Stick to what you know: Wiring diagrams complex as the trucks they document

Updated Mar 30, 2023
A black and white wiring diagram from Volvo.

Wiring diagrams trace the electrical system of the truck from beginning to end. Nowadays, they’re multipage, complicated road maps with specialized symbols for each manufacturer, and come electronically with diagnostic tools.

“In today’s vehicle, to go off the cuff on this stuff is really difficult to do,” says Duane Tegels, product marketing manager with Volvo Trucks North America.

Like many diagnostic tools, it’s critical to be trained on how to properly use a wiring diagram, Tegels says. Manufacturers each use their own set of symbols and color-coding and while some diagnostics companies supply generalized wiring diagrams, components are always changing.

Navistar officials say there are some industry standard symbols, but cautions each manufacturer displays their electrical systems a little differently. If the technician isn’t familiar with that manufacturer’s electrical systems, they could misread the diagrams and misdiagnose the problem, leading to extended downtime and the replacement of parts that weren’t broken.

Volvo’s Tegels also cautions against using diagrams that come from anywhere other than the manufacturer.

“The diagram could be 10 pages,” Tegels says, and if you’re using a PDF found online or from a source you don’t know, you might miss a page with critical information to your repair. “You’re going from the battery all the way to the component and it’s hit and miss sometimes.”

Correct, up-to-date wiring diagrams also should feature notes on different voltages and testing information. The connection points in particular are important, Tegels says, and the technician should know where they are and how to read where the grounds and splices are as well.

While updated diagrams are important to have, so are updated tools. Tegels says test lights aren’t really used in diagnosing electrical problems anymore due to the sensitivity to today’s circuits. Instead, technicians should use a digital volt ohm meter and make sure to consult that wiring diagram for the appropriate resistance values.

As far as the diagrams becoming more complicated for alternative-powered trucks, Tegels says the diagrams really aren’t any more complex than they already are, which is very, but technicians should be aware the voltages in alternative fuel trucks may be higher and more deadly. Especially in electric trucks.

The long and short of it, OEM experts say, is that while the premise for a wiring diagram — a map of a truck’s electrical system — is simple, the diagrams themselves are not. Using them correctly isn’t something for the shade-tree mechanic. It takes training in diagnostics, electrical theory and electrical systems to use modern wiring diagrams efficiently, avoiding downtime now and in the future.  

Learn how to move your used trucks faster
With unsold used inventory depreciating at a rate of more than 2% monthly, efficient inventory turnover is a must for dealers. Download this eBook to access proven strategies for selling used trucks faster.
Used Truck Guide Cover