For Mercedes W204 W212 W221 W164 W166, it’s necessary to have this adapter to use with the EIS ELV maintenance, Mercedes key programming and key detection.
Here are some diagrams for reference.
W164 / R251 / W221 / W216 with NEC inside
w203 / vito w639 with mcu HC908 inside
w203 with mcu hc912 inside
W204 / W207 / W212 with NEC inside
W210 / W208 / W202 with HC705 iside
W211 / W219 / W209 with HC912 inside
W220 / W215 / R230 with HC705 inside