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1999 Mercedes Benz ML430
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Discussion Starter #1
Hi, I am a newb to this forum so I apologize if this isnt placed correctly.

1999 ML 430, 107k miles. I have read several threads on p0410 and the air injection system. The Air pump fuse on my ML and the relay work. I checked the pump's harness and was getting about 12v across it when the engine cold starts. I was also getting vaccuum to the valves that open to allow the air injection to take place. I wasnt getting any air output out of the pump. I figure its the pump.

Here is where it gets weird. I took the pump off and hooked it up directly to the battery to see if it would work. Nothing happened. I order a new pump. I get the new pump and hook it up to the battery, doesnt work. So then I take the plastic cap off the top of the old pump and hook it the battery and it starts working. When I put the cap back on it stops. I try the same thing with my new pump and nothing happens, the new pump wont start with or without the cap on. My question is: Is my new pump no good? Also, why would the plastic cap, that comes on all of the pumps interfere with their operation? Does the fact that the old pump operates in this manner mean it is bad because it shouldnt do anything the way my new pump doesnt do anything?

I am really confused. I thought I had this solved. Now I am not sure if the new pump is faulty or if the pump was even the problem to begin with.
 

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Welcome to the forum.

You need to evaluate your connection method. You are dealing with low voltage high amperage connections. Pump at startup may take 50A. For such high amp connections MB uses 4mm round silver plated connectors. Male 013 545 79 28, female 003 545 26 26 soldered to 2.5 mm2 wire. Anythings less then that will make poor connection and component will fail to start.

Check and report resistance across pump pins. Both old and new.
 

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1999 Mercedes Benz ML430
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Discussion Starter #3
Hi Witek,

Thanks for the response. My multimeter is a pretty cheap analog one so when I have the dial set to 1k ohms and I read the resistance on the old pump its off of the scale. Unfortunately this is the max scale on my meter. This is a consistent reading on the old pump. When I try to read the resistance on my new pump it doesnt even register. I get tiny blips on the meter and thats all.

This also seems very strange. It seems like I should get consistent resistance from each of the pumps
 

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Air pump resistance should be less then 1 ohm (~0.7 ohm). You need to put your meter to lower scale.

To get consistent readings you need to make consistent connection.

Please, update your profile with vehicle and location info.
 

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Discussion Starter #5
ok. So I cold started the engine this morning and at the pump harness I get 14V which is within the specs I have seen. On the first cold start i hooked up the OLD pump and it began pumping air. I then hooked up the NEW pump and no air. I checked the harness and was still pulling 14v. I hook up the new pump again and no air.

I read your post and with my meter in the X1 ohm position I get a reading of like .1 ohm on my OLD pump. On the new pump though I get nothing in the
X1 position. When I switch to the X1000 ohm position I get a reading of like 5000 ohms. This doesnt make sense to me. First why would the old pump work one time and not the others? (I checked the harness output at the end and was still pulling 14v so they should have worked) Second what are the odds that my NEW pump is faulty? Third why would the resistance readings be so much different between the two?
 

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Go to the MB dealer in pick up two of each male and female connectors. They are few bucks. Get air pump relay as well. There is updated part number on this. Try to insert new connectors together. This will give you idea how tight they fit. Using new male connector test your air pump connector for tight fit. It is possible that connectors are spread out.

If we ware to trust your meter 0.1 ohm load would require 120A (12/0.1=120) to run. That would blow 40A MaxiFuse instantly.

When measuring voltage at pump connector you need to do it on loaded circuit. Tomorrow morning open plastic connector housing so you can connect test leads to the back with pump plugged in. Circuit that reads 12V may go dead when loaded.
 

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Discussion Starter #7
Ok. I took the harness apart and hooked up the pump. With the pump installed I started the car and read about 13.5v across the pump terminals. While this was happening the pump did nothing.

Does the fact that I get a reading rule out the relay/fuses? Or could it be the pump wont operate at 13.5v because that is too much voltage? I am confused as to what the relay actually does. I'll check the connectors but they feel tight.
 

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Ok. I took the harness apart and hooked up the pump. With the pump installed I started the car and read about 13.5v across the pump terminals. While this was happening the pump did nothing.

