Many BMW vehicles are equipped with rain sensors that activate the wipers when water is detected on the windshield. These sensors are major improvements over the standard, intermittent windshield wipers, but when they do not work properly, it can be very frustrating, as well as being a significant safety hazard.
A few years after purchasing a new 2000 BMW 528, the windshield wipers in the rain-sensing mode occasionally stopped operating. In all cases, the wipers worked when first turned on, but after a short amount of time, while in pause mode, they were no longer operational. Then, after turning the wipers to full-wipe mode (where they were fully functional), and then back to the rain-sensing mode, occasionally they worked as they should, at least for a short period of time.
After experiencing this issue a few times, I took the car to the local dealership for repair. However, since the windshield wipers malfunction occurred only sporadically, the problem would never happen while at the Service Department. So, I had a very little choice but to try to find a solution myself. Fortunately, after a lot of troubleshooting, I was able to pinpoint the defective part.
The root cause of the rain sensor malfunction was a defective windshield wipers relay module. Of the two electromagnetic relays in the module, the lower relay is the switch that turns the wipers on or off, based on the signal from the rain sensor in the windshield. Figure 1 illustrates the wiper module and shows a closeup of the degraded (blackened) relay contacts. Apparently, these degraded electrical contacts allowed the wipers to operate in the rain-sensing mode for a period of time until heat or other factors caused them to malfunction. Then, after cooldown, the relay would operate properly for a while, until they overheated and another cooldown was necessary.
The following steps are specifically applicable to the replacement of a module in a 2000 BMW 528. However, the replacement process should generally be the same for all BMWs that have this type of mechanical relay module. If you have a different model and year, you should get a diagram from your local BMW Parts Department that pinpoints the location of the module in your particular car.
In the case of the 2000 BMW 528 series, the only tools needed are pliers and a 3/16” (5 mm) hex driver (or Allen wrench).