Diagnosing Ford communication network problems

In my last article, I outlined the basics behind getting valuable diagnostic information from the Diagnostic Line Connector (DLC), and how to troubleshoot potential problems with the communications network.

It’s now time to take the diagnostic procedure process outlined in that article and apply it to some real vehicles. Let’s begin with the Ford network and apply what we have learned earlier to isolate the cause (s) of a network systems failure, staring with the Standard Corporate Protocol (SCP) which we will quickly review.

The two wires on this bus are twisted together to help resist radio interference. Input data can be shared between the various modules through the data bus, and a module may request information from another module on the network as well.

The data bus wires are designated:

Bus + …circuit # 914….and is found on Pin 2 of the DLC

Bus – …circuit # 915….and is found on Pin 10 of the DLC

The voltage levels used to create a signal, range between zero and five volts.

The voltage at “Rest” for Bus + is 0-.2 v.

The voltage at “Rest” for Bus – is 5 v.

Remember, to get these readings with the DLC and using a DVOM, remember to have the key ‘Off.’ When a message is being sent, the voltages are reversed. You will need a lab scope to see this.

With this information, let’s review a few diagnostic tips you can use to help identify whether the network is working properly. Using a lab scope, adjust the settings to read as follows:

Voltage (per channel) = 5Volts

Time-Base (per channel) = 100ms (recommended)

Now, hook up the lab scope to the DLC as follows;

Channel # 1 of your lab scope to Pin # 2

Channel # 2 of your lab scope to Pin # 10

Ground lead to Pin # 4 (or you can use a good chassis ground)

With the ignition switch OFF, you should have the voltage readings that were mentioned earlier. Now, turn ON the ignition switch and compare the signal(s) you see with the diagrams below.

The diagram, at left, reveals a problem with the network. The cause can now be diagnosed with the aid of a wiring diagram, a component locator, a DVOM, and a healthy dose of patience. And always remember … the network must be repaired FIRST.

Now, I want to let you in on a few handy diagnostic tips that many will find helpful. The first is that if damage occurs to one wire, communication should continue between the modules, but diagnostics will likely be affected. Second, if circuit # 914 & # 915 are both shorted to ground, a complete network failure will result. All control modules will operate in FMEM. And finally, if both wires are open in one of the branches of the network, only the modules that are on the open side of the network will be affected.

Another handy thing to remember is that newer Ford vehicles are often equipped with more than one network, the second being the ISO 9141 protocol. The ISO 9141 network, is a single-wire, and is often identified as Circuit 70. There is no module-to-module communication on this network. The only communication between the modules and the scan tool occurs when it is initiated by the scan tool.

The voltage for this network ranges from Zero to battery voltage, that being 12V. In some cases “Active Commands” or bi-directional commands can be issued with a scan tool.

To diagnose this network, you will need a high-impedance DVOM and a scanner. First, hook up the DVOM (positive lead) to Pin #7 into the back of the DLC connector, the ground lead attached to the chassis. Then, with the ignition switch OFF or ON, there should be NO voltage on the network. Finally, hook up the scanner to the DLC, and observe the voltage reading on the DVOM. The voltage should now be at or near battery voltage.

Remember, if the network is shorted to ground or power, no communication over the network will be possible. Depending on a particular vehicle’s options, the following modules could be included on this network system:

RCM – Restraint Control Module

LPSDM – Left Power sliding Door Module

RPSDM – Right Power Sliding Door Module

PAM – Parking Aid Module

RAP – Remote Anti-theft Module

IABDM – Integrated Air Bag Diagnostic Module

GEM – Generic Electronic Module

Happy Motoring!