Just bought an E320 that seems in good shape, but have a slight problem.
The check engine light is on, but when I press the button at #2 on the diagnostic box, all I get is a relay click after about half a second and then a solid LED. No blinks.

Do I still have to use jumpers to read codes with the onboard pushbutton reader?

I fiddled with the #3 fuse immediately before the CEL came on to see if corrosion or the dimmer pot was to blame for the instrument lighting not coming on.

__________________
RIP:
2011 E350 BlueTEC, Schwartz 81k (totaled by uninsured drunk driver)
1964 Unimog 404.1 (wrongfully towed by the city)
1994 E320, Brilliantsilber 208k (transmission shuffled off after sudden catastrophic fluid loss)
1982 300D, Silberblau under a blue repaint 256k (sold for parts)
1995 E320 Polarweiss, 131k (sold for parts)
1995 E300D Polarweiss, 287k (totaled by drunk driver)

Did some more research and discovered the onboard tester only deals with emissions. Plugged in my external tester and cleared the codes, who knows when they were last cleared.

__________________
RIP:
2011 E350 BlueTEC, Schwartz 81k (totaled by uninsured drunk driver)
1964 Unimog 404.1 (wrongfully towed by the city)
1994 E320, Brilliantsilber 208k (transmission shuffled off after sudden catastrophic fluid loss)
1982 300D, Silberblau under a blue repaint 256k (sold for parts)
1995 E320 Polarweiss, 131k (sold for parts)
1995 E300D Polarweiss, 287k (totaled by drunk driver)

Here's a document that may help you. Courtesy of GSXR.

http://www.w124performance.com/docs/mb/diagnostics/DTC_List_W124_M104.pdf