Get any teenager their first car and the first thing they want to do is modify it with new wheels, a different exhaust, seat covers or, the most important and popular upgrade, the stereo. You know, when they have to get to school with all their buddies in the car, they’ve got to have those tunes. They can’t be seen around town with a dull factory radio or wimpy stock wheels.
However, on today’s modern import vehicles, adding on some accessories can leave them stranded in dad’s driveway. When it comes to installing that new stereo system or video system, a lot of “pre-thinking” needs to occur before reaching into that dash and ripping out the old one. That’s because in many of today’s cars the stereo system is tied into the CAN Bus line. This line allows communication to and from the Body Control Module (BCM) and other modules.
Some manufacturers will use the left front speaker as the warning chime speaker for all the necessary functions such as “door open-lights on,” “key in the ignition,” etc. The BCM will tell the stereo to produce the appropriate bell or “ding” to inform the driver of the immediate problem of which they should be aware. Some will keep the radio in check with the security systems; others incorporate the stereo and the climate control into one unit and display them together on a screen in the dash.
The problem comes down to the fact that the communication between these units needs to be there, or at least in some way recognized, so the CAN Bus line is not lost or shorted. It could lead to a service light on the dash or, even worse, a no-start condition.
WHAT IS CAN?
Just like your cable TV, as a signal is sent down the wire from one communication device, there needs to be another at the other end that can “descramble” that information and turn it into readable information. These “lines” are generally referred to as Bus lines, or Data lines.
Most of the time they are in pairs of wires that are twisted together (less radio frequency [RF] interference). Some manufacturers use a two-speed CAN.
One line is for low-priority information, such as radio, windows, etc. A second, faster speed line is for things like the transmission, theft devices, etc. Both systems move along the same wires at the same time.
Each of the “modules” on the Bus line use the information that they are programmed to read, and any other information on the Bus is ignored and not read by that particular module.
My advice when it comes to diagnosing power windows, gauges, or, for that matter, just about anything these days, is to get your scanner out and look for codes, look for a Class 2 serial data line, read the Mode $06 information, or whatever that particular manufacturer is calling its CAN line information.
In 1983, Bosch Corporation introduced the CAN system to the world as a precursor to what they saw as an increase in the automotive electrical system advancements. In 1987, the first CAN system was officially called “CAN,” but it wasn’t until 1992 that a Mercedes-Benz CAN system was accepted as the first true CAN system. Early Saabs had a system that could have been called CAN back in 1987, but the only references were to call the lines “data lines.” It still worked about the same way, but wasn’t diagnosed the same way as it is today.
In 1995, Saab introduced Class 2 serial data lines that run at a speed of 10.4 kbps. In 2004, GM went to its next generation system called GMLAN (local area network) on Saab vehicles, which had a two-speed system: low at 33.3 kbps and a high at 500 kbps. Mercedes-Benz uses several Bus lines; on one car I counted five different CAN speeds.
With the speed and flexibility of these electronic systems that manufacturers can create in today’s cars, I can only imagine how far all this information is going to go. At some point in time, that wiring will also be a thing of the past. Everything in the car could someday go completely wireless; modules will get smaller, faster and less likely to fail. Scanning could be done without even seeing the car in a repair shop. Just dial your cell phone to your shop of choice and a complete diagnostics could be done right then.
JAMMING THE CAN
Here’s an example that happened at my shop with a car that had a no-start condition. When checking the service codes, I found the Class 2 serial data (Bus) line showed one to be shorted to ground. Tracing down the shorting bar for the Bus line, you can then individually test each Bus line for a grounded signal. One glance at the dash and it wasn’t hard to tell it didn’t have the factory radio in place.
After tearing out the aftermarket radio system and locating the Bus wiring, the “chase” was on. Whoever installed the aftermarket stereo knew enough that the factory radio still needed to be hooked up in order for the door chimes to work. The factory radio was still wired in and was jammed under the back seat cushion, which is where I found a crushed stereo system case caused by someone sitting in the back seat.
