Experiencing a no-spark issue in a classic Bmw 635csi can be frustrating, especially when the engine cranks vigorously but refuses to start. This problem, common in older vehicles like the BMW 635csi, often stems from issues within the ignition or fuel delivery systems. This guide, inspired by real-world troubleshooting experiences, will walk you through diagnosing and resolving a “cranks but no spark” condition in your BMW 635csi.
Understanding the Problem: Cranks But No Spark in Your BMW 635csi
When your BMW 635csi cranks but fails to start, and you suspect a lack of spark, it indicates that the engine is turning over mechanically but not receiving the necessary spark to ignite the fuel-air mixture in the cylinders. This can be due to a variety of electrical component failures or sensor malfunctions within the ignition system of your BMW 635csi.
Initial Checks for Your BMW 635csi: Fuel and Basic Electrical
Before diving deep into the ignition system, performing basic checks can save time and effort. In the case of our BMW 635csi, the owner initially confirmed good fuel flow, which is a crucial first step. Here’s a checklist for your BMW 635csi:
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Fuel Supply: Ensure fuel is reaching the engine. Listen for the fuel pump priming when you turn the ignition key to the “ON” position. You can also check fuel pressure at the fuel rail if you have the tools. In this case, fuel lines were pulled and cranking confirmed fuel flow, eliminating fuel delivery as the primary issue.
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Fuses: Inspect all fuses related to the engine management system. A blown fuse can cut power to critical components. The owner of this BMW 635csi confirmed all fuses were intact.
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Battery Voltage: A weak battery can cause various electrical issues. Ensure your battery is properly charged and providing sufficient voltage.
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Wiring and Connections: Visually inspect wiring for any obvious damage, corrosion, or loose connections, especially around the ignition coil, distributor, and ECU (Engine Control Unit).
Diagnosing the Ignition System of the BMW 635csi
With basic checks completed, the next step is to systematically diagnose the ignition system of your BMW 635csi. The troubleshooting journey for this particular BMW 635csi focused on the following components:
Checking the Coil and Relays in your BMW 635csi
The ignition coil is responsible for generating the high voltage spark needed to ignite the fuel. Relays control power flow to various engine components.
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Ignition Coil: The owner replaced the coil, but it’s worth testing if you haven’t already. Use a multimeter to check for primary and secondary resistance. Also, confirm voltage at the coil terminals with the ignition ON. This BMW 635csi owner confirmed power to the coil.
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Main and Fuel Pump Relays: These relays are essential for powering the ECU and fuel pump. Check if the main relay and fuel pump relay are clicking when the ignition is turned on. Testing for power at relay terminals is also crucial. For this BMW 635csi, relay operation and power were confirmed. However, as advised by experienced mechanics, relays can sometimes fail under load even if they seem to test okay with a multimeter. Substitution with a known good relay or load testing are more definitive methods.
Testing the Speed and Reference Sensors on your BMW 635csi
The Motronic engine management system in the BMW 635csi relies on speed and reference sensors (Crank Position Sensors – CPS) to determine engine speed and crankshaft position. These signals are critical for spark and fuel injection timing.
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Sensor Resistance: Using a multimeter set to ohms, measure the resistance across the sensor terminals. Consult a repair manual for the correct resistance values for your BMW 635csi. In this case, the owner found no resistance readings from the sensors, indicating potential failure.
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Sensor Signal: Ideally, use an oscilloscope to check for a signal from the sensors while cranking the engine. However, if you don’t have access to an oscilloscope, replacing suspect sensors is a reasonable next step, especially on an older BMW 635csi where sensor degradation over time is common.
An engine bay of a BMW 635csi, highlighting the areas where ignition system components like coil, distributor, and sensors are typically located, crucial for troubleshooting a no-spark issue.
Rotor and Distributor Cap Inspection on your BMW 635csi
The rotor and distributor cap distribute the spark voltage to the correct spark plugs.
- Visual Inspection: Check the rotor and distributor cap for cracks, carbon tracking, or corrosion. The owner of the BMW 635csi replaced the rotor and cap as a part of the troubleshooting process, finding them to be in poor condition.
The Solution for this BMW 635csi: Replacing Faulty Sensors and Components
In this specific case, after systematic checks and component replacements, the BMW 635csi was brought back to life by replacing the following components:
- Rotor Button and Distributor Cap: These were replaced as part of routine maintenance and troubleshooting.
- Crankshaft Position Sensors (Speed and Reference Sensors): Testing revealed these sensors were not functioning correctly. Replacing them restored the necessary signals to the ECU.
- Front Crank Sensor (Pulse Generator): Although sometimes debated whether this sensor is strictly necessary for running, replacing it, alongside the other sensors, contributed to resolving the no-spark issue for this BMW 635csi.
After replacing these components, the BMW 635csi started immediately, resolving the “cranks but no spark” issue.
Conclusion: Systematic Troubleshooting is Key for your BMW 635csi
Troubleshooting a “cranks but no spark” issue in a BMW 635csi, or any classic car, requires a systematic approach. Starting with basic checks and progressing through the ignition system components, as demonstrated in this real-world example, can effectively pinpoint the problem. For this BMW 635csi, faulty speed and reference sensors, along with routine ignition component replacements, were the key to getting back on the road. If you’re facing a similar issue with your BMW 635csi, follow these steps and consult your vehicle’s repair manual for specific procedures and values.
