Encountering a situation where your scan tool refuses to communicate with your 2005 Chevy Cobalt can be frustrating. If you’re using a scan tool and it’s failing to connect to the high-speed GMLAN (General Motors Local Area Network) modules in your Cobalt, this guide is for you. This issue often indicates a problem within the vehicle’s communication network, preventing you from reading diagnostic trouble codes (DTCs) and assessing your car’s health.
Several factors can contribute to a scan tool communication failure with high-speed GMLAN systems. During normal operation, your Cobalt’s modules constantly exchange information over this network. A disruption in this communication pathway is usually flagged by the system, potentially setting DTCs like U0001 (High Speed Communication Bus Malfunction) or U2100 (Lost Communication with Vehicle System). These codes are often stored as history, meaning they might not be currently active but indicate a past or intermittent issue.
To understand why your scan tool isn’t connecting, it’s helpful to know about the GMLAN system’s design. The high-speed GMLAN network relies on two 120-ohm terminating resistors to ensure proper signal transmission. These resistors are positioned at each end of the communication link, one near the Data Link Connector (DLC) where you plug in your scan tool, and the other at the engine management module, which could be the Engine Control Module (ECM) or Powertrain Control Module (PCM). In many cases, the resistor at the DLC end is integrated within the Electronic Power Steering (EPS) module.
When diagnosing a potential short circuit in the high-speed GMLAN (+) and (-) circuits, a specific resistance reading can offer clues. With the engine management module disconnected, a reading of approximately 120 ohms across the high-speed GMLAN (+) and (-) circuits is considered normal. However, for a comprehensive analysis, it’s recommended to temporarily disconnect the Body Control Module (BCM) along with all other modules and the scan tool from the network. After disconnecting these components and the DLC terminating resistor, a normal reading across the high-speed GMLAN bus should be infinite ohms, indicating no short circuit. Remember to reconnect the BCM after testing, as it’s crucial for the vehicle’s power management system.
A complete failure of the high-speed GMLAN serial data circuits will prevent the engine from starting when it’s not already running. The following conditions are typical culprits for a total loss of high-speed GMLAN communication:
- Short Circuit Between GMLAN Circuits: A direct short between the high-speed GMLAN (+) and high-speed GMLAN (-) wires.
- Short to Ground or Voltage: Any of the high-speed GMLAN serial data circuits shorting to ground or to a voltage source.
- Internal Module Malfunction: A fault within a module itself that causes a short to voltage or ground on the GMLAN circuits.
Distinguishing between a partial and total malfunction of the high-speed GMLAN serial data circuits is important as it dictates the diagnostic approach. The presence of a history DTC U2100 might be recorded but isn’t necessarily the root cause of a current communication issue. Your scan tool connects to the GMLAN serial data high and low circuits via terminals 6 and 14 of the Data Link Connector (DLC) respectively. Ensuring these connections are clean and secure is a basic first step in troubleshooting.
In conclusion, if you are facing a “2005 Cobalt scan tool won’t connect” problem, systematically checking for shorts, inspecting module connections, and understanding the GMLAN network layout are key steps. By following these diagnostic pointers, you can effectively troubleshoot the communication breakdown and get your scan tool back online with your Chevy Cobalt.
