Were you able to fix your problem? or is the check engine light still on?
"Looking under the hood" is not likely to tell you anything about this air injection system - unless it's on fire!
Sorry I missed your message back in April. Seems like you still have the problem now.
I have managed to fix the entire air injection system on our ML 320. I did not clear the computer check engine light (CEL) as it clears itself a few cycles after this system is repaired - a good verification for me.
The entire repair is very quick and inexpensive depending on how resourceful you are with finding a replacement pump and cleaning/repairing/purchasing replacement valves. I found a replacement pump very inexpensively on Ebay, and repaired the valves.
Ultimately it takes far more time to read about the system on the threads here than doing the actual replacement/repair. Reading the information on this site will save you lots of time and money! The beauty of the threads is that it arms you with the knowledge to tackle the job correctly, and it's clean too! There are three related threads you really need to study;
1. This thread.
In the end, I'm a bit skeptical of the entire air injection system. I had a mechanic explain that he felt the system is only to pass an emissions requirement on start. His explanation was simple. There is plenty of air in the intake combustion flow in all cases - even when cold. Adding additional air to the exhaust head during cold start in no way helps unburned fuel burn further. His simple conclusion is that all it does is dilute
the concentration (parts per million) of unburned hydrocarbons (gasoline). The dilution gets the concentration within government regulations and specifications! Crazy eh!
That explanation makes a lot of sense to me. The position of the automobile manufacturers is maybe that the catalytic converter needs the extra air to burn the unburned fuel. Problem with that explanation is that the cat is cold (and not doing a good job) anyhow at start. I know from watching a smog check in progress, that catalytic converters take some time to warm up. So I have pretty much concluded myself that the entire system is just to trick the emissions test - and a result of very mis-guided government regulation - what a surprise! If anyone else has a better theory, I'm all ears.
However, in any case you want to clear a CEL on your vehicle so this diagnostic system warns you of future problems. I fixed my air injection system. Maybe somebody here knows how to modify it so the diagnostic CEL system thinks it is working fine (even though it's dismantled). If my memory of Witek M's posts is correct, I believe it is measuring current to the pump during a fixed time. Tricking this may be more difficult than fixing it. Maybe the pump can be replaced and the outlet left unattached (along with sealing off the valves). If you live in a state where the emission system is checked regularly, you might get in trouble for this or any modification anyhow! So I'm happy I repaired it to work as it should - even though it's probably an entirely useless complicated system.
Read all three threads
in their entirety and you will be on your way.