As soon as the aftermarket radio was installed, there was still enough time for the vehicle owner to cruise down the street to show off for all his friends with the boom box cranked to max, and, of course, a car full of his buddies jamming to the music. Everything was fine until they stopped at the local hangout to look cool next to another car with the windows down and stereo still cranking out the latest tunes. When they all piled back into their “concert on wheels,” it wouldn’t start. That’s when it was towed to the shop where I had to find out what was wrong.
A stereo system is not the only thing that can cause this problem. But it’s sure the most common culprit. Theft systems that try to “piggy-back” the factory alarm system, GPS units, van conversions and aftermarket add-ons of all types can cause a problem when it comes to CAN systems.
These data information screens will give you the clues as to what to look for. The next stop is to go to your PC and look up the wiring diagrams. Codes are only a starting point. Remember, you still have to diagnose the cause of that code and what it means.
Be sure to read, follow and understand the circuits before you attempt to change or add something to these types of systems. And, if you get a no-start in the shop, don’t be afraid to ask the question…“So when did you install the stereo? It might be the “bling” in your diagnostics.
You Can CAN!
As great as multiplexing is, it can cause all kinds of strange problems when communications break down. If the system detects a loss of communication with a module, it will usually set a Diagnostic Trouble Code (DTC) to help you identify and isolate the fault. However, intermittent problems may or may not set a DTC, depending on their severity and duration.
A worse situation is when communications are distorted because of unwanted noise on the data Bus or because a module is generating a bad signal. This may cause other modules to ignore or misinterpret their instructions with unpredictable results. The vehicle may not start, it may stall for no apparent reason, lights may turn on or off, and accessories may operate intermittently. These types of problems may not set a DTC and could act as though the electrical system was possessed.
Spark plug wires routed too close to the wiring harness can sometimes generate noise. A defective alternator could create enough voltage variation to upset the operation of the system. Low battery voltage, loose or corroded connectors, poor grounds, shorts and opens will destroy the integrity of the data Bus circuit.
Tech Tip: If you have a vehicle with no serial Bus communications for any module on multiple serial Bus networks, look in the vehicle’s wiring diagram and/or theory of operation for a module that has a function known as a “gateway.” Simply put, if there is a module on more than one Bus and more than one Bus has problems, disconnect that module to see if the problem goes away.
Tech Tip: You can measure for the presence (or absence) of the 60 ohm resistors that terminate each end of a two-wire CAN Bus. Simply connect an ohmmeter between pins six and 14 of the DLC. Make sure the ignition is off and the Bus is not active (if in doubt, disconnect the negative battery cable) to ensure your ohmmeter can accurately measure resistance.
Tech Tip: The key to diagnosing multiplex electrical problems is to understand the system you’re working on, test for any data on the data Bus, measure the data Bus for voltage within specifications and isolate the fault by unplugging or swapping modules one by one until the fault is eliminated.
On connecting the oscilloscope to a data Bus wire and displaying the signal, you’ll see a digital wave pattern that changes in pulse width. The waveform confirms there is data on the Bus, however, the data can be meaningless because it’s in a binary code. If there’s no data on the Bus (a flat line), there is no communication and the module that generates the bias signal is most likely dead. Voltage and grounds for the device should be tested.
A good data Bus signal should have nice sharp corners, flat tops and bottoms, and vertical sides. Peaks, rounded corners, spikes and sloping lines are all indications of trouble because they distort the signal waveform. This can result from internal or external noise, poor connections, excessive resistance and voltage variations. The height of the waveform can be measured to test for proper voltage. It should be within the specified voltage range (typically 2.5 volts or 7.75 volts, depending on the application).
The same basic test can be made with a DMM, although due to the DMM’s speed of sampling a Bus waveform may cause voltage to vary greatly or only slightly depending on the meter.
A flat line on a lab scope checking a data Bus may indicate a shorted Bus. However, not every flat line scope pattern is a problem.
Delphi offers a four-hour training course for intermediate to advanced technicians, which will show you how to troubleshoot stubborn communications problems with electronic control modules. The course provides practical tips on the uses of meters, scopes, and factory and aftermarket scan tools are combined with theory and real-world case studies to demonstrate how to diagnose everything from U-codes to power mode masters to serial bus gateways.
Courtesy of Delphi Product & Service Solutions