Experiencing a BMW coolant pump that seems to run even after your engine is off and cooled down? This can be a perplexing issue for BMW owners, potentially indicating a problem within the cooling system’s sensor network or the Digital Motor Electronics (DME) control unit. Let’s delve into a systematic approach to diagnose this problem, leveraging the capabilities of a Schwaben scan tool to pinpoint the culprit.
The scenario described in a recent forum discussion highlights this exact problem. A BMW owner noticed their coolant pump running continuously after engine shutdown, even after the car had been sitting for hours. Initial temperature readings seemed normal, leading to some confusion. However, understanding the intricacies of BMW’s cooling system, particularly the dual temperature sensor setup, is crucial for accurate diagnosis.
Early BMW models, like the 2006 325i mentioned in the forum, often employ two coolant temperature sensors. These sensors play distinct roles in engine management and cooling system operation.
Firstly, there’s the Engine Coolant Temperature Sensor (ECTS). Typically located on the front of the Oil Filter Housing (OFH), this sensor measures the coolant temperature as it exits the cylinder head. This is the primary temperature reading used by the DME to monitor engine temperature and trigger overheat warnings. Generic scan tools commonly access and display the data from this sensor.
However, there’s a second, equally important sensor: the Radiator Outlet Temperature Sensor (ROTS). Positioned on the Lower Radiator Hose, or Radiator Outlet Hose, this sensor plays a critical role in post-shutdown cooling. The ROTS informs the DME about the coolant temperature exiting the radiator. This information is used to determine if and when to activate the electric water pump and radiator fan after the engine is turned off to prevent heat soak and potential component damage.
Image: A representation of BMW coolant temperature sensor locations, highlighting the ECTS near the oil filter housing and the ROTS on the lower radiator hose.
If the coolant pump continues to run excessively after shutdown, especially when the engine is cold, a malfunctioning ROTS could be the cause. A failure within the ROTS, such as an internal short, might send an erroneous signal to the DME, indicating a high coolant temperature even when the engine is cool. This false reading can trick the DME into continuously running the coolant pump and fan.
Diagnosing a Suspect Radiator Outlet Temperature Sensor (ROTS)
Here’s a step-by-step diagnostic approach, incorporating the use of a Schwaben scan tool or similar BMW-specific diagnostic tool:
-
Initial Observation and Basic Checks: Confirm that the coolant pump is indeed running excessively after engine shutdown and cooldown. Listen for the pump noise and check for vibrations.
-
ROTS Connector Disconnection Test: Locate the Radiator Outlet Temperature Sensor (ROTS) on the lower radiator hose. Carefully disconnect the electrical connector from the sensor. Observe if the coolant pump stops running. If the pump ceases operation upon disconnection, this strongly suggests a fault with the ROTS or its associated wiring.
-
Resistance Measurement of ROTS: With the connector disconnected and the engine off, use a multimeter to measure the resistance across the two pins (spades) of the ROTS sensor itself. Simultaneously, use an infrared (IFR) thermometer to measure the temperature of the lower radiator hose near the ROTS housing, or estimate the ambient temperature.
Compare the measured resistance to typical ROTS resistance values. A healthy ROTS should exhibit resistance that varies inversely with temperature. For example, a general guideline is around 2,500 Ohms at 20°C and approximately 250 Ohms at 90°C. A significantly lower resistance reading than expected at the measured temperature could indicate an internal short within the sensor.
-
Schwaben Scan Tool for Real-Time Sensor Data: This is where a Schwaben scan tool or a similar BMW-specific diagnostic tool becomes invaluable. Connect your Schwaben tool to your BMW’s OBD-II port. Navigate to the engine control unit (DME) and look for live data or sensor readings. The Schwaben tool should allow you to monitor the real-time values reported by both the ECTS and the ROTS sensors to the DME.
Compare the temperature readings from both sensors. If the ROTS reading is significantly higher than expected (especially when the engine is cold and the ECTS reading is normal), this reinforces the suspicion of a faulty ROTS. The Schwaben tool provides direct insight into what the DME is “seeing” from each sensor, eliminating potential wiring or connector issues from skewing your diagnosis.
-
DME Violet Wire Check (Advanced): If disconnecting the ROTS connector does not stop the coolant pump, a more complex issue might be at play, potentially within the DME itself. The violet wire at pin #3 of the coolant pump connector (X6035) is the control signal from the DME to activate the pump.
As an advanced diagnostic step, with both fuses F02 and F09 (related to the coolant pump circuit) still installed, carefully disconnect the violet wire from pin #3 at the coolant pump connector. If the pump stops running after disconnecting this wire, it suggests that the DME might be erroneously sending a continuous pump activation signal. This scenario is less common but possible.
Caution: Working with electrical wiring requires care. Ensure the ignition is off and consult a repair manual if you are not comfortable with electrical testing.
-
Utilizing BMW Diagnostic Software (INPA or ISTA): For the most comprehensive diagnosis, BMW-specific software like INPA or ISTA offers deeper access to the DME and cooling system diagnostics. These tools can read fault codes, display detailed sensor data, and even perform activation tests on components like the coolant pump and fan. While Schwaben tools offer excellent DIY capabilities, INPA/ISTA provides dealer-level diagnostic depth.
Beyond Temperature Sensors: Considering Other Diagnostic Avenues
While a faulty ROTS is a prime suspect for a continuously running coolant pump, remember that BMW diagnostic tools like the Schwaben scanner are capable of much more. These tools can also be used to:
- Read and clear fault codes: Identify any stored diagnostic trouble codes related to the cooling system or other engine management components.
- Monitor VANOS system parameters: Although not directly related to the coolant pump issue, Schwaben tools can also provide insights into the functionality of your BMW’s VANOS (Variable Valve Timing) system. This is relevant for overall engine performance and efficiency diagnostics. While “VANOS activation reset” might be a separate procedure, understanding VANOS operation is part of comprehensive BMW engine diagnostics.
- Perform service resets: Reset service indicators after completing maintenance tasks.
- Adaptations and calibrations: In some cases, Schwaben tools can perform certain adaptations or calibrations necessary after replacing components.
Conclusion:
Diagnosing a BMW coolant pump that runs continuously after engine shutdown requires a systematic approach. Starting with basic checks and utilizing the diagnostic capabilities of a Schwaben scan tool to monitor sensor data is crucial. By understanding the roles of both the ECTS and ROTS sensors, and following the diagnostic steps outlined, you can effectively pinpoint the source of the problem and restore your BMW’s cooling system to proper operation. Remember to prioritize safety and consult repair manuals or professional technicians when dealing with electrical systems or complex diagnostics.
