CAN/Bus Issues: Following The Learning Curve
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CAN/Bus Issues: Following The Learning Curve

Several times last summer, I was confronted with a lack of product familiarity when diagnosing CAN/bus circuits on several late-model import platforms. The most frustrating part came when I had to spend hours educating myself on how a specific body control system worked by looking at wiring diagrams and going through trial-and-error diagnostics.


Several times last summer, I was confronted with a lack of product familiarity when diagnosing CAN/bus circuits on several late-model import platforms. The most frustrating part came when I had to spend hours educating myself on how a specific body control system worked by looking at wiring diagrams and going through trial-and-error diagnostics.


The good news for diagnostic techs is that on-board body control diagnostics are becoming far more comprehensive than in years past, which reduces diagnostic time. The bad news is that, due to the many safety-related and passenger comfort systems being added to current CAN/bus architecture or “topology,” the body control learning curve is becoming longer, especially when we begin servicing multiple nameplates.

CAN/bus systems aren’t new, especially among Euro manufacturers. Mercedes-Benz began using Controller Area Network (CAN) technology during the early 1990s. Many other manufacturers followed suit by introducing their own CAN/bus technology in the early 2000s. In 2008, CAN systems were mandated as standard equipment on vehicles manufactured in the U.S. to accommodate the many powertrain and electronic safety systems coming onto the market. Although CAN/bus systems have been around a long time, their reliability is such that we rarely see them in the service bay unless the vehicle has suffered collision damage or, worse still, wiring modifications done by an amateur technician. See Photo 1.

Photo 1: It’s much easier to diagnose a body control module by using a scan tool than by taking the vehicle apart.

At the very minimum, most entry-level platforms employ at least a dozen different modules that communicate with each other through multiple bus communications protocols. The more electronic “gadgets” a vehicle has, the more modules there are communicating with each other on the CAN/bus. When we get into high-end luxury platforms, we’re looking at as many as 60-80 different modules communicating on a half-dozen or more different CAN/bus protocols. Confusing it is, and confusing it should be. But, whether it’s electric, hybrid or gasoline-powered vehicles, we’re going to be looking at more body control diagnostics in the years ahead. So be prepared to extend your learning curve.

Why CAN/BUS Systems?

During the 1980s when auto manufacturers began adding passenger comfort and safety systems to their vehicles, it quickly became apparent that traditional hard-wired electrical systems were too bulky to fit into a modern automotive chassis. Engineers abandoned “hard-wired” electrical systems in favor of “multiplexed” or bus communications systems that use electronic modules to operate systems like exterior and interior lighting, door locks, power windows, heating/air conditioning, etc.


As the number of body control functions increased, engineers created bus communications systems that share information among modules. The anti-lock braking system (ABS) module might, for example, simultaneously share vehicle speed data with the engine, transmission, air bag and body control modules. While the sharing of data simplifies the topology of the bus communications system, it does create some confusion as to the origin of a specific bit of data.

Serial Data

To summarize the theory of bus communications, let’s say that electronically bundled data bits allow data from a specific module to be transmitted at very high rates in a sequence of digital on/off or high/low voltage signals. These data bundles can flow through a single data bus wire or a twisted pair of data bus wires. Each data bit package contains an identifier that allows each module to retrieve its own data from the datastream. Using the bundling method, multiple modules arranged in data bus “loop” or “star” configurations can communicate with each other at very high speeds on the same data bus wire or pair of wires. Fiber optic systems used on European nameplates operate in the same manner, with light pulses being used to transmit serial data. So far, so good. See Photo 2.

Photo 2: Breakout boxes that connect between the scan tool and diagnostic link connector (DLC) greatly simplify bus communications testing.

Tooling Issues

Since the operating protocols and electrical architecture or “topology” of each body control system varies widely among vehicle makes and models, tooling is becoming a major issue. The aftermarket has responded by offering a wide array of scan tools. Some manufacturer-specific aftermarket scan tools can nearly duplicate the OEM scan tool’s capability to display body control data, so don’t discount the ability of an aftermarket scan tool to fill your CAN/bus diagnostic needs.

That said, and aside from retrieving diagnostic trouble codes (DTCs), many general-purpose aftermarket scan tools can’t efficiently display all of the body control data needed for an in-depth diagnosis. In some cases, a “work-around” solution can be found, but requires more time and effort. A shop owner who wishes to provide bumper-to-bumper service for one or two nameplates will no doubt find OEM scan tooling more suitable for addressing the complete range of body control issues. Last, but not least, among tooling issues, OEM service information and diagnostic tables are usually written for OEM scan tools. If a shop is doing enough volume on a specific nameplate, the OEM scan tool might well pay for itself, thanks to less diagnostic time and shorter learning curves.


Module Programming Issues

Today, just about all body control modules must be “programmed,” “initialized” or “configured” to the individual vehicle. At the one end, aftermarket J-2534 “universal” programming equipment was originally designed to update calibrations on engine control modules (ECMs), which is why the ability of some “J”-box tools to program body control modules might be limited. At the other end, while OEM scan tool systems are simpler and much easier to use for body control diagnostics and programming, they’re also manufacturer-specific, which leads to vehicle specialization. See Photo 4.

Photo 4: Programming requires an OEM-spec battery in good condition to maintain system voltage. A “clear” OEM-spec battery charger is also required to provide a voltage supply that is free of electrical interference.

In either case, calibration downloads for reprogramming must be obtained from manufacturers’ websites, which bill by the hour, day and year. Some high-volume import specialty shops find it more economical to subscribe to a single manufacturer’s website by the year. Depending upon their frequency of use, low-volume import repair shops might find it more economical to purchase by the hour or day.


Speaking from experience, I believe that no single diagnostic technician can become an expert on each body control issue and that no single tool can serve a diverse import market. The fact that OEM tooling requirements can change every few years also adds to the cost of servicing body control issues on multiple nameplates. I therefore believe that, when addressing body control issues, smaller shops are better served when they invest their operating dollars and manpower hours into supporting a limited number of nameplates. That’s difficult to do, but it can be done if a shop presents itself as a one-stop solution for the owners of a particular import nameplate. Yes, marketing to a narrower customer base requires more effort, but the results are more rewarding in the form of reduced tooling costs and shorter product learning curves.



Getting Started With CAN/Bus Diagnostics

1. Since you’ll be measuring key-on, engine-off (KOEO) voltage, begin with a fully charged battery in a good state-of-health. All aftermarket electronics must be CAN/bus compatible. So always inquire if any aftermarket devices are installed on the vehicle (see Photo 3).

Photo 3: I use an inexpensive 8-amp electronic battery maintainer to keep the battery alive during key-on, engine-off diagnostics.

2. Use a scan tool for the initial diagnosis, which includes measuring battery voltage data from the ignition switch. Measure key-on at the battery terminals and compare that reading with battery voltage data displayed on the scan tool. Both readings should be within a few tenths of a volt.

3. Check all modules for communications issues and any DTCs. This “polling” or “pinging” process is usually automatic on newer imports. Record all DTCs in the scan tool memory and in a notebook. Consult service information for DTC-specific diagnostic procedures.


4. Worn ignition switches can cause intermittent “U-code” bus communications problems. Faulty connectors can also cause intermittent U-code issues. The four-digit number in a U-code indicates which module has the bus communications problem.

5. Test for worn ignition switches by comparing KOEO voltage at all key-on fuses with battery voltage. All key-on fuses should measure within 0.5 volts of battery voltage.

6. Multiple U-codes usually indicate a power or ground problem.

7. A module ground problem might be the cause of a bus circuit measuring higher than specified voltage.

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