Gaining information from the DLC

No matter how long that I have been in the automotive trade, I am still at amazed at how much diagnostic information can be gained through the Diagnostic Link Connector (DLC). Yet none of the manufacturers (Ford, GM or Chrysler) are consistent in their use of the various pins at the DLC, especially when it comes to communication (network) systems.

To help technicians, I’ve compiled a list of the different communication (network) systems used by the Big Three, highlighting the following information:

A. Pin Designations

B. Diagnostic Procedure(s)

C. Diagnostic Tips

In this article and the next, I’m going to focus on Ford vehicles, breaking the article into how to make sure the communications network systems are functioning correctly, and then knowing how to diagnose what the DLC is giving the technician. In subsequent issues of SSGM, I’ll cover GM and then Chrysler. So, without much further adieu, let’s begin by quickly looking at that what the PIN designations are for the DLC with a Ford, starting from 1999 to the newer Ford P/U, including the popular Windstar:

Pin # 2….. Standard Corporate Protocol [SCP]….Circuit # 914…[Bus +]

Pin # 4…..Chassis Ground

Pin # 5…..Case Ground

Pin # 7…..International Standard Organization [ISO – 9141]…..Circuit # 70

Pin # 10…Standard Corporate Protocol [SCP]…..Circuit # 915…[Bus -]

Pin # 16…Fused Battery [Hot – at – all – times]

Diagnosing whether the network system is functioning

In some circumstances, diagnosing communications network systems can seem overwhelming. Yet, if you have a clear understanding of how the different systems function, sorting out the problem(s) can easily be accomplished, and without having to send the job to a dealer. You will, however, need the following test equipment: a dual-channel lab scope, a high-impedance DVOM, a test-light, a scanner, and a current wiring diagram for the specific vehicle you are working on.

If, a vehicle arrives at your shop, with let’s say, the “Service Engine” light ‘On,’ hooking up a scanner to the DLC might produce a ‘No Data’ reading, instead of something you can work with. So, what might cause you to check for a possible communications network failure? To begin answering that, let’s look at how the different manufacturer’s network system(s) function, because, if the communications (network) system fails, you must repair it first, before attempting any other computer/sensor diagnostics.

The Standard Corporate Protocol (SCP) on a Ford has two wires on the bus that 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 it is found on PIN 2 of the DLC; Bus – …circuit # 915, and this one 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 zero-.2 v; and the voltage at “rest” for Bus – is 5v.

So to begin testing whether the communications network is working, first bring out the lab scope as mentioned earlier and adjust the settings to read as follows: Voltage (per channel) = FiveVolts; Time-Base (per channel) = 100ms. 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; and 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:

Diagram B 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! Remember, the network must be repaired first, before you can move onto anything else, which is why it is important to run this test should you get a vehicle coming in with a possible problem, but produces a ‘No Data’ signal.

In my next column, I’m going to outline some helpful diagnostic tips and a give a closer look at the ISO 9141 protocol used in some of the new Fords on the road today.

Happy Motoring!

Mike Duguay has been a licensed technician since 1974, opening Duguay’s Auto Repair in 1992. He has also been a substitute instructor for the Vancouver Community College Automotive Department. In 2003, Mike began working as an instructor with Injectronics Training www.injectronicstraining.com.