Can This Jeep Start?

Posted 1/14/2010
By Keith LeMay

Understanding the type of communication system on a vehicle is essential in the process of diagnosis and repair.

I recently received a call on a 2005 Jeep Grand Cherokee Laredo with a 3.7 V6 with a no-start condition. This vehicle has a controller area network (CAN) communication system. This serial bus system was developed by Bosch in the early 1980s and has been used in automotive applications for more than 20 years. Chrysler first introduced a partial (inter-module) CAN bus in 2003 on the Grand Cherokee Export and the DR trucks with a Cummins diesel engine.

When the technician tried communicating with the system using his scan tool, he found that it wasn't CAN-compatible, so he borrowed a scan tool from another shop. Once the scan tool was hooked up, he found he could communicate with some modules but not with others.

This vehicle has two systems: CAN C and CAN B. He could get all the data and talk to all the modules on the CAN B network but nothing on CAN C. These two systems are similar in that they communicate over two wires but are different in many ways. The CAN B network is a medium speed system and is fault-tolerant. If one of the wires is cut, shorted to ground, shorted to power, or if the wires are shorted to each other, the network will switch and communicate over one wire.

As long as one of its conductors has a potential difference from vehicle ground, CAN B continues to operate and communication is possible. There will be no "Check Engine" light or loss of any communication, and unless a scan tool finds fault codes for the CAN B system, no one will be aware of a problem. Each of the CAN B wires has different voltages. The CAN B positive (+) has 4.5 volts and the negative (-) has 0.5 volt.

The CAN C is the high-speed bus network and is not fault-tolerant. If any wire gets cut, shorted to ground, shorted to power, shorted to each other, or if a module shorts out, the modules on the CAN C will not communicate with each other or to the central gateway computer, which is the front control module on this vehicle.

The CAN C bus only functions with the key "on" and transfers data at real-time rate. The CAN B network cannot transfer data as quickly, but can remain active with the ignition "off" if individual modules require it to be active.

On this vehicle, there is a CAN C diagnostic circuit. This is the circuit the scan tool uses to communicate with the central gateway computer (the front control module), which in turn communicates with the CAN C and B.

In this case, we knew the CAN C diagnostic and CAN B systems were working but the CAN C system was down. On the CAN C, there are two ways of checking the operation. The first is to check voltage on the bus wires. Both should individually have between 2.4 to 2.6 volts with the key on. The second is to check the termination resistance by measuring the resistance across the two bus wires with the battery disconnected. There should be approximately 60 ohms of resistance, where the CAN B's resistance cannot be measured.

While checking the bus network on the Jeep, we found 0.5 volt on both bus wires, indicating a wiring or a module problem. The next step was to check the termination resistance. On these systems, there are two dominant nodes or modules. Each has 120 ohms of resistance so when they are in the same network, they bring down the total resistance to approximately 60 ohms. With the battery disconnected, checking the resistance showed 110 ohms. This pointed to one of the dominant modules because on the modules that aren't dominant, their termination resistance is only 5 to 10 ohms each, and they only drop the resistance by a few ohms.

The dominant nodes on the Jeep were the central gateway (front control module) and the powertrain control module (PCM). I asked the technician to unplug the PCM to see if the voltage went to 2.5 on the bus wires (with the battery connected and the key "on") and if the resistance would go to about 120 ohms with the PCM and battery disconnected.

After unplugging the PCM, the voltage did return to 2.5 volts and the resistance was 120 ohms, so we tested the PCM powers and grounds and found them to be good. Once the PCM was replaced (under warranty), the vehicle was back on the road. Without knowing the type of communication system or how to check it, this process could be time consuming and expensive - with many misdiagnosed parts installed.

Keith LeMay is an Identifix Chrysler specialist. He is an ASE master with L1, A/C recovery and recycling certifications.

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