BMW E39 REST auxiliary water pump not working
What is the E39 REST auxiliary water pump and when should it run?
The auxiliary water pump circulates the engine's residual heat in REST residual heating, not normally at idle. It is under the water valves and is not fitted to all cars. REST uses only heat from the engine's cooling system, and it is not the same as latent heating that works with a separate heat accumulator.
Why does the E39 REST water pump not run?
The most common cause is worn-out brushes. If the pump does not run, the car stores the fault code Auxiliary water pump, or for a short circuit the code short circuit to ground. The pump can also seize, and then REST heating blows cold.
How is the REST water pump repaired?
The pump can be opened and the brushes replaced, but it is awkward and the brushes break easily. If the impeller is broken, replacing the brushes is not enough and the pump must be replaced. The hose clamp bracket of a new pump may need thinning so that the hose fits in place.