This is a issue I see a lot when people are using aftermarket parts to save money. I am not going to preach about having to use OEM parts only. In this post, I would just like to explain why some of these parts will cause faults and how to avoid it. With this knowledge you might even save a few dollars!
The two most common components I see issues with are: alternators and electric water pumps. BMW uses a single wire bus system called BSD (bit serial data). This is a bus system that the DME (digital motor electronics) uses to communicate with alternator, water pump, IBS (intelligent battery sensor) and oil condition sensor. The DME receives information from all these components and also can send commands to the alternator and water pump. Let’s start first with the alternator: the DME gets information about voltage level and battery charge rate and makes decision on how the alternator operates and charges. By doing it this way the vehicle only charges how much and when needed. This saves engine drag and fuel economy. The same happens with the water pump. The DME makes a calculation on water pump speed according to many engine sensors, temps and modes of operation; such as sport mode engaged or high driving. Again, this is done to save on engine drag and fuel economy.
All these components on the BSD network have a computer processor in them in order to send and receive messages on the BSD network. A lot of the super cheap water pumps and alternators that are out there do not have this processor on them and thus can not communicate on the network. Some can even bring the whole network down and cause some problems…but I will get into that in a minute. The aftermarket parts will work, don’t get me wrong, other wise companies wouldn’t sell them. But they don’t work the same as BMW designed ones. So for instance lets say you replace the alternator with OE rebuilt one from aftermarket reseller. Yes, it will look the same and probably be rebuilt with parts from which ever alternator you have Valeo or Bosch. The OEM regulators have a computer processor. A processor will not be in the rebuilt version in order to offer a much cheaper part to consumers. This after market alternator will still work with your car and it’s quite clever. When the BSD communication fails on the car, the alternator fail safe will charge at 14.5v constantly. The aftermarket alternator will still charge, but will not be able to vary the charge level or recharge well if the battery is depleted (such as cold weather). This same principal is used for the water pump. Keeping it a constant flow rate to keep the engine from overheating.
Here are some issues I have seen from the aftermarket components: when plugged into the BSD network it can actually cause a short circuit in the network and bring the whole network down. The components on the network can no longer function properly when the network is compromised. In this scenario, I have seen a car randomly overheat and some have charge malfunction lights come on even when charging properly. The BSD fault codes will not cause any malfunction lights and can only be read out with BMW specific software. The worse case scenario is when that aftermarket component can cause damage to the DME. This case is very expensive and very rare, but I have seen it happen.
I cannot tell you to not use aftermarket parts, but just use reputable brands. Do your research on that component to make sure it works fully with your car. If something is too good of a deal to be true then there is a reason why.