1993 300E 2.8l OBD1 Diagnostic Port
 

Hi Everyone,
I have a 1993 300E with a 2.8l that's having issues when at idle and has a tendency to hesitate when you step on the accelerator. I suspect it's the throttle body, but want to check the codes before I start tearing it apart. That brings me to the issue at hand. I created a tester with diagram attached below.
I have a 16socket diagnostic port The positive lead was in socket 16, black in socket 1, and test lead in socket 8. I pushed the button for 3 seconds, the light came on, but then nothing happened. I checked the fuse (fuse 9) and it was good.
https://cimg6.ibsrv.net/gimg/www.mbw...20a9942541.jpg

So what can be the problem? Any ideas where to check?

You should have really put a 200ohm resistor in series with the LED/switch. That way you are not relying on the "on" resistance of the LED to not blow a circuit in the car or the LED itself.
I would suggest you do that and repeat the test. See if you can light up that LED again.

Other than this the only other thing I would double check is that the test lead in your case is indeed pin 8. I have an '89 CA model that has the LED and a push button built in.
I'm surprised by 1993 Mercedes did not have this built in for the entire production.

Also, check to see if you a static duty cycle first. Emission issues generally show up there already.

Thanks for that!
For the resistor, should it after the test lead, or before it?
I actually tried it today for the SRS (socket 6) and the tester worked. So now I can say the tester works.

As I read more diagnosis procedure, I get more confused as to which socket I need to test. I believe it's 8, but maybe 3 or 14. I can't seem to get a straight answer. It's a 123.028, M104 series engine.
Socket 3 is for continuous fuel injection, which I believe is the old style mechanical injection systems with electronic sensors and actuators.
Socket 8 still doesn't respond, and I have yet to try socket 14.

Thanks for the help!

I would put the resistor before or after the LED in series, either side does not matter.
If you have $4 to spare watch this video. Used to be free from Kent Bergsma.
I think it covers everything for the KE-Jetronic.

But is seems you may have electronic injection not continuous KE-jetronic from your comment above. Is this the case?

Assuming yours is an HFM not KE-Jetronic, you most likely have harness issues if they are original, and of course the duty cycle measurements will not apply to you.
The Harness on my '95 E320 started going bad when the car was only 10 years old. Eventually replaced in 2010. This should have been a recall.