![]() Remove resistor 2 out of the harness and measure it, let’s say it measures 119 ohmsģ. Remove resistor 1 out of the harness and measure it, let’s say it measures 118 ohmsĢ. Note: these were actual measurement on a truck that had a problem.ġ. Here are the steps for checking each resistor so you know what you should be measuring at pins C and D for the J1939. Here is the simplest formula and you will need a calculator. My experience tells me there is a problem in the J1939 data link.īefore I get into actual troubleshooting you will need to understand how to calculate the resistance in the J1939 harness. In most data link troubleshooting text books this is an acceptable measurement and falls within the 60 ohms + or - 10 ohms. The trucks that were checked had measurements that were about 65 to 68 ohms on pins C and D of the 9 pin data link connector. What I see all the time are inactive codes in the ABS for J1939 time outs on Freightliner Columbia. When checking the J1939 on pins C and D make sure the ignition is off and no modules are communicating when you check the resistance. When the data link is in good condition the total resistance will be approximately 60 ohms on pins C and D of the 9 pin data link connector. The J1939 data link has two 120 ohm resistors in parallel in the data link. This troubleshooting method will work for all truck makes but I will use a Freightliner Columbia in my example. I will show you how to find the potential problem in the J1939 data link. Most of the time these codes will get erased and minimal or no troubleshooting will be done. Trucks will come into the shops all the time with inactive codes for data links, such as ABS J1939 time outs, communication errors, and Engine ECU’s data link failures. The most common data link used is the high speed J1939. Let me explain: Modern day trucks have multiple data links used by all the electronic modules. What I am going to share with you about J1939 data link diagnostics is from life experience not text book. One of the most valuable things I will teach technicians is how to use a calculator to troubleshoot the J1939 in my Advanced Data Link and Fault Code Diagnostics Course. I have to teach everything on the vehicle from bumper to bumper, which could be 3 axle alignments to troubleshooting a data link. Working for a major fleet as a Technician Instructor I have taught many courses. Why I use a calculator to troubleshoot a J1939 data link, yes I said a calculator. J1939 Data Link Troubleshooting with a Calculator ![]()
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