This month’s Tech Talk is a short continuation from Novembers data link article. I am going to discuss how to quickly check to see if there is an issue with the data link in your DTNA (Daimler Trucks of North America) vehicle.
A digital multimeter is required for this test. It does not have to be an expensive tool, as long as it is capable of measuring VDC (voltage direct current), VAC (voltage alternating current) and Ohms. It does not have to be an auto ranging meter, however, I do find the auto ranging meters to be easier to use. Most auto parts stores, hardware stores or any other business that sells hand tools will have a digital multimeter to purchase. Some places may even rent the tool.
On the last page I have laid out a quick test that can be performed if there is a data link issue or a module is not responding. First, check to make sure there is battery voltage at pins ‘B’ and ‘A’. Compare this to the source voltage that is measured right at the positive and negative terminals at the battery. There should be greater than 12.4 VDC, but the difference between the battery and diagnostic plug should not be more than 0.05 VDC.
Next, follow the chart, putting the red lead and black lead of the meter to each specified terminal at the diagnostic connector. I have also presented a diagram of that connector, it is the same whether it is the black 9 pin or the green 9 pin that came with OBD16 vehicles starting in January 2016.
I suggest taking all the voltage readings first before disconnecting the batteries and performing the Ohm checks. The batteries MUST be disconnected, simply turning off the cab disconnect switch (if equipped) will NOT give accurate results. If the diagnostic connector is green, it will NOT have the J1587 installed in pins ‘F’ and ‘G’. However it will have a “Diagnostic CAN” installed in pins ‘H’ and ‘J’. This can be checked the same way as the J1939 on the Ohms check, 55-65 Ohms. This Diagnostic CAN will NOT have fluctuating voltage when checking for voltage. The Ohms check should be the only test on this data link.
Line ‘14’ on the quick test I have given is only to be done if line ‘11’ is out of specification. Each terminating resistor is 120 Ohms and there are two of them, which is why there will be half of the 120 Ohms at each of the diagnostic connector on pins ‘C’ and ‘D’. These resistors are placed in each truck differently, depending on the number and location of each “node”. If this spec is out I recommend consulting your nearest DTNA dealer.
This test does work with black 9 pins as well as green 9 pin connectors. The J1939 in the black and green connector travel at different speeds but the protocol is the same. If there is a green connector, however, and there is an issue with the 250kps data link, you will have to test it at one of the splice packs that were mentioned in Novembers Tech Talk. This is because the 250kps data link is NOT in the green 9 pin diagnostic plug.
Once it has been determined that there is in fact an issue with one of or multiple data links the best thing to do is get it to an authorized DTNA dealer where factory trained technicians can trace the issue quickly and get the unit back to OEM specification. Another option is to start unplugging each module one at a time and redoing the test. Once the issue goes away, that will be the “node” that is affected and it needs to be determined if it is the wiring or the module at that “node”.
A third option is to use a P.C. diagnostic toll that has a “data link” tester. What this tool does is takes a “roll call” of all the components that are reporting on a data link. The missing components in the roll call will be a place to start looking for the issue in the data link structure.
If there is an issue with the wiring of a data link there must NOT be any “soldered” butt splice repairs. If a repair is needed, a solid, sealed connector with gold terminals must be used while keeping the one twist per inch protocol. If this is not done correctly, especially with the 500kps data link, extra resistance will be introduced and data will be slowed, and corrupted.
Data link issues can be sophisticated and difficult to diagnose. I always recommend that a factory trained technician at an authorized DTNA dealer be consulted to diagnose and repair data link issues.