the code reader is rather trivial. simply a momentary push button switch
wired in series with an LED (+resistor if it's not internal to the LED). Then
the other end of LED goes to batterie's +'ve and the other end of switch to
battery's -'ve. The connection between LED and switch has a third connection
added (either at LED or switch terminal) that will go to the pins of X12
connector (it has a plastic cap with the sockets for the wires) and is on
secondary firwall near battery. Again, I know for certain this is the case
for 88 and 89 W124's (103 engine) but not sure of other years. One then
simply presses switch for a few seconds to activate ECU that then blinks the
LED a number of times to spell out the difficulty it thinks exist. One blink
means no problems. Moreover, you can then also erase any stored codes again.
I can send you more detailed information/pictures if you need
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