This is the biggest problem with M104's that do not have 722.6 transmission. Mechanics do this jumper thing, but what I have found is the computer module that drives the smog-air pump shorts-out, but only smog-air channel shorts out, and register a "check engine" light. Note, the smog air pump channel is tied in parallel with the egr control air control solenoid. The solenoids that enable the EGR valve, and the smog AIR valve are located under the black plastic cover at the front of the engine, and one can test each solenoid by applying 12v and blowing on one side of each solenoid. The node of vacuum that feeds the solenoids that ultimately drive egr, and air valve is located under the front of the intake manifold. follow the plastic line that leaving the solenoids to the right, and make sure it is not cracked, or broken, yet the tubing where it ties under the intake will have a nipple, and the connection is a piece is made from rubber which tears, or gets soggy like a marshmallow. Also a rubber WYE is right there too, check the wye for integrity, and blue little inline looking filter which is a check valve.
Now, the vacuum lines to the left leaving the solenoids will disappear into the valve cover top cover. You will need to take that cover to see if the plastic lines are complete, and in good standing, and those two lines are the ones that drive the EGR and Smog air valve.

On the passenger side of the engine right below the center of the valve cover, their exist two diaphragm valves exist. One is the EGR, and the other is smog-air valve, yet each valve has a little rubber hose attached to each nipple. If you disconnect the hose, attach a three to five foot piece of vacuum hose to the nipple of each diaphragm, and suck on each one. Each should hold vacuum. I want to say the rear-most valve is the EGR. If the engine is started, the engine RPM settles after a minute or so. The hose long vacuum hose is attached to the valve if you suck on it. The engine rpm will start to cough a bit, and maybe it will even cough to the point that the engine has a very rough idle. Wants to turn off that indicates the EGR is working, and the EGR channel is not blocked.

The smog-air valve will only work when the smog pump is working, so to turn on the smog pump the drive signal voltage from the computer need to drive the clutch on the smog pump. If that does not work, then one can drive the clutch from a external 12v source, and then apply vacuum to the smog air valve, and the pitch/tone will change...

Now, you can have another problem, so have your codes read, and reconnect the MAS-airflow, and get a one day permit to drive it for 30 minutes to 1hr to get the code, but this is the most common problem to smog problems in California, and 1/3 of the cars I see have some kind of problem with something I just outlined..

If you find the smog pump is new(or the electrical connection disconnected), or one or both solenoid blocks are new. That is a clear indication that the computer has shorted channel, and the failure is Mercedes should have driven a relay to drive the smog pump, but on the M104 the smog pump is driven directly from the computer... In most cases you can use the computer module from a C280 to a S320. Just make sure you are in the range of 94 to 96, ANDDDD the car you are taking this from does not have new smog air pump or solenoid block, but at the local Pick N Pull junk yards in the San Francisco Bay have these cars, and these computers are rarely missing on those junk cars. Check the row52 site for cars...

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I have been through this twice, and the very first time was a brain-busting for a few months. The second time was a easy fix because I knew what to do...

Enjoy,

Martin