“Symptoms might range from no-start to malfunctioning light systems. It is impossible to predict how the vehicle will react to a CCD problem because it varies from vehicle to vehicle, and depends on what is causing the CCD failure.”

Chrysler's Collision Detection (CCD) multiplexing system allows all connected modules to communicate with each other.

Symptoms might range from no-start to malfunctioning light systems. It is impossible to predict how the vehicle will react to a CCD problem because it varies from vehicle to vehicle, and depends on what is causing the CCD failure.

Most of the time, the bias voltage is produced from the body control module (BCM) and is terminated with the powertrain control module (PCM).

If you know which modules are used for biasing and termination, you can unplug them to see if their internal circuits are OK. On a Grand Cherokee, for example, the two modules would have a resistance of 120 ohms when checked unplugged with an ohm meter across the CCD circuits. This would be good for doing a quick check on the bias and termination modules, but that's all.

Otherwise, what really matters is that the CCD has the correct voltage, which in theory is 2.51 for the CCD minus and 2.49 for the CCD plus. This sounds backward, doesn't it? The key words are “in theory.” Whenever the system is powered up, there will be communication on the wires, which causes a slight variance in voltage, so chances are you will never see this exact voltage. But you should see about 2.5 volts on each, with about two-tenth's voltage variation. Since these two wires are carrying such low voltage and even lower amperage flow (.006 amps), there is a potential for the system to be affected by electro-magnetic interference. To prevent this, the wires have been twisted together with one twist for every 1.75 inches. To check power, a digital volt/ohm meter (DVOM) will do the job as well as a labscope since the only concern is that the voltage stays around 2.5 volts on each wire.

I worked on a 1998 minivan with a start and stall problem. After hooking up the scanner to verify the theft alarm system was not allowing fuel, I was surprised I could communicate with the modules despite the CCD being the problem. But the PCM was not receiving the OK to start messaging from the BCM. I checked the bus voltages, which were out of range (though not drastically). Voltage on both wires was steady at 2.8 volts. I was skeptical because it was borderline, and decided to test the system since the BCM was seeing the “unlock” switch state from the door lock cylinder and wouldn't tell the PCM to allow fuel.

It turned out that the overhead console was raising the voltage above normal, and when I unplugged it, the voltage dropped back down to 2.5 on each wire and the vehicle started. I verified this by plugging the overhead module back in and resetting the alarm. The car wouldn't start so I repeated the process several times, with the same results. I replaced the module and that fixed the problem.

Normally, with a bus problem, there are multiple problems and no communication with any modules other than the PCM. The PCM communicates with the scanner on a separate pair of wires called the “SCI” and “SCI Receive” - unless the PCM is the problem.

To test the CCD, check the voltages on the two wires. If they are out of range, simply start unplugging modules until the voltage returns to normal. Make sure the biasing module is the last module unplugged, which almost always is the BCM. If the vehicle doesn't have a BCM, then the voltage could be from the PCM or cluster or central timer module - but you still need to unplug it last. Test the two wires for shorts to power, ground, each other, or opens. If the wires are OK, then the last thing to check is the biasing module connector for pushed or damaged pins, and then powers and grounds.

Todd Plourde is an IDENTIFIX DaimlerChrysler specialist with 11 years of diagnostic repair experience.

This information is provided by IDENTIFIX®. IDENTIFIX® resources cut diagnostic time and provide repair solutions that increase the shop's bottom line. From Repair-Trac pattern failure quick fixes, to Diagram-Online wiring diagrams by fax, to the Repair Hotline staffed by 32 master techs who specialize in diagnosing complex problems by phone or fax, IDENTIFIX® helps techicians fix more cars in less time. For more information on IDENTIFIX, call (800) 288-6210, 8 a.m. - 6 p.m. Central Time. www.identifix.com. © 2008 IDENTIFIX. All Rights Reserved. © 2002 IDENTIFIX. All Rights Reserved.

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