The BMW 635csi, a classic grand tourer from the E24 generation, is renowned for its stylish design and robust performance. However, like any vehicle of its age, 635csi models can encounter mechanical issues. One common problem that can prevent your 635csi from starting is a lack of spark. If your engine cranks but fails to ignite, a no-spark condition is a prime suspect. This guide, drawing from real-world troubleshooting experiences, will walk you through the steps to diagnose and potentially resolve a no-spark issue in your beloved 635csi.
Understanding the No Spark Condition in a 635csi
A “no spark” situation means that the engine is not receiving the necessary electrical spark at the spark plugs to ignite the air-fuel mixture in the cylinders. Without this ignition, combustion cannot occur, and the engine will crank but not start. Several components within the ignition system of your 635csi can contribute to this problem. From simple issues like a faulty ignition coil to more complex problems with sensors or the ECU (Engine Control Unit), a systematic approach is crucial for effective diagnosis.
Step-by-Step Troubleshooting for Your 635csi No Spark Problem
Before diving into complex diagnostics, it’s essential to rule out the basics. Here’s a step-by-step approach to pinpoint the cause of a no-spark condition in your 635csi:
1. Verify Fuel Supply
While this guide focuses on spark, it’s always wise to quickly check fuel delivery. As mentioned in a forum discussion by a 635csi owner, ensuring good fuel flow is a preliminary step. Listen for the fuel pump when you turn the ignition key to the ‘ON’ position. You can also cautiously check for fuel pressure at the fuel rail, but always prioritize safety when dealing with fuel systems. If fuel delivery is not the issue, proceed to investigate the ignition system.
2. Check the Ignition Coil and Related Wiring
The ignition coil is responsible for generating the high voltage needed to create a spark at the plugs. A faulty coil is a common culprit in no-spark scenarios.
- Initial Coil Check: As one owner initially did, trying a new coil is a straightforward first step. While it might not always be the solution, it’s an easy component to test.
- Power to the Coil: Use a multimeter to verify that the wires going to the coil are receiving power when the ignition is turned on. Specifically, check for voltage at terminal #15 on the coil with the ignition in the ‘ON’ position. Also, ensure terminal #1 is properly grounded.
3. Inspect the Distributor Cap and Rotor
The distributor cap and rotor are crucial for distributing the high voltage spark from the coil to the correct spark plugs in the firing order.
- Visual Inspection: Remove the distributor cap and inspect it for cracks, carbon tracking (black lines indicating spark leakage), or moisture. Check the rotor for damage or corrosion.
- Replacement: As seen in the troubleshooting process of a 635csi owner, replacing the rotor and cap can be beneficial, especially if they are old or show signs of wear. These are relatively inexpensive parts and are part of regular maintenance.
4. Examine Relays – Main and Fuel Pump Relays
Relays are electrical switches that control power to various engine components. The main relay and fuel pump relay are essential for engine operation.
- Relay Function: Confirm that the main relay is clicking when the ignition is turned on, indicating it’s likely functioning.
- Voltage Check: Verify voltage at pins #86 and #30 on the main relay socket.
- Load Testing/Substitution: As suggested by an experienced member in the forum, simply checking relays with a multimeter might not be sufficient. Relays can fail under load. Try substituting the main relay with a known good relay or temporarily bypass it with jumper wires (with caution and proper knowledge) to rule it out.
5. Investigate the ECU (Engine Control Unit)
The ECU is the brain of the engine management system. While ECU failures are less frequent, they can cause a no-spark condition.
- Visual Inspection: Check the ECU connections for corrosion or damage.
- ECU Replacement (Testing): In some cases, as a troubleshooting step, owners have tried swapping the ECU with a spare known good unit to see if it resolves the issue. However, this should be done cautiously and with the correct ECU for your 635csi model year.
6. The Crucial Role of Speed and Reference Sensors
For Motronic engine management systems found in the 635csi, the speed and reference sensors are critical for ignition and fuel injection timing. These sensors tell the ECU the crankshaft position and speed. If these signals are missing, the ECU will not trigger spark or fuel injection.
- Sensor Location: The speed and reference sensors are typically located near the flywheel or crankshaft pulley. In the 635csi E24, they are often found near the bell housing.
- Resistance Testing: Use a multimeter to check the resistance of both sensors. A forum user found that their sensors showed no resistance, indicating a failure. Consult a repair manual for the correct resistance values for your 635csi model year.
- Sensor Replacement: If the sensors fail the resistance test, replacement is necessary. It’s important to note that the speed and reference sensors for the 635csi (part number 12141708619) are often the same part, simplifying replacement, as confirmed by forum experts. Given their age and critical function, replacing both sensors is often recommended when one fails, especially if they are original.
An engine bay of a BMW 635csi E24, showcasing the classic inline-six engine where many ignition components are located.
7. Consider the Crank Position Sensor (Pulse Generator)
While the front crank position sensor (pulse generator) in the 635csi, part number 12521287784, is primarily for diagnostic purposes and not essential for the engine to run, a faulty signal could potentially interfere with the Motronic system in some scenarios. Although less likely to be the primary cause of a no-spark issue, it’s worth considering, particularly if other sensor checks are inconclusive. One owner in the forum replaced this sensor along with the speed and reference sensors and successfully started their 635csi.
Key Takeaways and Solutions for 635csi Spark Issues
Diagnosing a no-spark condition in a BMW 635csi requires a methodical approach. Here’s a summary of the key areas to inspect:
- Start with the Basics: Fuel supply, fuses, and easily accessible ignition components like the coil, distributor cap, and rotor.
- Electrical System Checks: Verify power to the ignition coil and injectors. Test relays, especially the main and fuel pump relays, under load if possible.
- Sensor Importance: Prioritize testing the speed and reference sensors. These are often the root cause of no-spark issues in older Motronic systems.
- Systematic Approach: Work through the troubleshooting steps logically. Don’t overlook simple solutions before moving to more complex components like the ECU.
In conclusion, if your BMW 635csi cranks but doesn’t start, systematically investigating the ignition system is crucial. By following these steps, starting from basic checks to sensor and ECU evaluations, you can effectively diagnose and address the no-spark issue, potentially bringing your classic 635csi back to life. Remember to consult your BMW 635csi repair manual for specific procedures and values relevant to your model year.
