BSD and waterpump

This entry is written, because:

a) a problem was caused by a non-typical (rare) defect;

b) I think, it’s worth to tell about symptoms, which are characteristic in case of BSD problems for modern petrol engines.


Patient: E9X, with N55 series petrol engine. Problems – even new AGM battery, encoded two times (“just to make sure”, the battery was encoded for the second time, when a while after first described problems appeared), but still – the battery was on the edge to “fainting”.

One morning, the owner of the car noticed, that, when he unlocked the car, the water pump started to work immediately, and it continues to work even a while after the car is locked. Shortly after the appearance of this symptom, problems with electrical supply began.

In the error message memory of DME – all possible error messages related to the BSD data network.

And now it’s time for a short story regarding consequences, which can suffer the cars with N series petrol engines if BSD data network “dies”.

Things, which are connected to BSD: IBS, oil quality/level/temperature sensor, alternator, water pump.

In case, if the water pump doesn’t receive a command via BSD (including – required flow-rate), it starts to work in emergency mode, with power – close to max. This is done for the purpose – not allow the overheating of the engine. In case of damaged BSD the water pump don’t know, when the engine is started and when – stopped; it starts to work at the moment, the power supply voltage is supplied.

The connection of the water pump (as an example: N52 series engine, E60 LCI):

As we see, the pump has 4 connections:

1. Ground

2. Powerful +12 (30-th bus)

3. Power supply for electronics

4. BSD data connection


And in this image: IC block schematics of typical LIN driver (electronic hub: for communication with  the “outside world”):

LIN output: connection with the LIN network.

Passive level (the unit “sleeps” or sends “0”) – close Ubat (12 V);

Active level (Master or some unit sends “1”) – close to Ground.

Such LIN drivers are in each of LIN clients and in Master. If some unit loses supply voltage, the LIN drivers of this unit are in passive status and don’t disturb all LIN network.

If by some reason, the unit doesn’t start (for example, the MCU clock generator of the unit is damaged), also then LIN drivers are in “passive” status and don’t interfere with communication of other clients.

The only reason, when LIN driver can paralyze communication of other clients, is damage (damages of TXD buffer are theoretically possible, but – very, very unlikely) of its TXD (transmit) transistor.

In the block schematics, the transistor, which damage causes “blocking” of all LIN customers, is marked. One more note:

a) if only the transistor is damaged, oscillogram of LIN signal will display max voltage around 0.5 .. 0.8 V (a drop of the voltage on the diode, which is connected in transistor’s Drain);

b) if also the diode in the series with the transistor is damaged, the signal of LIN will not be visible at all.

Such defect is not typical, because LIN drivers are safe (their output transistors can handle the voltage of 50 V in the LIN line, to 1000 V human model static discharge). I can imagine only one possible cause of damage – possibly: the signal (its amplitude can reach several hundreds of V; for ignition coils – also high dU/dt) of injectors or ignition coils has hit LIN data line.

Similar problems have been observed – in such way, for example, are the CAN lines of NOx sensors damaged, also temperature sensors of an exhaust. Unfortunately, BMW has not intended double insulation of wires of increased voltage nor for ignition coils, nor injectors.


But now – returning to the problem.

Observing LIN signal with an oscilloscope, I concluded strongly lowered level of signal (which corresponds to passive):

Reducing the vertical scale to 500 mV/section, we see the amplitude of the signal is around 0.8 V (instead of the necessary 12 V).

Yes, in this time the damaged LIN driver was in the water pump itself: when the water pump was disconnected, the level of the signal in BSD network became correct:

Also checking rise/fall time of BSD signal, verdict – correct (no any “half-level” signals, no load of increased capacity):

In the end – returning to problems of the water pump and BSD. If the water pump doesn’t receive commands via BSD, it works all the time (as already mentioned before), as soon as it receives supply. The power of these pumps can reach 500 .. 700 W (depending on model). Accordingly – current consumption can reach 50 A or even more. In addition – as the BSD doesn’t work, DME doesn’t receive (don’t calculate) energy, left in the battery. When the car is tuned out, the water pump will stop to work (the 30-th bus will be disconnected), when the voltage in DME/CAS will reach the critical minimum – such an emergency mode is intended if the IBS system doesn’t work or fails. Even an ideal (and completely charged a battery) will be discharged to almost empty state very quickly.

This entry is one more example, how important is correct performance of the IBS/BSD system.


After the repair (water pump was replaced):