ASE Truck and Diesel Mechanics

Why I use a multi-meter and a calculator to troubleshoot a J1939 data link, yes I said a calculator. Working for a major fleet as a Corporate Technician Instructor I have taught many courses. I have to teach everything from bumper to bumper and that could be 3 axle alignments to troubleshooting a data link. 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 Course.

What I am going to share with you about the J1939 data link is life experience versus text book. Let me explain: Modern day trucks have multiple data links used by all the modules. The most common one used is the high speed J1939. Trucks will come into the shops all the time with inactive codes for data links, ABS J1939 time outs, and Engine ECU’s data link failures and so on. Most of the time these codes will get erased and no troubleshooting will be done. I will tell you how to find the possible problem.
This troubleshooting will work for all truck makes but I will use a Freightliner Columbia in my examples. The J1939 data link has two 120 ohm resistors in parallel in the data link. 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.

Locations of the 2 resistors on some different trucks:
Freightliner Columbia: R1 left hand frame rail, front cab mount and R2 right hand frame rail near frame mount ABS module
Freightliner Century Class: R1 left hand frame rail, front cab mount and R2 left hand B pillar
Volvo with Cummins Power: R1 near engine ECM connector and R2 cab fuse panel
Volvo with Volvo Power: R1 built into engine ECM and R2 cab fuse panel
IHC ProStar: R1 left hand frame rail strapped to airlines near transmission and R2 strapped on top of transmission towards the rear

Before I get into actual troubleshooting you will need to understand how to calculate the resistance in the J1939 harness. Here is the simplest formula and you will need your calculator.
Product over sum
R1xR2
R1+R2

120x120 = 14400
120+120 = 240

14400/240 = 60 ohms


I see all the time inactive codes in the ABS for J1939 time outs on Freightliner Columbia. In about a 3 month period I can count at least 15 trucks that I had to walk someone through these diagnostic steps to find the problem. 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.

The trucks that were checked had measurements that were about 64 to 68 ohms. In most data link troubleshooting text books this is an acceptable measurement and falls within the 60 ohms + or - 10 ohms. My experience tells me there is a problem in the J1939 data link. If you had measured 120 ohms then one resistor is missing or there is some heavy corrosion in the connector to one of the resistors.

Here is what you need to do to know what you should be measuring at pins C and D for the J1939. Note: these were actual measurement on a truck that had a problem
1. remove resistor 1 out of the harness and measure it, lets say it measures 118 ohms
2. remove resistor 2 out of the harness and measure it, lets say it measures 119 ohms
3. Plug these resistances into your formula to know what the actual measurement at pins C and D should be.

118x119 = 14042
118+119 = 237

14042/237 = 59.24 ohms

As you can see we should measure approximately 59 ohms on this data link you can add a small amount for the resistance in the wires but should still be very close to 59 ohms. In everyone of these Columbia’s the resistor near the ABS module on the right hand frame had some green corrosion in the 3 way connector that holds the resistor. Seems the o-rings that should seal out the moisture do a better job sealing in the moisture that got inside of it. After cleaning up the corrosion in the connector the resistance measurement on pins C and D of the J1939 connector dropped from 68 ohms to 59 ohms.

I have used this diagnostic method on many trucks: Volvo, ProStar and Freightliner. I would practice this on a good truck and you will see that using a calculator to calculate the resistance should match what your multi-meter shows you. You will repair many data link problems if you learn this method.

Note: There are some trucks that the J1939 data link resistance can’t be measured unless the batteries are disconnected. IHC ProStar recommends it but I have checked the data link without disconnecting the batteries. Freightliner M2 and C2 chassis you must disconnect the batteries to check the J1939. These trucks and/or school busses have bulk head and chassis module that communicate all the time even with the ignition off and the data link can’t be checked as long as module are transferring data on the J1939.