How to Get Your Scanner to Communicate with Your Car Without a Key

Encountering issues when trying to connect your OBD2 scanner to your car, especially when you’re not sure if the ignition key is the problem? This guide will walk you through essential steps to diagnose the communication pathway between your car’s computer and your scanner, even when you suspect key-related issues. We’ll focus on checking the fundamental electrical connections that enable your scanner to talk to your vehicle’s systems.

Checking CAN Bus Resistance

The Controller Area Network (CAN bus) is the communication backbone of your car. A proper resistance reading here is the first step in ensuring the physical integrity of this network.

1. Disconnect the Battery: For safety, always start by disconnecting the negative terminal of your car’s battery. This prevents any accidental electrical shorts during testing.
2. Locate the DLC (Diagnostic Link Connector): This is usually found under the dashboard on the driver’s side. It’s a 16-pin connector.
3. Measure Resistance: Using a multimeter set to Ohms, check the resistance between pin 6 (CAN High) and pin 14 (CAN Low) on the DLC.
4. Expected Reading: You should ideally find a resistance of approximately 60 Ohms. This indicates the CAN bus termination resistors are correctly in place and the wiring is likely intact. If you find significantly different readings (like very high or very low resistance), it suggests a problem within the CAN bus wiring or a connected module.

Verifying CAN Bus Voltage

With the CAN bus resistance checked, the next step is to verify the voltage levels when the car’s ignition is turned on. This confirms power delivery to the CAN system.

1. Reconnect Battery and Turn Key ON: Reconnect the battery negative terminal. Turn your car key to the “ON” position (do not start the engine).
2. Check CAN High Voltage: Using your multimeter set to DC Volts, measure the voltage at pin 6 (CAN High) of the DLC. A common and easily accessible point to check voltage on a CAN module is often the power steering pump. Look for a pink wire, which is frequently the CAN High wire.
3. Expected CAN High Voltage: You should expect to see around 2.75 Volts on the CAN High wire.
4. Check CAN Low Voltage: Similarly, measure the voltage at pin 14 (CAN Low) of the DLC. A blue wire is often used for CAN Low.
5. Expected CAN Low Voltage: You should expect to see around 2.25 Volts on the CAN Low wire.
6. Alternative DLC Check: If accessing module wires is difficult, these voltage checks can also be performed directly at the DLC connector pins 6 and 14.

Proper CAN bus voltages are crucial for scanner communication. Deviations from these expected values can pinpoint issues within the vehicle’s network.

Testing ECM Power and Ground

If the CAN bus checks are within range, the next critical area to investigate is the Engine Control Module (ECM) power and ground supply. The ECM is the central computer, and without proper power and ground, it cannot communicate.

1. Access ECM Connectors: Locate the ECM, usually in the engine bay or under the dashboard. You’ll need to access its wiring connectors.
2. Back-Probing for Connectorized Checks: Ideally, use back-probing techniques to measure voltage without disconnecting the connector. This involves inserting a thin probe alongside the wire into the back of the connector. If back-probing is not feasible, carefully identify wire locations.
3. Voltage Drop Testing for Power: To accurately test power, use a voltage drop test. Connect the positive probe of your multimeter to the positive battery terminal. With the ECM connector plugged in and the key in the “ON” position, use the negative probe to check for voltage at the ECM power supply wires. You are looking for a voltage drop as close to 0V as possible, ideally less than 0.100V. A higher voltage drop indicates excessive resistance in the power circuit.
4. Voltage Drop Testing for Ground: Similarly, test the grounds. Connect the negative probe of your multimeter to the negative battery terminal. With the ECM connector plugged in and the key “ON”, use the positive probe to check for voltage at the ECM ground wires. Again, aim for a voltage drop close to 0V.

Consistent and clean power and ground are essential for the ECM to function correctly and communicate with your scanner. Problems here are common reasons for scanners failing to connect.

Conclusion

These diagnostic steps provide a solid foundation for troubleshooting scanner communication issues, even when you are considering key-related problems. By methodically checking CAN bus resistance and voltage, and verifying ECM power and ground, you can isolate many electrical faults that prevent your scanner from linking with your car’s computer. Remember to consult your vehicle’s wiring diagrams for specific wire colors and locations for accurate testing. If you identify any wiring issues, ensure to properly repair and seal any wire damage to prevent future problems. If these checks do not resolve the communication issue, further, more in-depth diagnostics may be required, potentially involving professional automotive technicians.