Tech Info


EWS – Electronic Immobilizer on CAN-based BMW motorcycles

What does "EWS" stand for?

It is German for “Elektronische Wegfahr Sperre

The most common symptom:

The motorcycle does not start and the “EWS” is intermittently or permanently displayed on the instrument cluster (KOMBI)

The problem

Since the systems inception in 2004, it has been plagued by various problems, the biggest by far being the (un)reliability of the Ring Antenna and Evaluation electronics. This of course is a single point of failure and if it does not work, there is NO chance of authenticating the Key and no chance in hell that your engine will start and run.

The Fault Code diagnosis

According to several dealerships and dealer techs we spoke to, a faulty ring antenna raises a Fault code within the BMS-K(P) only 50% of the time…

General description of the EWS system in CAN-bus based motorcycles

The electronic immobilizer consists of 4 parts:

1RF Transponder in the Key
2The Ring antenna
3The Evaluation electronics integrated into the Ring antenna module
4The Engine Control Unit (BMS-K or BMS-KP)

When the ignition is switched on, the Engine Controller initiates communication with the transponder chip in the key. Data is exchanged and if the challenge is authenticated, the BMS-K(P) will unlock the ignition (spark), fuel injection, fuel pump and starter motor.

The Transponder chip

The “chip” in the Key does not contain a battery. In stead it is powered by the magnetic field of the Ring antenna, which also acts as the RF communication medium. In essance the transponder is a wireless read/write EEPROM with a unique serial#.

The Ring antenna and evaluation electronics

The Ring antenna both powers and communicates with the Transponder chip in the Key. The max communication distance between Key transponder and the Ring antenna is just short of an inch (2cm) and that is the reason why there should NEVER be any other Keys with transponders in close proximity, as they would both be powered by the Ring antenna’s magnetic field and both try to communicate, thus essentially corrupting each others signals.

The Evaluation electronics

This is integrated in the Antenna ring unit and conditions both the RF and data signals to the BMS-K(P) in order to communicate with each other (read: rf <-> digital conversion). The Evaluation electronics is powered by Ignition (Terminal 15), and has the following pin-outs:

pin1 : Terminal 15 (ignition)

pin2 : Terminal 31 (ground)

pin3 : data

pin4 : data


A certain portion of the BMS-K(P) software is dedicated to the EWS. This block of software integrates the data from the Key (by means of the Evaluations electronics, to the rest of the engine management functions of the ECU. It’s primary tasks are:

  • To retrieve the identification data from the Transponder in the key and validate its authenticity
  • To release the rest of the engine management functions, if the they key is authenticated
  • It manages up to a total of 10 keys
  • It manages the keys that have been blocked (typically if you’ve lost a key, you can have it blocked)

The EWS procedure at start-up is as follows:

  • When the ignition is switched on, it powers the Evaluation electronics and in turn the Transponder chip in the Key
  • The BMS-K(P) authenticates the key data against its corresponding data within the BMS-K(P)
  • Based on a secret algorithm, a random number is manipulated by both the Transponder as well as the BMS-K(P). The Key’s answer is transmitted back to the BMS-K(P), and if they are a match, the rest of the engine management functions are released.

The dreaded "EWS issue" has a silver lining?

What to do when things go wrong

Unlike, loosing your key (quite probable) or an electronic failure within the BMS-K(P) (highly unlikely – cars have been using these modules for years!), the unreliability of the Ring antenna and Evaluation electronics, does have a silver lining… Anyone can replace it! (no special matching by the BMW diagnostic computer required) – unlike the learning of a new key or the replacement of a BMS-K(P) control unit!

That’s why you should have at least ONE spare unit amongst your riding party… AND ALWAYS carry your spare key… The spare key is essential as it is required when your loose (misplace, of course) your current key, however also required for the emergency Ring Antenna replacement procedure (the original ring antenna is difficult to replace, hence you will only be unplugging the wires from the original connector and plugging them into your replacement unit. However your replacement unit will not reach your key in your ignition switch – hence you require two keys… one in the ignition switch (to turn the ignition) and the other taped or cable-tied into the replacement ring antenna, to authenticate and enable your engine controller…)

What you require…

  • A spare ring antenna (New part#: 61 35 7 705 247 ) released 06 2008
  • Your spare ignition key!
  • Some cable ties and/or tape (to fasten the spare key to the replacement ring antenna)

Is the ring antenna always at fault!

NO! in some occasions the symptoms of the motor not starting and “EWS” displayed intermittently or permanently on the instrument cluster are NOT the result of a faulty Ring antenna!

If the Low beam comes on and “EWS” is displayed, when the ignition is switched on the first time after the battery has been removed, the Ring antenna is NOT faulty!

This happens when the BMS-K(P) boots up incorrectly… A good booted BMS-K(P) always shows the gear indicator in the instrument panel, provided the gear selector potentiometer is NOT faulty (GS-911 will tell you this).

Once the ignition is switched on a second time, the EWS should function normally!

What to check before replacing the Ring antenna?

  • Cycle the ignition again, and if the problem persists, then
  • Check the wiring and wiring connector to the Ring antenna for:
    • Any damage (possibly caused by chaffing etc. or rodent damage)
    • Over tightened cable ties that might have cut into the wiring or are preventing good contact at the connector (due to lack of slack in the wiring)
    • Poor connection at the connector to the ring antenna
  • Read the fault codes with GS-911. The ring antenna might not be the problem that is preventing your motor from starting!