What does it mean?
NOTE: B2107 is an SAE, manufacturer specific code ( all B2XXX codes SAE defined as manufacturer specific ) that is used by many car makers to indicate a fault in the front windshield wiper control circuit. However, due to make and model-specific differences in wiper control systems, the information in this guide is presented for general informational purposes only, and should therefore NOT be used in any diagnostic or repair procedure that involves code B2107.
ALWAYS refer to the relevant technical manual for the application being worked on, or refer the vehicle for professional diagnosis and repair. OBD II fault code B2107 is (mostly) defined as “Front Wiper Motor Relay Circuit Short to Vbatt” and is set when the PCM (Powertrain Control Module) detects a short circuit in the front windshield wiper control circuit. Not that “Vbatt” refers to “voltage battery”, meaning that the wiper control circuit is shorted to the positive battery terminal.
When the wiper control switch on most vehicles is activated, the switch energizes an electromechanical relay that connects the wiper motor directly to the battery. This relay is a normally open, heavy-duty switch that is forced to close, and thus complete the circuit by the action of a coil that creates an electromagnetic field to force the high current contacts to close.
Thus, for as long as the coil is energized, the high current contacts will complete the circuit between the battery and the wiper motor. This contact will only be broken when the coil is de-energized, allowing a small spring to pull the high current contacts apart.
The advantages of using a relay lies in the fact that relays are better able to cope with high amperages, thereby protecting the delicate contact points in the wiper control switch on the one hand, and that several high-current circuits and/or components can be activated simultaneously with a single low current switch, on the other.
When a short circuit occurs in the relay control circuit, the coil might be activated permanently, meaning that the wipers could work even if the control switch is not in the “ON” position. However, depending on the application, and where in the circuit the short occurs, the relay could be damaged, or prevented from closing, which means that the wipers might not work. NOTE: On many applications, such a VAG vehicles (Volkswagen Group) the wipers are not controlled by a relay.
Instead, the wipers and other electrical functions are controlled by an electronic control module that may, or may not be incorporated into the PCM, depending on the particular application. The image below shows the construction of a typical 12 volt automotive relay. Note the position of the coil in relation to the high current contacts.
Symptoms
- Wipers run continuously and will not switch off, even with the key removed on some designs
- Wipers sweep on their own without any request from the stalk
- Wipers do not park properly and stop mid sweep
- Wipers completely dead because the relay was damaged or is being held open
- Wiper fuse blowing repeatedly
- Battery going flat because the wiper circuit stays live
Likely causes
- Chafed insulation letting the relay control wire touch a permanently live circuit in the same loom
- Wiper relay with welded or internally shorted contacts feeding voltage back into the control side
- Water or road salt inside the engine bay fuse and relay box bridging adjacent terminals
- Damaged wiring in the cowl area, where the loom crosses to the wiper motor and is exposed to water
- Multifunction wiper stalk switch with an internal short or carbon tracking between positions
- Corroded or pushed back terminals in the relay socket letting pins touch
- Incorrect previous repair, such as an added accessory or a bridged relay socket
- Failed low side driver inside the body module that no longer isolates the coil circuit
How to fix it
- Look up the manufacturer wiring diagram and identify which pins are coil control and which are the high current side
- Pull the wiper relay and the wiper fuse, then measure the coil control pin in the socket with the key off, any voltage there is the short you are looking for
- If the socket is clean with the relay out, refit a known good relay and repeat the measurement
- Inspect the loom in the cowl and along the plenum for chafed insulation and green corrosion
- Check the relay box for water intrusion, dry it and inspect the terminals
- Disconnect the stalk switch and see whether the voltage disappears, that isolates a shorted switch from a shorted harness
- Verify the module ground and supply before condemning the module
- Repair the damaged section of harness, then operate every wiper mode and confirm correct park behaviour