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.

The impulse test should not be done on the round diagnostic connector. There are no impulse self-diagnostics in 1987.

There are duty-cycle (on-off ratio)diagnostics. The 230E is a European model (or world model) that probably didn't have a closed loop lambda control system. The 300SE does.

Seventy percent, key on engine off, is the check sum for a properly running Federal version closed loop system (85% would be a CA version). This code just tells you that the system is federal and is working!! The rest of the diagnostics on this system are real time only with the engine running. No remembered faults, only real time.

As to "30 up or 70 down", go to your 70% key on engine off situation and set your equipment to read 70 (not 30).

Thanks for the reply Steve.

Your are correct the 230e has no catalyst and the 300se does, both these cars are European models running in the UK. What sort of 'real time' tests can be performed?

The 300se is running pefectly except it will sometimes hesitate when accelerating hard especially around 2000 -> 4000rpm. This seems slightly worse when the engine is cold. Plugs and distributor cap + rotor have been replaced. New fuel filter going in next week.