Tool Briefing: CAN bus communication failure

Feb. 12, 2015
Steps to pinpointing a wiring issue in a 2009 Ford Crown Victoria Police Interceptor.

Vehicle affected: 2009 Ford Crown Victoria Police Interceptor currently operated as a patrol vehicle

Symptoms: Service Engine Soon and Battery Lights On, Multiple DTC’s, No Drivability Issues, Dash Gauges Inoperative

Tools Used:

  • Bi-directional scan tool
  • Digital multimeter
  • DLC break out box
  • Various test leads
  • Vehicle information source
  • Waveform viewer
  • Micro torch 

With all of the wiring on a vehicle it is not uncommon to have circuits that either short to power or ground. The common shorts to power or ground can be caused by relays or actuators, components like alternators, or a wire that has worn through the insulation and is allowing current to travel where it was not meant to.

With the advent of CAN data busses another type of short can occur which will create situations where modules cannot communicate or have drivability issues, and even no-starts. One of the problems with diagnosing CAN data shorts is that since there is not a lot of current travelling through the communication lines, typical signs of shorted wires such as blown fuses or burned wires are not a common indicator of where the short is located, or even which module the problem is located in.

In this Tool Briefing we will discuss a shorted CAN bus that has caused a lack of communication between modules and we will take you through the diagnostic test process as well as the repair procedure.

It is a good idea to have a regular diagnostic strategy that you use every time you diagnose a vehicle. When you perform a brake inspection you follow specific steps such as removing the wheels and inspecting the calipers, pads, rotors, etc. in order to provide all of the information you need to form your conclusions on the condition of the brakes and what will be necessary to repair them properly. In electronic diagnostic procedures you must follow specific steps to assure the electronic system is working properly, but you also need to remember that electronic components that monitor or control mechanical components need fully functioning mechanical devices and it may be necessary to confirm that the mechanical components are working as well. In the case of a CAN system you must eliminate the possibility of wiring, multiple modules, and mechanical systems causing a failure in the system.

Step 1: Pull codes and perform a vehicle health check

A proper diagnostic strategy includes the use of a good bi-directional scan tool. In this case we will be using a Ford IDS Scan Tool (While this is the factory scan tool, techs can also use aftermarket scan tools such as the Delphi Scan Tool or Snap-on Solus). Our strategy starts with observing and recording codes, noting both monitor status and pending code status, and performing a “Health Check” which is a test that determines which modules are or are not communicating.

With a CAN equipped vehicle that is showing symptoms of issues in multiple modules such as a “Check Engine” light (Powertrain Control Module) and an ABS light (ABS Module), it is a good idea to start your diagnostic testing by determining if there is a direct link between the affected modules or if a different module is the communication link between them.

 Our DTC Health check showed multiple “U” codes which are communication fault codes. We had the following codes:

  • B1352 LCM
  • P1000-FF PCM
  • U0100-20 ABS
  • U073-20 ABS
  • U1155-20 ABS
  • U1041 DDM
  • U1059 DDM
  • U1135DDM
  • U11041 LCM
  • U1059LCM

The following results were obtained by performing the module health check test.

Standard Equipment Modules:

  • Fail IC (Instrument Cluster)
  • Fail PCM (Powertrain Control Module)
  • Fail VDM (Vehicle Dynamic Module)

Optional Equipment Modules:

  • Fail ABS (Anti-Lock Brake Module)
  • Fail FSSM (Fire Suppression Module)
  • Fail HVAC (Heating/Ventilation/Air Conditioning Module)
  • Fail MC (Message Center Module)

When diagnosing a CAN data issue a complete understanding of the communication network map is essential. It is also necessary to understand the vehicle you are working on to determine if all of the modules listed on a network map and the Health Check are installed on the vehicle. In the case of this vehicle, the FSSM and MC Modules will show no communication because the modules are not installed on the vehicle. Additionally there are codes for the Driver Door Module (DDM) and Lighting Control Module (LCM) but the modules are not showing either a “pass” or “fail” in the Health Check because their line of communication, the Instrument Cluster, is failing the check and is unable to report their status.

The fault codes for the DDM and LCM were most likely set when communication was still present. In order to determine what the network communications looks like, we use Mitchell 1 ProDemand and looked at the wiring diagram section, Computer Data Lines Circuit diagram (see Fig. 1). This diagram provides the wiring between modules so you are able to see how the modules are connected as well as determine which module is the “Master” Module. The Master Module is the module that is powered by the ignition being turned on, then the Master turns the other modules on.

Determining which module is the master will allow you to make sure that the master is getting power and is able to send the signal to the other modules in order to activate them. The other benefit of obtaining the wire diagram is that you will be able to assess how the modules are connected and determine if there are sub-networks allowing a single module to communicate information from multiple modules to the master module and look for shared connections or splices that may be causing the communication error. A sub-network could consist of a Dash Integration Module carrying messages to the Body Control Module from various sources such as a radio, instrument cluster or A/C controls.

