Going Back to the Basics Pays Off in Testing BMW

Posted 4/14/2010
By David Tidaback

I took a call the other day on a 2002 BMW 325i that had two problems: The warning chime for the key in the ignition would sound with the door open, whether the key was in the ignition or in the owner's pocket. Second, the central locking would not work on either "Lock" or "Unlock" with the remote buttons on the key. The doors would lock and unlock normally using the key in the driver's door, and a second key with the remote buttons also would not lock or unlock the car using the remote.

The technician had looked for a switch in the ignition assembly for the "key in ignition" signal and had not found one. He also knew that the car is equipped with Elektronische Wegfahrsperre (EWS). EWS is the immobilizer system that keeps the car from starting if an incorrect signal is present for park/neutral (or in the case of a manual transmission, clutch switch), if an incorrectly coded key is used, or a locked signal from the central locking system is present.

Knowing this, he unplugged the antenna ring in the ignition lock trim ring, cycled the key, the chime went off and the central locking resumed working. The car, of course, would not start with the antenna ring unplugged because the EWS could not get a key code signal. The test confirmed that the "key in ignition" signal came from the antenna ring and was transmitted by EWS to the central body electronics (ZKE), which controls the central locking, and the instrument cluster, which controls the key chime.

Since the car started and ran perfectly, indicating that power, ground, park/neutral, key code, and lock/unlock signals all had to be good, it seemed that a bad EWS module was getting the key signal, but not sending it along to the other modules. In the spirit of thoroughness, however, I suggested that the technician do a pin check of the EWS module connector. When he tested pin 10, which should have constant battery voltage from fuse 67, imagine his surprise when he found 0 volts; fuse 67 was missing from its holder. When it was replaced, the system went back to normal operation. The EWS module has to have a constant power source so that it can see the key in the ignition before the ignition is turned on, and also see that the key is removed after it is turned off. The programming of the system is such that it will not allow remote central locking to operate with the key in the ignition. A better understanding of the EWS system and all of its functions would have been a big help here, but so would the elementary principle of testing the basics, such as powers and grounds, before doing more involved testing.

David Tidaback is an Identifix European specialist. He is BMW, ASE master and L1 certified. He is also certified in Volvo engine and fuel injection and Bosch BSC 1, 5.

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