By Dave Hobbs. Car audio? Been more interested in R.P.M.s than in watts R.M.S.? Better rethink that philosophy with today’s factory car audio and entertainment systems. The technology that used to be a handful of simple circuits like “power, ground, speaker and antenna” and totally independent of other electronic systems has given way to vehicles sprawling with wires and data busses interacting with the audio system.
Everyone knows that some of today’s so called “radios” are also brimming with features like voice recognition GPS Navigation, DVD Video, satellite digital radio, multifunction steering wheel controls and closed circuit TV for backing the vehicle up.
New Honda minivans now allow for cylinder cancellation to increase fuel economy. This process makes the engine a bit noisy so the factory audio system comes to the rescue by emitting an audio frequency that cancels out the sound waves created by this new engine feature. Pretty slick! And what about the new Saabs with their “Infotainment” system? “What the heck is ‘Infotainment” you ask? It’s a combination of entertainment system components like the radio, CD changer, satellite radio, GPS Navigation and palm pilot interface (all voice activated) that will allow you to keep in touch with the high tech world of multimedia entertainment and internet communications (all while you are driving I might add) thanks to a new communications fiber optic data bus boasting speeds well over 500 KPS…over 10 times faster than most of the data buses of the 1990’s. “Well” you say, “that’s just those blasted new luxury cars. We don’t get too many of those in our shop.” OK, how about a more down to earth example? On 2000 model year and newer Chevy Impala and Monte Carlos the radio and BCM are very interested in each other. The lack of either one communicating with the other will result in a U-codes (communication problems) and/or B-codes. (body electrical problems) Failing to properly run “Radio Setup” on one of these factory radios with the Vetronix Tech 2 scanner could not only cause the radio to display “LOCKED” and not play music due to the lack of the proper VIN being sent on the class 2 bus, but the oil change and tire store techs won’t be able to reset the Oil Life and Tire Inflation monitors because the radio does that too! And let’s not forget the fact that a lack of chimes (or a customer complaint of the chimes being too loud) on most 2003 or newer GM trucks and SUVs will send techs into the service manual’s radio diagnostic section because the radio produces the chimes through the left front door speaker!
Want more examples? Let’s look back into the 1990’s. The VSS (Vehicle Speed Sensor) circuit started being connected to GM radios over 10 years ago to allow for operation of the SCV (Speed Compensated Volume) feature. That means that the radio can be a cause for a powertrain DTC or be the cause of an inoperative speedometer or cruise control complaint. Since the 2000 model year, many GM radios have been tied to the class 2 data bus for this SCV feature. The radio reads the vehicle speed right off of the data bus just like your scan tool does. You go faster, and the radio turns up its volume automatically to compensate for the inevitable increase of passenger compartment noise. Since the radio is now connected to the class 2 data bus, this means that you could get class 2 problems from it too, and vice versa. If you’ve seen the AVI training video “F.R.E.D. Take The Bus” you know that the radio is just as an important of a player on the GM class 2 data bus as the instrument cluster, body control module, HVAC head, airbag module or PCM. Take one of the very extreme cases of data bus interaction mentioned in the “FRED” video as an example. I’ll call it the case of the haunted radio. The customer had a 2002 Chevy Malibu with the ever-common HVAC evaporator odor problem. You know, a little too much stagnant water from A/C condensation in the evaporator case and before you know it your car smells like a pair of worn out sneakers. The solution is to spray some cleaner/disinfectant across the evaporator coils and install an after blow module to run the blower motor for a short period after the ignition is shut off to dry out the evaporator case to prevent a comeback. So on this car, the after blow module was installed and everything worked fine. Or at least the tech thought so until the customer called back with an excited tone reporting, “ever since you fixed my smelly air conditioner, my radio acts like it’s haunted.” “Haunted?” the tech asked. “Yeah, we can be sitting in the house and all of the sudden we hear the radio playing in our car parked out in the garage…and there’s no one around it…and the key is not even in the ignition…it’s kind of spooky!” To make a long story short, what was happening was the blower motor was being turned on when the after blow module commanded it. When the module turned on, it isolated the factory blower motor circuitry from the vehicle and ran the blower directly with its added B+ power source. That part was fine. It was only when the after blow module turned off that we had a problem. When it turned off, it dutifully reconnected the blower motor circuitry back the way GM had wired it. The problem was, the blower motor was still spinning for a few revolutions after the after blow module switched it off. What does a blower motor do when it’s spinning without power applied to it? That’s right…it becomes a generator. This short period of time spinning to a stop created enough voltage to back feed power into the HVAC control head, and even further upstream to it’s fuse. That fuse was on a buss bar on the same fuse panel that shared a power strip with still yet another fuse. That other fuse was the fuse that sent ignition power to the BCM. So what was happening was that the BCM was going live (thinking the ignition switch was on) and sending a “wake up” message (via Class 2) to the radio whenever the after blow module stopped running the blower. A simple relay was added to the after blow module’s wiring harness and the problem was fixed. No more “haunted radio”.
The valuable lesson learned in that “Haunted FRED” story was how interconnected the audio systems on today’s vehicles have become with the rest of the vehicle. Stories like this one aren’t alone. I see them all the time on IATN and on the hotline I answer calls on. One recent case was a Malibu Max with a no start complaint because the radio was shorting one of the data busses. The BCM on the low speed bus couldn’t communicate with the VTD (Vehicle Theft Deterrent) in order to let the PCM (by passing that message along on the high speed GMLAN bus) know it had the right key in the ignition it to start. The radio was shorting that GMLAN low speed bus. Another strange one involved a Trailblazer that had its radio relocated to remote spot on the vehicle in order to have an aftermarket radio installed without removing the factory radio from the data bus. This is a common stereo shop trick. Every module was happy until there was another shorted bus due to water intrusion in the rear compartment where the factory radio had been relocated to. (FRED doesn’t swim well) Just when I thought I had heard it all, I received news from my friend down the street at the local Chevy dealership reporting a new problem. On late model trucks some BCMs are going to sleep (permanently) after being flashed because they didn’t see the radio active on the class 2 bus during programming. Again, aftermarket radios were the cause of those dead “FREDS”. And finally you, my friend will wind up with a few strange stories of your own. So remember to start building up you “car audio IQ”. You might just find yourself fixing your next drivability problem by replacing the radio!