Yes, 89-91 are the same. The 88 is also much the same except it has a few less codes since it only has a drivers side airbag. Before 88 there are no available self-diagnostics and the system must be tested with a box that plugs in with the control unit unplugged.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician