2005 Buick Lacrosse, U1300 Follow Up Testing

This repair information is an addition to a previously published article. To see that article please click here. The code U1300 turned out to be the most relevant code but SI was severely lacking in background information. SI states that the code U1300 can only be seen as a history code. The U1300 was stored in the ECM and since I was seeing it, it had to be a history code, at least according to SI. Again according to SI when the code U1300 was active I should not have been able to communicate with the ECM. At no time was communication with the ECM lost.

In the next picture is a fault tester for the class 2 data bus. Although it makes testing easier it is not absolutely necessary.

At least if you are not afraid to cut and splice a wire or two. Note the dark green wire that has been cut and spliced back together. This connector is located under the dash on the driver’s side of the car. The lower hush panel needs to be removed to access it.

With the tool connected to the class 2 data splice pack a switch can be depressed to break the circuit. It is switch “B” which I am pointing to in the next picture. The “B” designation can also be found on the splice pack body and it corresponds to the dark green wire.

Note that the lighter green wire had already been cut and spliced back together. The light green wire is positioned at terminal “J” and is the connection to the Vehicle Communication Interface module (VIM). By either switching off terminal “B” on the tester or cutting the wire a voltage test can be performed. Voltage on the class 2 network cycles between 0 and 7 volts. The extremes of this voltage range can only be seen with an oscilloscope. With most digital voltmeters you would see a range of slightly above 0 volts and just below 4 volts. The value should also be constantly changing. At bare minimum each module on the network will send out a state of health message every two seconds.  On the terminal “B” data line there was constant movement with the circuit in tact. With the circuit separated the main bus connection was still cycling but the wire that was still attached to the ECM  was stuck and a -.67 volts. There was no cycling of the state of health signal.

Now that I knew what the problem was, I needed  to confirm where it was located. Was the ECM itself faulty or was there a problem with the wire in between. The ECM is located on the passenger side front of the engine.

The wire that I needed to find is in the outer harness connector. The one on the left in the following picture.

The metal lever on the connector has to be slid backwards slightly before it will rotate forward and release. Look closely at the slot near the pivot point of the lever.

The rubber cover has to be slid back and a wire tie will have to be cut and removed. Then the outer cover can be removed with some gentle prying. Now that the cover was removed I could read the numbers on the back of the connector body. The dark green wire at terminal 48 can be located as shown below. I did test the wire terminal on the other side of the connector body  for continuity to ground (none present, good) and continuity to terminal B of the splice pack (less than 5 ohms, good). It takes a very gentle touch and the right kind of probe to prevent terminal damage. Again, there was no continuity to ground and there was continuity to terminal “B”  in the under dash splice pack.

The option is to cut the wire in such a way that it is easily repairable. As I did in the next picture.  As it turned out severing the wire allows for a very definitive test. It is important to cut the wire and then temporarily connect the two halves back together. The ECM cannot receive the anti theft password from the BCM if the wire is cut and the engine will not start.

So strip the two wires and twist them back together, then start the engine. Check the voltage where the wires are twisted together. It should be varying as mentioned earlier. Now separate the twisted wires while the engine is still  running. Both halves should maintain a variable voltage. On this car the wire that was connected to the ECM harness connector body flat lined at 0 volts. The wire that runs back into the harness continued to fluctuate. Keep in mind that all harness connectors have to be attached to do this test. I had them removed from the ECM so that I could locate the proper wire and remove the covers.

This test proved that the ECM could receive data on the dark green wire at terminal 48 but it was incapable of sending data on this wire circuit. Very much like a phone that will allow you to listen but not allow you to talk.

Replacing the ECM and performing all of the programming corrected all of the issues with this 2005 Buick Lacrosse.

Now to provide some clarity on this vehicle’s problem and what exactly was going on in testing.

The ECM can communicate in two different methods. It communicates with the TCM and the Tech 2 scan tool on the CAN communication wires. It communicates with all other modules on the Class 2 data lines. So when it was unable to communicate on the Class 2 network the code U1300 was set inside the ECM. The code was reported to the scan tool on the CAN circuit which was still active.

Now to make this more interesting, what caused this problem in the first place? The ECM that I installed was the fourth one in the life of this Lacrosse. There is a cause and that will be in another post coming soon.

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