If you aren’t familiar with truck datalinks, we suggest you read our post regarding the Introduction to Truck Datalinks. In this post, we are going to get more in-depth on the J1708 datalink, also referred to as J1587. This is the primary protocol found on the 6-pin diagnostic connectors that were used from the early 1990s to the mid-2000s.

Background on J1708 (J1587)

The J1708 network was the first real industry-wide accepted standard for commercial trucks. The wiring for the datalink itself is not too terribly complicated. The wiring itself is done in a 2-wire twisted pair of 18-gauge wires that can run up to 130 feet in length. There is also the standard of requiring 1 twist per inch of cable.

The J1708 network itself is very slow compared to today’s standards. Data on the J1708 moves at 9600 bits per second, which is much slower than today’s modern J1939 network. There can also be a maximum of 20 ECMs, or “nodes”, on the J1708 network. This is rarely a problem, as most J1708 networks consisted of no more than 4 ECMs (Engine, Transmission, ABS, & Cab Controller). You will also not find any termination resistors on the backbone wiring.

Troubleshooting J1708 Issues

Before we dive into how to troubleshoot the datalink, let’s take a look at the older 6 pin connector you’ll find on older J1708 only trucks. There are only 4 pins you need to be aware of, which are:

• A Pin = J1708 Data Link +
• B Pin = J1708 Data Link –
• C Pin = Power
• E Pin = Ground

This graphic gives you a representation:

As a first step, when you plug your diagnostic tool into your dataport it should have power. All datalink adapters will have a power indicator light, so you want to make sure that is on. If your adapter is not powering up, you will need to check for battery power and ground at the diagnostic port.  You are looking for Battery voltage (minimum 12v) between Pin E (Ground) and Pin C (Power). If you have no voltage or low voltage, you will want to check Pins E and C separately to determine if you have a loss of ground or loss of voltage.

The most common place to start with is the fuses. Head to your fuse box and start checking all the fuses. It varies by manufacturer, but often they will tie in the diagnostic connector power with other circuits such as radio or cigarette lighters. These circuits often have failed fuses, and the fix is relatively easy.

If you do have the correct voltage to those pins, and the device is powering up, you should look at the software side of the equation. Most manufacturers of datalink adapters such as Nexiq, Noregon, Diesel Laptops, and DG Technologies will provide you with a software tool to check for communication traffic. This tool will show you where the problem is – between the laptop and the tool or between the tool and the truck. You can usually select which protocol you are wanting to test, such as J1708. Here is a sample screenshot from our Diesel Laptops DieselLink Adapter:

If you are having no communication between the laptop and adapter, and you have power at the adapter, it is either an adapter malfunction or a driver` issue. Try to remove the drivers, reboot, and re-install. If you need help with this, feel free to reach out to our technical support department.

If you do have good communications between the laptop and adapter, but there is no communication between the adapter and truck, you need to look at the actual datalink on the truck. The step here is to make sure the key switch is in the “ON” position. You are now going to measure the voltage between Pins E and A. You are looking for voltage between 3.00V and 4.50V. You are looking for voltage readings to fluctuate in the thousands position, as this indicates the databus is working properly.

As you may remember from our other blog post, datalink traffic is a series of 1s and 0s. Depending on your multimeter, you may not see these fluctuations in real-time due to how fast it’s happening. For the hard-core diagnostic technician, this is where oscilloscopes would come into play, but that is a conversation for another blog post.

If your readings are good here, start checking the low voltage circuit of the datalink between pins E and B. The voltage here should be between 0.500 and 1.500V. As with the high voltage mentioned above, it will fluctuate in the thousandths position.

At this point, if you have correct voltage on all wires, the module you are trying to communicate with is more than likely either not powered up or has an internal fault. Another possible scenario is that one or both wires with zero voltage needs to be checked for a shorted module or wires.

If you are at the point with no voltage, there are some common causes that we see with our clients. The first is aftermarket modules that were added to the database. Maybe someone added an aftermarket safety system, ELD, or installed a radio improperly. Look for something that could have possibly NOT come from the factory.

If you are at the point with no voltage, here are some common causes we see with our clients. The first is aftermarket modules that were added to the database. Maybe someone added an aftermarket safety system, ELD, or even installed a radio improperly. Look for something that could have possibly NOT come from the factory.

ELDs have become prevalent in today’s commercial trucks, and we often see installers that will splice into wiring or try to install “splitters” behind the dash panels. You should look near where the diagnostic port is, remove some dash panels, and see if there are some non-factory wiring repairs or aftermarket equipment installed. The reason behind this is that these devices are sending and receiving datalink traffic, just like your diagnostic tool, but sometimes they send corrupt data through the network which causes issues.

The other strategy here is to start unplugging other ECM modules. Unplug each ECM on the vehicle one at a time, and re-test. Eventually, you’ll find the ECM module causing the problem, and that should lead you to a more specific system to troubleshoot.

J1708 Troubleshooting Shorted Wires

Let’s go back to the issue with no voltage on either wire. If that is your scenario, you either have a short between datalink wires or a short to ground on both wires. What you will want to do is unplug all modules, and check resistance between Pins A and B. Resistance should be OL. If your resistance is less than an open circuit, this means you have a short between J1708+ and J1708- datalink wires.

You should also check for a shorted wiring to ground on both Pins A and B. A short to ground on either wire could bring down the entire datalink bus. Unplug all modules, check from Pin A to E. Resistance should be OL. Also, check from Pins B to E. You want the same results, resistance should be OL.

9 Pin connector w/J1708

The J1708 protocol although much slower and not as robust as the newer J1939 protocol is still used by some manufacturers today in the industry standard 9 pin connector.  The good news is that now that you know how to troubleshoot the J1708 datalink on a 6 pin connector, the same troubleshooting steps can be used on the more modern 9 pin connector where J1708 is normally used on pins F and G.   The diagnostics will be the same as described above. 

Wiring Help

If you’ve narrowed your problems down to a wiring issue, you may need a wiring diagram to trace down and locate the bad circuit. Customers of Diesel Laptops can find this information in their Knowledge Base application, or by purchasing the DTC Solutions Software. We will also be providing wiring diagrams to clients on DieselWiring.com shortly.

Summary

Hopefully, you learned how to properly troubleshoot your J1708 databus after reviewing all this information. These are things we see every day in our technical support department. For customers of our tools that stay within their support, this kind of information is available via phone with live, trained diesel technicians waiting to serve you. Learn more at www.DieselLaptops.com.