E90 No Crank After Disconnecting Battery? OBD2 Won’t Connect? Diagnose and Fix

Experiencing a no-crank issue with your BMW E90 after disconnecting the battery can be frustrating. You turn the key, the lights come on, but all you hear is a single click, and the engine refuses to turn over. To make matters worse, you might find that your OBD2 scanner won’t connect to the vehicle to help diagnose the problem. This article, written by a BMW repair expert, will guide you through the diagnostic process to identify and potentially resolve this issue.

It’s crucial to understand that a “no crank” condition can stem from various underlying problems. Don’t assume there’s only one possible cause. Instead, systematically investigate the potential culprits, starting with the most likely scenarios. Modern BMWs, like the E90, rely on a complex network of modules and sensors, and pinpointing the exact fault requires a methodical approach.

The Car Access System (CAS) module plays a central role in starting your E90. It’s responsible for activating the starter solenoid, which in turn engages the starter motor to crank the engine. For the CAS to initiate this process, it needs to receive specific signals:

• Automatic Transmission (AT): The CAS needs confirmation from the brake light switch that the brake pedal is pressed and a signal (via the bus system) indicating that the shift lever is in Park (P) or Neutral (N).
• Manual Transmission (MT): The CAS requires a direct signal from the clutch switch confirming that the clutch pedal is depressed.

If any of these signals are missing or if there’s a fault in the wiring or components related to the CAS, starter, or battery connection, you could experience a no-crank situation, and potentially issues with OBD2 connectivity as well if the underlying electrical problem is severe.

Initial Troubleshooting Steps for E90 No Crank Issues

Before diving into complex diagnostics, perform these basic checks:

1. Battery Condition: While seemingly obvious, ensure your battery is properly connected and has sufficient charge. Disconnecting and reconnecting the battery, especially if it was old or weak, could highlight an existing battery issue. Use a multimeter to check the battery voltage. A healthy battery should read around 12.6 volts at rest.
2. Battery Terminals and Connections: Inspect the battery terminals for corrosion and ensure they are clean and tightly connected. Check the ground cable connection to the chassis as well. Loose or corroded connections can impede current flow and prevent the starter from engaging.
3. Fuses and Relays: Consult your E90 owner’s manual or a fuse diagram to locate the fuses and relays related to the starting system and the OBD2 port/diagnostic system. Check for blown fuses and ensure relays are properly seated. Pay particular attention to the CAS relay and the DME (Digital Motor Electronics) relay.

Advanced Diagnostics with BMW Diagnostic Tools

For more in-depth diagnosis, BMW-specific diagnostic software like INPA or ISTA is invaluable. These tools provide dealer-level diagnostics, allowing you to read fault codes, access live data, and perform component-level testing. While generic OBD2 scanners can read basic engine fault codes, they often lack the depth and BMW-specific functionality needed to diagnose complex issues like CAS or starting system problems. If your OBD2 scanner is failing to connect, it could indicate a power or communication issue that these tools can help pinpoint.

Step-by-Step No Crank Test Procedure

Perform these tests systematically and record your results to help narrow down the problem:

1. Vehicle Wake-Up: Open the driver’s door, sit in the driver’s seat, and close the door. The “P” light on the automatic transmission gear display should illuminate, indicating the car is waking up from sleep mode.
2. Terminal R Activation: Insert the remote key into the key slot. The radio should turn on, and the radio panel lights should illuminate. This confirms that Terminal R (Radio) is activated by the CAS.
3. Brake Light Check: Verify that the brake lights illuminate when you press the brake pedal. This confirms that the brake light switch is functioning and that Terminal R is powering the brake lights, sending a signal to both the CAS and FRM (Footwell Module).
4. Terminal 30g and Terminal 15 Activation: Without pressing the brake or clutch pedal, press and release the START button. The instrument cluster should light up, and all accessories (windows, A/C, etc.) should become operational. This indicates that the CAS has activated Terminal 30g “Accessory” Relay (a large black relay on the right of the Junction Box fuse panel) and Terminal 15 Ignition Relay (soldered to the Junction Box circuit board).
5. Gear Selector Signal (Automatic Transmission): If you have an automatic transmission, press the brake pedal and move the shift lever out of Park to Reverse (R) and Neutral (N). Check that the gear display on the lower instrument cluster (and the gear display light by the lever) changes accordingly. This confirms the BUS signal from the gear selector is being received.
6. Starter Solenoid Click Test: Press the brake or clutch pedal and then press the START button. If the starter does NOT crank the engine, have a helper listen near the rear of the intake manifold, above the left fender/wing, for a “CLICK” sound from the starter solenoid as you press the START button. Note the sound(s). A single click might indicate the solenoid is engaging but the starter motor is not receiving power or is faulty. No click could suggest a problem with the CAS, wiring to the solenoid, or the solenoid itself.

Reporting Your Test Results

To get further assistance, be prepared to provide the following information:

• Last-7 Characters of your VIN (Vehicle Identification Number): This is crucial for identifying the correct wiring diagrams and component locations specific to your E90 model year and build date, as there were changes during the 2007 model year.
• BMW Diagnostic Software: Indicate if you have access to INPA or ISTA BMW diagnostic software.
• Windows Laptop: Do you have a Windows laptop available to run the diagnostic software?
• Multimeter: Do you have a multimeter for electrical testing?
• Scan Tool Make & Model: If you have a scan tool, provide the make and model. Can it read live data or parameters from the CAS module?

Next Steps

Based on your test results and the information you provide, further diagnostic steps can be recommended. This might involve:

• Checking for fault codes using INPA or ISTA.
• Inspecting wiring and connectors related to the CAS module, starter, and battery.
• Testing the starter solenoid and starter motor.
• Verifying power and ground circuits to the CAS module and starter.

Professional Assistance

If you are not comfortable performing these diagnostic steps yourself, or if you need immediate vehicle operation, it’s recommended to have your E90 towed to a qualified BMW independent repair shop or dealership. Attempting to diagnose electrical issues without proper knowledge and tools can potentially lead to further complications.

By following these diagnostic steps systematically, you can increase your chances of identifying the cause of your E90 no-crank issue and get your vehicle back on the road. Remember to prioritize safety when working with electrical systems and seek professional help when needed.