But how do I read codes from the Alarm (ATA) system or the convenience system? If those systems (or others) are generating codes, I would NEVER know about them, as the check engine light would never be turned on for those.
According to Steve Brotherton's DIY article:
http://www.peachparts.com/Wikka/EngineControls
Quote:
This process of reading codes for these early systems works up to 1994 for all systems, and later on some systems. Basically, it will work up to OBD II systems, and applies to all systems, not just engine management systems. The code retrieval method is very simple. The communications take place over a single line on the appropriate diagnostic connector. On the car I used (a 1991 300TE), the connector (X11/4) is located next to the battery, next to the right side hood hinge.
The KE controller is accessed on data terminal #3. With the key on, engine off, self-diagnostics is activated by grounding the #3 pin for two to four seconds. Once this takes place, the control unit transmits the codes by itself, grounding the #3 terminal. (Remember that all systems work the same; all you need is a directory of which pins to activate and the code table to interpret.)
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So, all I'm looking for are instructions on how to read codes for all of the other modules.. either using a DVM with duty cycle or building one of these LED things.
I'm sure that the following pictures from JimF's site hold some clues.
https://www.k6jrf.com/MB_S500.html