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Avoid Getting Lost on Electronic Highways

By on March 1, 2019

By: Homer Hogg,
Director, technical service

Understanding Twister Pairs, Datalinks & Fragmented Messaging

Compared to the simplicity of 30, 20 or even 10 years ago, the control systems you’ll encounter on new trucks have drastically changed. Now, most truck systems are electronically controlled. This not only changes the way a driver might add accessories to a truck, but it also changes the way technicians diagnose and repair today’s electrical and electronic systems. 

You may recall, as I do, how easy it was to add an electrical component like a CB radio or a dash fan. Today, that could be a serious challenge if the truck manufacturer hasn’t provided an accessories outlet in the dash. In order to reduce weight and improve efficiency, design engineers moved away from electrical circuits to electronic circuits. While reducing the number of wires and components on a vehicle, tasks that were previously very simple can now be very complex.

Twister Pairs Form Datalinks

If you look under a dash panel on a new truck, you’ll very quickly see twister pairs of wires routed throughout the vehicle. These twister pairs are called datalinks or electronic highways. Computers use these electronic highways to send and receive messages traveling across the network. Other computers or components connected to the network pick up these messages and use the transmitted data to make decisions.

For example, the A/C computer responds when the driver simply turns on the A/C control, sending a message to the A/C compressor to engage the A/C compressor clutch. This message may be automatically delivered to another computer that will give the command to the A/C compressor in the form of voltage. In fact, this message may go through a device called a gateway, which will convert the message to a language that other computers can understand before it gets routed to the A/C compressor. Are you following this sequence or are you getting lost?

The Dangers of Fragmented Messaging

The problem we see today is conditions may arise that can fragment the messages moving on the datalinks. If a datalink has corrosion, the message can be corrupted, resulting in poor performance of the affected truck systems or, worse yet, some systems may not function at all. If a component or a computer connected to a datalink develops an electrical short, it may negatively impact some or all components attached to that datalink.

Recently, I received a call from a technician who had a truck that came in with a blown fuse in the electronic logging device. After the technician replaced the blown fuse, the transmission would not come out of 1st gear. Since the truck had an automatic transmission, it had a transmission computer. The transmission computer and the electronic logging device (ELD) were on the same datalink.

After hooking up a diagnostic computer and reading the codes, we saw that the transmission triggered a code indicating that the datalink communication update rate was not correct. We unhooked the ELD and all of the problems disappeared. Because of a short, the ELD had taken down the entire datalink.

Overcome Challenges

Typical of modern trucks, this can be a perplexing challenge to those who drive them and the technicians who diagnose problems and repair them. You can no longer just wire a new component into the truck, nor can a technician grab a test light and locate the problem. If the problem is related to a given computer or the network, the technician must hook up a computer to the truck’s diagnostic datalink and begin assessing the communications or the flow of data on the electronic highway.

If a driver wants to add an accessory, he or she must use the manufacturer’s designated circuit and recommended connecting procedure. Never, ever splice into a datalink! This seemingly logical and easy connection may cause communication errors and could permanently damage the datalink.

So, before you add an accessory, be sure to read the truck manufacturer’s service manual or request assistance from an experienced technician at a TA Truck Service location. Relying on a professional to help you decide the best method will prevent you from getting lost on the electronic highway. In turn, you’ll keep your truck running reliably while pocketing more of your hard-earned money. 

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