Okay, so I can't figure out what this part is, nor can I figure out what it does, other than make a noise when the engine starts up, only when it's cold. Once warm it shuts off.

Mike

 

Auxiliary Air Pump.

It's what you (should) hear whining when you start 'er up. For ~ 15 seconds or so.

 

This is a smog pump aka secondary air injection pump. The part number is 05098830AA.

Here's what the MB folks say about it:

An air pump is used on the vehicle to lower tail pipe emissions on start-up. The Powertrain Control Module (PCM) grounds the Air pump relay control circuit which energizes the Air pump. The PCM also grounds the Air combination valve vacuum control solenoid circuit, which energizes the Air vacuum control solenoid. Vacuum is then applied to both Air combination valve diaphragms which opens the shut off valves. The PCM enables both circuits simultaneously when Air system operations id desired. When the AIR system is active, then Air pump forces fresh air into the exhaust stream in order to accelerate catalyst operation. The AIR combination valves replace the conventional check valves. When the AIR system is inactive the shut off valves prevent air flow in either direction.

If you're seeing a Check Engine light or are experiencing hesitation at take off they recommend replacing it.

 

If you have the CEL then there are much cheaper options available than replacing it. The problem is most likely a bad ground, or a bad relay. The cost of these is zero or $6, if you want to replace the pump you are talking hundreds of dollars. Take your pick.
The fault does not affect performance at all, you just will not pass any required pollution tests.

 

It's a good thing nothings wrong with it, I was just curious what it was.

Thanks guy!

 

What if it won't turn off?

 

Most commonly, this is caused by a stuck relay on the RCM. Yank the RCM and send it to DJ for reworking, he replaces all suspect relays and reflows the bad solder joints, etc.
There is NO WAY the issue can be anything other than a stuck relay. The only alternative is that the output transistor on the ECU that pulls that relay in, is bad. So far, in 12 years here, I've not seen that happen.

 

cleaning the relay worked. Thanks

 

Cleaned relays do not last too long.

 

Good morning,

Secondary Air Injection Smog Pump kicked on during the last few minutes of my drive home today, and continue to run after I turned the car off. Disconnected, let cool down, reconnected, but still runs without key in ignition.

Please provide a bit more info about resetting relay, or cleaning.

Can I still drive it while having PMC reset?