secondary air injection system trouble
 

I have a 98 SL500. Ocasionally the check engine light comes on. I have a code reader and the error mesage is "secondary air injection system malfunction". I can reset it and it will be fine for a while. Have not noticed any pattern of when it comes on, maybe at idle when humidity is high. I've heard this can be very expensive and hard to solve. Anyone know of a good system check to isolate problem.

MANY scanners have the ability to activate that emission system(and most other systems as well). You need to do that in order to diagnose the problem.

When the air pump runs the FRONT O2 sensors must react to the oxygen from the pump & stop cycling. They MUST stay at 50mv or less durning that test!

check out this thread
http://www.peachparts.com/shopforum/showthread.php?p=1019089#post1019089

That thread is for the old belt drive pumps.

Your chassis has an electric pump

http://catalog.eautopartscatalog.com/mercedesshop/sophio/image.jsp?title=Air%20Pump&url=http%3A//img.eautopartscatalog.com/live/B601070965BOS.JPG
Check if it comes on and check fuse power.

if the air pump works
possibly clogged/not opening air valves or egr ports in the head. I just fought an E320 with the same problem. It would turn the light on after 100 miles or so.

pay close attention to the o2 voltages

One thing to check is the vacuum line from the switch to the bypass valve. I had that occur on my SL and that is what it turned out to be. Check out our website at www.mbca.org/northtexas, go to tech stuff, then engine technical, there is location of engine controls. Fully inspect the vacuum lines to the bypass valve, the switch, and even check the valve to see if it is operating. Get this fixed as if it is broken you could ruin your air pump, or even worse ruin the converters. Good luck!
Brian