| This dashboard didn't just puke its guts out trying to swallow some aftermarket solution that didn't sit well with it. Just another day at the office for Rene Ortiz as he installs Viper's 791XV.
Integrating an aftermarket security system into a new vehicle can be like mixing milk and coke: You can do it, but it'll leave a bad taste in your mouth. Because, unlike a stereo system, which simply needs power from the vehicle's charging system, a security solution has to interact directly with the vehicle's electronic infrastructure: automatic door locks, trunk release, ignition, keyless entry, and anything else that a user might want to automate. That used to be a fairly straightforward proposition. An installer would use his knowledge of basic electronics and a wiring diagram to hardwire a security system's brain and other components into a vehicle, and it worked. Now, with the automakers' databus approach, computers control a vehicle's subsystems by speaking a proprietary language over the same wires whenever possible. Plus, there's that pesky immobilizer system that's just as much an anti-convenience as it is anti-theft. So what was once a matter of determining voltage or polarity in order to make a vehicle dance at the touch of a button is now as frustrating as hacking into the Pentagon's mainframe
It's not as though the automakers are doing this in order to thwart aftermarket meddlers. They're just trying to save materials, weight, and-ultimately-money by maximizing the efficiencies of computerization and multiplexing. And it's up to the aftermarket manufacturers and installers to keep up
A while ago, Directed Electronics installed a Viper 791XV into the Chevy Avalanche owned by veteran photographer Peter Linney, whose photos often grace our pages. We won't go into every single aspect of installing the Viper because much of what you would see are photos of Directed Electronics technical support specialist Rene Ortiz lying on the front seat of the Avalanche with his face stuffed under the dash. Therefore, we will focus on the narrow topic of how this product incorporates some technologies designed to get around GM's electronic pitfalls
The 791XV is a two-way remote start and security system that uses extreme-range, 66-bit, code-hopping responder technology. But the Viper needed a little help to work well with the GM system, so Directed whipped out two extra modules for the install: the 456G and 555L
The 456G '03 GM Fullsized Databus Interface is essentially a little black box that cracks the code on Hummer H2s and Cadillac, GMC, and fullsized Chevrolet trucks. A single databus connection allows the Viper system to arm and disarm, bypass the dome light, and to control the door locks. To accomplish this, Directed engineers studied the 56-bit digital "words" used by the vehicles' onboard computers and transmitted on the J1850 databus, which is the OBDII protocol used by GM vehicles. Here's an example of what happens when Peter presses the keyless entry button on his keyfob: Commands are sent from the Viper's brain via the door trigger wire to the 456G, which translates the analog command into a digital command word data string that is 56 bits long. That data string is then transmitted on databus at the specific frequency of 10.4KHz, the frequency the vehicle computer recognizes as the command to unlock the doors. The 456G implements bit collision detection to keep commands from multiple nodes from interfering with each other. And there you have it, a databus interface that's the equivalent to the easily understood international language of love.
Another OEM integration issue has to do with what are called immobilizer systems. Common to many factory anti-theft systems, one of the current GM versions is called Passlock II. Passlock is based on VATS (vehicle anti-theft systems), which GM introduced in 1984 in the Corvette. When a key with an embedded resistor pellet is inserted into the ignition cylinder, the VATS module measures the resistance of that pellet via contacts in the lock cylinder. If that resistance matches one of 15 possible values, then the vehicle starts
Passlock I and its successor Passlock II superceded VATS almost 10 years ago. Passlock works in a similar way to VATS, but the resistor has moved from the key to the ignition switch. Therefore, the key itself has no special properties. A well-equipped thief could carry 15 VATS keys with all 15 of the possible resistor codes. But VATS is considered to be a better anti-theft concept than Passlock, because a thief could force a locked ignition cylinder to turn, and that's as good as a key to Passlock, so it gives the go ahead, and the vehicle starts. Why did GM switch to Passlock? Apparently, wear and tear on the VATS key can lead to a bad connection between the resistor in the key and the VATS module, so Passlock is what Peter had to deal with on his Avalanche
Unfortunately, your typical remote start, including the one on the Viper, won't work on this setup without a little help. What you need is something like the 555L. Normally, when Peter inserts his key into the ignition and turns it, an analog voltage code is sent to the Passlock II module, which compares it to the most recently learned voltage code. If they match, the module tells the PCM to enable the fuel injectors. But if Peter tries to start his Avalanche remotely, the 555L module remembers the code used during the last one keyed so that the Passlock II module thinks that everything is just fine. The 555L only bypasses Passlock II during remote starts, so the vehicle retains the factory anti-theft deterrent, and OE and aftermarket can walk hand in hand into the sunset.
