Re: Auxiliary air pump
This is off the top of my head so it may not be 100% accurate but I think the ECU commands the Relay Control Module which tells the relay to close and the pump comes on. If the relay does not reopen the RCM (if it is working correctly) will tell the ECU and the check engine light will come on. In an ideal world the driver will stop immediately when the check the engine comes on. He will open the hood while exclaiming, “Heavens to Betsy my secondary air pump didn’t stop when commanded to". What happens in real life is the check engine light comes on after a delay of a couple of minutes (just guessing), and the driver continues down the road, hoping for the best. At that point the relay will either start working again, the thermal shutoff safety switch (i.e. high temperature) on the pump will activate, or the pump will continue to run until it burns up. This system is a classic example of German over-engineering which is one reason why their awesome cars depreciate so precipitously. Most people are afraid of the maintenance issue on these overly complicated cars, which works out great for someone like me because I am finally able to afford some very expense cars, after a few years and about 80K miles. My SRT-6 only had 11K miles when I bought it. There is a person on one of the Benz forums that put together a 10 page dissertation on trouble shooting the secondary air system. If anyone is interested, there is also a link to it on an older post in this forum. I would recommend to anyone having problems to study his work because they could save potentially hundreds of dollars. I just look really smart. The smart people were the pioneers that came before me.