BMW Z4 cooling system components include:
- Radiator and coolant overflow tank
- Belt driven coolant pump bolted to the front of the engine block
- Electric cooling fan, DME controlled
- Electric (auxiliary) cooling fan attached to front of radiator - The cooling fan is controlled by the engine control module (ECM) via an output final stage.
- Electrically heated thermostat
- Automatic transmission cooler or heat exchanger (if equipped)
- Heater valve and heater core (for climate control)
- Coolant level sensor inside expansion tank
- Coolant temperature sensor at cylinder head
- Coolant hoses and lines
The radiator outlet temperature sensor, located in the right side radiator hose, monitors coolant temperature as it leaves the radiator. The sensor reading is used by the engine management system (DME) to determine when the electric cooling fan is activated. The DME uses other inputs as well to control cooling fan operation and speed: coolant temperature at the cylinder head and AC operation.
The radiator outlet temperature sensor can fail electrically; a fault code will be stored and the MIL (malfunction indicator lamp) will be illuminated. The sensor can also leak coolant from the sensor sealing O-ring. I have replaced this sensor and soon after developed a leak at the lower radiator hose. If it's within your budget, replace the lower radiator hose also as preventative maintenance. Replacing the radiator outlet temperature sensor is a snap and can be done working above the engine on the right side of the radiator.
In this article I will describe how to replace the radiator outlet temperature sensor.
Be sure to work with a cool engine and confirm the cooling system lacks pressure before opening the cooling system.
Keep in mind that when your car was serviced before, parts may have been replaced with different size fasteners used in the replacement. The sizes of the nuts and bolts we give may be different from what you have, so be prepared with different size sockets and wrenches.
Protect your eyes, hands and body from fluids, dust and debris while working on your vehicle. If you're working with the electrical system, disconnect the battery before beginning. Always catch fluids in appropriate containers and properly dispose of any fluid waste. Recycle parts, packaging and fluids when possible. Do not work on your vehicle if you feel the task is beyond your ability.
Vehicle models change and evolve, as they grow older, so the vehicle shown in our illustrations may vary slightly from yours. If something seems different, let us know and share your info to help other users. Do you have questions or want to add to the article? Leave a comment below. When leaving a comment, please leave your vehicle information.
Lift and support the front of the vehicle safely. See our tech article on lifting your BMW Z4. Wear safety glasses anytime you're working under your car.
Remove the engine splash shield. See our tech article on splash shield removal if needed.
Drain the cooling system. See our tech article on draining and filling your cooling system.
The radiator outlet temperature sensor (red arrow) might be the cause of a hard to find coolant leak. The sensor is mounted in a plastic section of the right side radiator hose. There is a sealing O-ring that normally prevents leaks in the system. Over time, this O-ring fails and creates a coolant leak. At first, it will be a small leak. You may notice a small amount of coolant under the right side of your vehicle or the coolant reservoir coolant level slowly dropping. This photo shows the beginning of a failing O-ring. You can see coolant seeping out through the sensor mounting bung (note wet area around sensor). If you have a small, yet hard to locate coolant leak, start by inspecting this sensor.
Working at the radiator outlet temperature sensor, disconnect the electrical connector by pressing the wire release tab (red arrow) and pulling the connector off in the direction of the green arrow.
To remove the sensor from the hose, squeeze the plastic locking tabs on the sensor (red arrows) and pull the sensor out of the radiator hose. Be prepared to catch any excess dripping coolant in a container when you pull the sensor out of the hose. I like to keep my drain pan under the area of the hose. To install the new sensor, lubricate the sensor O-ring with clean engine coolant. Then push it into the radiator hose until both plastic mounting tabs engage. Then connect the electrical connector and reassemble the splash shield. Fill and bleed the cooling system.
If you suspect the sensor is good and the wiring or voltage to be faulty, disconnect the electrical sensor as noted above. Using a DVOM check the voltage to the sensor. Pin 1 should have 5 volts. Pin 2 should have close to zero volts. If your voltage readings do not match, you may have a faulty wire or control module. Consult the most current wiring diagram for your vehicle.
You can also measure across the sensor terminals (red arrows). As the temperature of the sensor increases, resistance should decrease. You can test this by placing the sensor in hot water with a DVOM set in OHMs.