I believe I've located the X11/4 diag port along the firewall (contains the LED and button) and the X11 diag port on the left fender (round connector with spin-on cover, looks like a short Diesel tach amp). Can someone confirm?

As far as reading duty cycle to determine how the fuel system is functioning, which pins do I measure? Do I need a reference ground?

I tried my DMM in duty cycle/frequency mode at X11/4 pin 3 and get sporadic readings in the 30s Hz. Sometimes it reads zero. That's with the + probe in pin 3 and the - probe loose. If I put the - probe on chassis ground or battery - post, it'll only blip to some reading other than zero for a second or two per minute.

I tried also at X11 pin 3 and get continuous readings around 81.35 Hz. 81.35 seems to be a number it likes but it'll go a little above and a little below.

Obviously I have to learn how to use this DMM to get duty cycle (another post) and more to learn about applying the theory Steve B presents in his article on evaluating engine controls. But any advice is helpful.

The specific problems with my 91 300SE are:

- vacuum is only 12"Hg at idle, capping the 4 manifold vacuum ports improves nothing

- idle speed is steady at 600 or 700rpm but the engine shakes (tolerable in N, annoying in D)

- pulling the idle air valve connector causes idle to drop to 500rpm or so, not rise. Spritzing carb cleaner around the intake manifold gasket and idle air valve hoses indicates no leaks.

No wonder I like Diesels

Thanks,
Sixto
95 S420
91 300SE
87 300SDL
83 300SD