I can't say with 100% certainty the pumps are the same internally? I just know that my superior Mercedes secondary air pump burned up when the relay stuck closed for about 10 minutes (to its credit, it also had 8 years of service). However, my potentially inferior Audi pump hung in there. When the relay stuck closed, the Audi pump was running for about 15 minutes; hot as a firecracker before I got pulled over in Houston traffic where I could safely unplug the pump; saving its life from an evil relay. After changing the relay my Audi pump runs fine. I was pretty proud of my research and decision to not pay the extra money for a Mercedes or a SRT-6 secondary air pump. I feel like a made the correct choice and got exactly the same pump but like you said there could be differences that I am not aware of. Even if there are subtle differences the Audi pump is a good and proven option for the less purist among us. I am only trying to offer my learning experience as a potential option.

 

And the knowledge that it works so far is great for those who may want to try your solution. Have you tried to pull any codes to see if anything is listed that doesn't trip a CEL? That would help others (maybe reassure) that it could be worth a try? The cfm/rpm may be different, as well as any number of other things like current draw. I don't know, knowing things like this helps others determine if the part is a replacement (more robust, or not). Just food for thought!

 

The code cleared with the new pump. I don’t' remember the code number because it’s been a while since I made the replacement. I considered putting a resistor there instead of the pump just because that system is unnecessarily complicated and for the most part useless. I couldn't find any information but I didn't look at the schematics either. That is about all I know on the subject except that if folks are burning up pumps they may have a relay problem because those relays are prone to stick at times.

 

Is it possible to put a small indicator light from the pump when it is running? This way you could see it running and if it's on for a long time you could unplug it before it burns up. Just a thought.

 

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.

 

Per a number of well published posts, and a few very helpful dedicated members, we know those relays are sometimes not the best for the application (bad contacts/excessive current draw which pitts the contacts excessively/defective relay which 'hangs' before complete mechanical throw/cheap low quality solder used during assembly/anything not yet mentioned). There are a couple of members who volunteer their time and efforts for quite the low price, that installs (or has sources for those who are capable of replacing themselves) and modifies a couple of places on those RCM's including new relays that are either damaged beyond repair or just not serviceable at all. One thing I would do before sending an RCM (and thats just me, and I would send it only because I am now at the age of physical decline that my close-work soldering skills may not be what they used to be given the location where the work is needed, to put it mildly) a voltage/current check of the air pump, at the air pump [and] the rating of the part itself (if there is a specification available to compare it too). This small step would help insure any relay repair/replace will have the chance of being more successful given the contact rating doesn't approach approximately 75-80% of rated current at applied voltage (yes, 75-80%, because as voltages vary across the operational rpm's of the engine, higher or lower current is required vs the voltages available at the pump motor, to run the pump as neccessary). Main reason I would do this, is if the part is drawing too much current, the whole process will have to be repeated again down the road. Now, if that replaced relay is of a higher current rating (for its contacts) you still have to be aware of the tracks on the circuit board (cross-sectional totality of conductors) and that they are also able to handle the increased current draw (if present). Whole circuit concept, sometimes these little fine details get overlooked in the rush of the moment. When something fails, its easy to fix, when a few more have the same problem then something slipped by the engineers or the builders. Sucks having to back-design (so to speak) but we all help each other to solve problems across the platform. I can use a good example where people put those mega amplifiers in their cars and wonder why the battery and alternators are constantly needing replacements.

 

One possible solution to the air pump relay issue is for some genius like DJ ( tighed1 ) to do a circuit analysis of the RCM/K40 board and see if it would be possible to leverage the empty relay location to have 2 parallel relays for the air pump.
Or, alternatively have the existing relay drive an exterior big honkin relay
.... just a thought.

 

Which relay - the K40 relay or a relay within the pump mechanism?

 

The relay would be the one inside the K-40 module.

 

The aux air pump is driven by a relay in the RCM/K40 module :

 

That photo pops up all the time, I should be getting royalties for it.

 

I do say a silent "Thank you, Your Highness" every time I post it.
Isn't that royal-ty enough ?

 

No thanks required sir, just send money.

 

Yes, certainly, the parallel relays are quite interesting. If I had another (I wouldn't want to screw up the one in my car) I would check and see if there are any conflicts involving the tracks on (and in, I understand that is a multilayered board?) the existing. Of course one MUST insure there aren't other board revisions, as these also must be completely checked to insure any tracks are identified (or not) that would interfere with other circuits (isolation). Quite an interesting thought George!

 

I must be missing something, if I had two or more relays in parallel I would expect the failure of one, that would be welded contacts, to keep the pump going all the time.
Two or more in series would do the trick, until they all screwed up.
Or have I got my series/parallel circuits mixed up? I know little about sparky stuff.

 

2 NEW relays in parallel would provide twice the contact area, therefore dropping the amp load on a given set to 1/2 of what it was.
In this way, I don't think we would ever see the contact erosion problems currently being experienced.

That being said, I am inclined to think that the best solution would be to use a 100 amp relay driven by the existing relay.
( or remove the existing relay and wire in the larger one )
This : Sales Hot Car Motor Automotive 12V 100A 100 Amp SPST Relay 5 Pin 5P Heavy Duty | eBay or
http://www.grainger.com/product/GRAI...AS01?$smthumb$ ( 70 amp, but much better than the current 40 amp one )
may even fit inside the RCM so no external wiring would need to be done.

 

Spot on George IF the tracks on all the RCM's are isolated from each other AND there isn't other layers involved with any of the tracks. . Simple jumpers could easily pull in that extra relay spot with the air pump relay.


One question (since I haven't taken my car's air pump system apart for any sort of testing) is, how much current does the air pump require, at idle (and say) 2000 rpm? I don't know if that pump actually kicks in at speed, I know it kicks in after deceleration to a stop, or deceleration to almost a stop (due I believe to the extra fuel residuals being sent through the cats/sensors telling the ECU to command the pump on?). Does anyone know how often that pump kicks in (say) 20mph and up during casual driving? I would think it kicks in more often if aggressive driving is present (ie speed up quickly, unload drivetrain, then reload drivetrain again repeatedly). Reason I ask, a 40 amp relay is a hefty relay unless the contacts/spring/response time aren't designed/engineered very well. I've had no reason to tear into mine yet so I haven't looked into it. Next time the hood is open, I might go ahead and remove my RCM and inspect the contacts (and see if the board has any telltale signs of it being multilayered). It'll be on my FMI 'to-do' list. Its parked right now after this mornings garage repairs and yard maintenance (I put the passenger side as far to right as I can to have added access to high traffic area in garage so the passenger side of car is inaccessible right now). So, whats that pump motor rated at, and how much is it drawing under various loads?