MOST, BEAN, LIN, LAN, CCD, UDS, Byteflight, Flexray, and PCI to name a few. If you want to repair today’s vehicles, your equipment must be capable of communicating with all of the different protocols.

Developed in 1984 by Bosch/Intel and first used by Mercedes in 1992, the CAN system has become a common communication protocol for most of today’s vehicles. The CAN system may use one or two wires to communicate between the various modules.

Commonly, two-wire twisted pairs are used to connect the modules in a loop or bus as it is more commonly known. Communication baud rates vary based on the bus rating, while the voltage levels on the bus also vary by manufacturer even though a standard exists for this protocol. Data is sent in a series of bits — 1=HI 0=LO — and in packets which combine various types of information.

The standard CAN system uses a CAN HI and a CAN LO circuit to communicate information between the modules. The standard voltage on each circuit is 2.5 volts unless communication is occurring.

If one module is communicating with another, the CAN HI circuit will be pulled up to 3.5 volts and the CAN LO circuit will be pulled down to 1.5 volts. The voltages on the CAN HI and CAN LO circuits will be toggled high and low.

This high/low sequence creates a digital signal on both the CAN HI and CAN LO circuits that are opposite one another. In other words, when the CAN HI circuit goes high, the CAN LO will go low. The controllers communicate with each other via the transitioning voltages they’re sending and monitoring on the bus.

Communication among the various control modules through the CAN system helps simplify the wiring and reduce the number of components needed for vehicle operation. Like a typical job site, you must have a boss to control the system operation.

The manager or traffic boss for a typical CAN system is the body control module (BCM) or the instrument panel control module (IPC). Let’s look at how the typical communication process operates in an example A/C system.

  1. The driver/passenger presses the A/C switch on the HVAC faceplate.
  2. The HVAC faceplate sends a signal over a local interconnect network (LIN) to the HVAC control module.
  3. The HVAC control module receives the A/C request and changes it to a Low Speed CAN message to the BCM.
  4. The BCM receives the A/C request signal and changes it to a High Speed CAN signal. The BCM forwards the request on to the ECM.
  5. The ECM receives the A/C request, evaluates the sensor inputs, and, if all the required conditions are met, the ECM will engage the A/C compressor clutch.

CAN is the most common protocol used on today’s vehicles. Problems with the CAN network, its wiring, or its controllers can cause several hard-to-diagnose issues, including:

  • Won’t crank, won’t start, or start/ stall issues.
  • Dead batteries (aftermarket equipment that was tied into the bus, keeping modules awake, or faulty modules).
  • Warning chimes, messages, warning lights.
  • Inoperative or improperly operating systems and accessories (transmission and driveability issues, vehicle won’t move, indicators and displays that don’t respond).
  • Intermittent or ghost U-codes.
  • No scan tool communication with some or all of the modules on the bus.

When it comes to computer communications, two issues commonly occur:

  • U-DTCs may set.
  • No communication between your scan tool and one or more modules.

No communication issues can be frustrating to diagnose as you may receive little to no help from your scan tool in isolating the cause.

The typical CAN LO and CAN HI circuits are connected in parallel by two, 120-ohm resistors (figure 1). This creates an effective circuit resistance between the CAN HI and CAN LO circuits of 60 ohms.

The resistors can be externally mounted in the harness or they can be located within one or more of the modules on the bus. Depending on the vehicle, you may have a resistor in the harness and a resistor in a module on some applications (figure 2).

Knowing that resistors are used in the CAN system can really help when it comes to isolating the reason your scan tool won’t communicate with the vehicle. This is especially true if you don’t have a specialty software program or the equipment to help isolate CAN communication issues.

So let’s look at how to proceed when all you have are basic electrical test tools. Most scan tools have the ability to “poll” the modules on the bus to find out which modules aren’t communicating, but let’s look at a worst-case scenario: no communication with any module on the CAN system.

I’ve run into this on numerous vehicles, especially MD/HD Allison applications, as the CAN bus is quite extensive and most of the controllers and wiring are exposed to the elements.

Here are the basic tools you need to help isolate the issue:

  • Battery tester
  • Digital multimeter
  • Wiring schematics for the vehicle’s communication network

The CAN system and its controllers are very sensitive to battery voltage issues. Before you attempt to diagnose a problem with the CAN bus, start by checking the condition of the battery, battery connections, vehicle grounds, and the charging system operation.

If all of those areas check out, locate the DLC and use a digital multimeter to help isolate the reason for no communication.


Here are seven quick tests you can use to help isolate the issues with a conventional CAN, 2-wire, twisted-pair data bus or its modules when you have no communication to your scan tool.

Allow the system power down at least 15 minutes or, better yet, disconnect the battery prior to starting these tests.

  1. Connect your ohmmeter between pin 6 and 14 of the DLC (CAN HI and CAN Low). 58-64 ohms is good (figure 3). Wait for the modules to power down or false readings may occur.
    If you measure 116-128 ohms during the pin 6 and 14 resistance test, there’s an open circuit past the first module and you’re seeing the first terminating resistor.
    If the meter reads OL during the pin 6 and 14 resistance test, the open is before the first terminating resistor. If the meter reads 0-2 ohms, the CAN HI and CAN LO circuits are shorted together. Unplug the modules one at a time until the short goes away to help isolate the short.
  2. Connect your ohmmeter between pins 6 (CAN HI) and 16 (B+). It should read OL (figure 4).
  3. Connect your ohmmeter between pins 14 (CAN LO) and 16 (B+). It should read OL.
  4. Connect your ohmmeter between pins 6 (CAN HI) and 4 (GRD). It should read OL.
  5. Connect your ohmmeter between pins 14 (CAN LO) and 4 (GRD). It should read OL.
    A reading other than OL indicates a short to ground (6-4 or 14-4), or a short to voltage (14-16 or 6-16).
    If you don’t find an issue with the bus resistance measurements in step 1-5, the bus circuits are likely in good condition. You may be dealing with a problem with a module or your scan tool.
    Next, connect the battery, and turn the key on, engine off.
  6. Connect your meter between pins 6 and 14. Set the meter to DC Volts, Min/Max snapshot mode. It should read 0-2 volts (Min/Max) which indicates data is present.
  7. Run these checks:
    • Voltage at pin 6 should be about 2.5 volts.
    • Voltage at pin 14 should be about 2.5 volts (system not talking).
    • Voltage between pin 6 and ground should be +3.1 volts average.
    • Voltage between pin 14 and ground should be –1.8 volts average (system talking).

If step 5-7 check out, try a different brand of scan tool, as the bus and at least some of the modules should be able to communicate with your scan tool, based on the readings you received.

Without scan tool data, diagnosing today’s vehicles is next to impossible. As circuit and component operation continues to evolve, the use of a scan tool will become even more critical.

As you can see, isolating the reason for no communication, even when you don’t own any high-tech test equipment, really isn’t that difficult. The biggest challenge is to identify the actual location of the open, short to ground, or short circuit.