I have been trying to perform on-board diagnostics on my 230E 1987 and 300SE 1987 without much success. The 230E has a 9pin round diagnostic connector but no connection on pin3. Is it possible to perform any diagnostics on this car and if so how?
The 300se also has a 9pin connector but this time pin3 is present. With the ignition on and engine off I get a duty cycle of 70% (faulty engine speed sensor?) from pin 3. Is this correct? should the engine be running or off? and is duty cycle, time pin high(12v) or time pin low? (gnd) (Steve Brothertons article seems to imply that 30% duty cycle would be 30% time signal low and 70% time high).
I have also tried to read fault codes by grounding pin3 for 3s and looking for flashes on an led placed between 12v and pin3. The led remains on and I get no flashes.
I would be most grateful for any advice on how to test these cars and what on board diagnostics are available on the 1987 models.