On my '94 E320 wagon, I periodically get a check engine light with codes 5 and 26 at hot start-up. The check engine light only comes on and stays on when I start the car hot, never when cold. And if it doesn't come on at start-up it won't come on later while the engine is running. Also, it only comes on if I turn the key very rapidly from off to on. It doesn't illuminate if I turn the key slowly (except for the bulb check sequence).
I have replaced the key switch and the overvoltage protection relay at the advice of my mechanic, but no improvement.
Suggestions?

Use the search feature .
Lots of info on your problems, which are a possible blocked EGR tube [5]and a bad shift delay vac actuator element. [26]


Here is an MB Diagnostis starter kit for your E320...
http://www.peachparts.com/shopforum/showthread.php?t=107630

Art,
Thanks for the help. This info will keep me busy for a while. On the tester you sketched, which bananna plug goes into which diagnostics pin number to read the codes? I sure don't want to short something out inadvertently.

The access pin #s are specified on the code link I posted for you..
The Blk [neg] will go to 1 and the Red will go to 16 [ pos] .. that powers the tool w/12v.. the odd color bananna goes to the pin # of the specific module you want to check for codes. That is why I had the ODD color bananna in the tool design..so you could not screw up
{ Red/Blk are usually pos/neg at a battery, so you can't get that confused either]

Your main concern on the tool is to make sure you have the LED correctly wired , as they are polarity sensitive.. you can test that by simply hooking it to 12v power .. if you have assbackwards, it won't light.

Spectacular. The tester works great! It is a great service to us cheap DYIers for you to provide the design of this super simple device and the codes to go along with it. Thanks again.
My specific problem giving a code 26 is a leaky delay actuator diaphragm at the transmission. I think I will put off this repair until later as it looks to be a pain in the a--.
The code 5 EGR problem, on the other hand, is not apparent. With the engine hot, the pin on the EGR valve moves up and down when I goose the throttle indicating that the solenoid and vacuum lines are OK, and if I suck on the EGR valve with the engine at idle, the idle speed drops about 100 rpm indicating that is operating OK and that the steel tube tube to the intake manifold is not clogged.
Now what?

May be hanging up.. there was a change on the original EGR valve for sticking.. they modified the shaft sizing for better clearance..
Should drop to a real rough idle.. if not , might still have partial blockage

If you let the 26 go, at least BB the line.....

also, see how much vac you have at egr vac port line