Step 2: Check CAN data line communication

The next step in the diagnostic strategy is to check for communication on the CAN Data lines, in this case pins 6 and  14 of the DLC. Using the AES Wave LineSpi Breakout Box and the Matco MDAutowave waveform viewer is one of the easiest ways to do this.

The LineSpi connects to the data link control (DLC) and provides LED indicators which will allow you to observe if the DLC has proper power and ground in addition to activity on the CAN bus. Additionally, the breakout box has connections to allow you to use your scan tool while testing as well as having jacks to connect other test equipment directly to the DLC without taking the chance of damaging terminals on the connector.

Connect the breakout box and use the waveform viewer connected to the high-speed CAN terminals 6 and 14. As you can see in the waveform screenshot (Fig. 2) the line is relatively flat which indicates that there is no data travelling across the circuit. Before assuming that the issues are only due to a lack of communication, make sure that there is power and ground at the DLC terminals by observing the LEDs.

If they are on, you need to allow the CAN system to send a message along the communication line such as opening a door or turning on the A/C which requires modules to communicate with the PCM and look for activity on the bus. When modules communicate you will see a waveform similar to the one shown in the (Fig. 2) screenshot.

Observing signals will not allow you to determine what message is being sent, only that there is traffic on the communication bus. It is also necessary to use a DMM ohmmeter, such as the Fluke 88V, to determine if there is a short to ground in the data circuit. Set the ohmmeter to measure resistance and check both data lines on the breakout box. Make sure to check this with the ignition in the “off” position. You should not see any continuity between a communication line and ground. Continuity indicates that there is a short to ground either in a module or in the wiring.

The next step is to check the resistance between the two communication lines on the DLC, you could use the breakout box but it has some resistance built into it and may change your readings. Most vehicle manufacturers place a resistor in the communication circuit to stabilize communication and eliminate electrical interference. It is necessary to use your repair information to determine what this resistance should be. In this case, you should see a total resistance of 60 ohms. Upon testing this vehicle we saw no resistance between the two terminals which would indicate a short between the two circuits.Since there is no activity on this bus the next step is to determine if disconnecting any modules will allow communication to resume. If a module is disconnected it means that a short or open is either within that module or others connected directly to it, or that there is a wiring issue within the sub-network connected to it.

Step 3: Test individual modules for failed communication

 In the case of this Crown Victoria there are many modules that are indicating failures to communicate so we need to look to see how they are connected as well as determine if there are any TSBs for this vehicle matching the symptoms.

Using the Data Line Circuit wiring diagram you can see that there is a link between the Driver Door Module, the Lighting Control Module, Instrument Cluster, ABS Module and the PCM. It is easiest to disconnect each module while observing the data lines using the breakout box and waveform viewer to see if messages resume when an individual module is taken out of the circuit.

After disconnecting modules we observed message traffic resumed when we disconnected the ABS Module. At this point we cannot assume that is where the problem is because there are other modules connected to the PCM through the ABS module. In order to isolate the issue, reconnect the ABS Module then disconnect each sub-module, one at a time, to see if data communications return. After disconnecting each module in the sub-system, communication did not return so that indicates an issue either in the wiring, the ABS Module itself, or possibly issues with multiple modules.

Step 4: Find the short in the data lines

At this time it is necessary form a strategy to determine how you will find a short in the data lines shared by many modules and consists of many feet of wiring and multiple connection points. Since this vehicle is in use as a patrol vehicle and because we have built quite a few of them, we have seen many cases where an Upfitter will splice into the vehicle factory wiring harness to activate some add-on components. One such component includes the speed sensor, which allows some add-on functions to turn on or off at a specific speed.

The Crown Victoria has a rear speed sensor that sends signals through the CAN Bus using the ABS Module, so we decided to check this connection first. When we observed new electrical tape added to the wiring harness at the ABS Module we removed the tape and discovered that when the Upfitter added the connection to the speed sensor, the insulation on the communication wires of the CAN Bus were damaged and were touching causing a short in the CAN system.

Step 5: Repair the wiring

Once we found the issue we repaired the wires by cleaning them and inspecting to make sure there was no damage or corrosion to the wires. When we determined that the wires were okay, we took a picture of the wires and terminal to make sure we put the connector pins back correctly. You should note that communication wires are twisted together to assist in eliminating radio, magnetic, and electrical interference.

We then used the Matco Terminal Pin Remover Set to disconnect the terminals from the ABS Module connector in order to install a 1" piece of glue-sealed heat shrink tubing over each wire and used the microtorch to heat the tubing to provide a weather-proof seal. Make sure that you re-twist the wires after repairing them in the same manner as they were before starting the repair. After resealing the connections we installed a good quality electrical tape over the harness and used a dielectric compound to the connector to create a waterproof seal.

After the repair we re-tested the communication lines with the LineSpi and waveform viewer to make sure communication returned as well as installed the scanner to perform the health check again. Both tests showed communication and all of the modules passed the health check with no codes present.

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