1989 300SE computer diagnostics?
 

I'm trying to find a plug that I can use with my diagnostic machine to read the codes on my '89 300SE. I've heard I can make one? Or is there one on the market somewhere?

I've looked at the plug-in on the car (near the firewall) and it looks like it's a 3 pin plug?

Yes, Pin #3 of the ignition module round connector to ground monitoring duty cycle will give you the few diagnostic codes that are available. You will need the FSM to decipher the codes. If you don't have a duty cycle meter, you can get one for cheap at Sears or if you have an old dwell meter, you can make do with it and dividing the 90 degree scale number by nine to get the multiple of 10%.

There is also a diagnostic connector near the battery that will flash codes, but not for the engine. There are codes to be had there for the automatic climate control, air bag system and so forth.

Yes this has 6 pins in it, but only 3 apear to have connections. Does that make sense?

Is "Pin #3" identified as such? Does it have markings?

If you look very closely there is a number by each pin. Your positive lead goes in the #3 slot and negative in the #2 slot.

I've been fixing W124 Mercedes for 15 years. Why have i never had to use a code reader????? Can you even read any codes from them???

yes, code readers
 

not really required at all...