Does the fact that I get a reading rule out the relay/fuses? Or could it be the pump wont operate at 13.5v because that is too much voltage? I am confused as to what the relay actually does. I'll check the connectors but they feel tight.
If you are reading 12-14V with pump connected then pump is bad.

Relay allows to control high current consumers using tiny current while providing complete isolation between the two. Short in air pump circuit will not fry engine control module.

Relay - Wikipedia, the free encyclopedia
 

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Discussion Starter #9
Ok so we can rule out the relay.

So if pump is bad why does it start pumping when I hook it to the battery directly?

If I am reading 13volts at the pump terminals it should run the same as when its hooked to the battery right?
 

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Discussion Starter #10
Hi Witek,

So I went and bought a new digital multimeter. I tested the resistance across the pumps. For the NEW pump I got 430,000 ohms which is way way off, so I have come to the conclusion that the pump I bought is faulty. For the OLD pump I get readings ranging between .2 and 1 and it usually settles near .2. This is odd because like you said that would mean I need 60 amps of current which would blow the 40 amp fuse.

So I decide to keep the plastic cap off (that goes on at the electrical terminal side of the pump with three torque screws) and install the old pump. When I do this and start the engine the pump begins to operate blowing tons of air. At this point I am figuring that for some reason the plastic cap interferes with the pump function. I am going to install the pump with the spacers between the cap and the pump body and hopefully it will reliable start at cold starts.

I appreciate your help. Please let me if you have any information or other insights that might apply.

Thanks again
 

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1999 ML320
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Why is the pump bad when reading 12-14V?

If you are reading 12-14V with pump connected then pump is bad.

Relay allows to control high current consumers using tiny current while providing complete isolation between the two. Short in air pump circuit will not fry engine control module.

Relay - Wikipedia, the free encyclopedia
I am tracking a similar problem. The fuse had been blown, and the replacement blew also. Now I will hotwire the pump to see if it works or is causing the overload itself. I don't understand why you say a voltage reading of 12-14 volts indicates a bad pump? That is the voltage range that the battery supplies to this pump is it not?

Any advice on how to check the relay with my multimeter?

Thanks
 

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I'm convinced

I'm convinced there is an overload as mentioned in my previous post. However I don't know yet if it's a bad pump/motor, or possibly the wiring, the relay, or something clogged downstream as suggested by others here. So I am still troubleshooting and am curious about your previous comment that 12-14V indicates a bad pump.

I do my own car repairs as much as possible for economic reasons and I enjoy the challenge most of the time. We recently purchased this vehicle and it's our first Mercedes. This is also our first CEL on the vehicle. The fault code "P0410 Secondary Air Injection System" has led me here. All the advice here has been very educational and useful. I'll know more about the pump and plumbing tomorrow.

This web site is a fantastic resource, but a little difficult to navigate with my limited experience at this point. As an example, I don't know where to ask for advice on obtaining a good service manual. To use this example, I would like one that shows me the diagnostic test procedures for this pump and relay. Having an index of fault codes that would lead me there would be great too.

I've had several VW Westfalia campers, and the Robert Bentley service manual that covered them had instructions for removing and testing/measuring just about every component. I'm sure this is not the right thread for a service manual inquiry, but maybe somebody can point me in the right direction. Thanks for any help and suggestions. John
 

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Thanks

Witek M - Thanks for the link on relays. I'll study this and check the one for my air pump. John
 

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Comment was in response to:

Originally Posted by buster777
Ok. I took the harness apart and hooked up the pump. With the pump installed I started the car and read about 13.5v across the pump terminals. While this was happening the pump did nothing.

If relay was bad we would have no voltage at all. If relay contacts were tarnished we would have partial voltage like 5V under load.

If you have circuit that blows 40A fuse hot wiring is not a best idea. Unless you use jumper cables, wire that you use may start melting in your hands. Clamp on amp meter is only safe way to do it. If you want to check for short before the pump simply disconnect it and connect test light across removed connector.

http://www.mbwholesaleparts.com/vcm/MBWholesaleParts/Files/Feb09StarTuned-AnotherChance2Burn.pdf
 

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MB stopped publishing printed manuals 15 years ago. Old stuff up to '85 got scanned and put on CD/DVD. Newer stuff is in electronic format only.

From your local MB dealer order P-2700-163-05 Star Service DVD.

Star Service CDs and DVDs
 

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1999 ML320
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Witek - Thanks Again!

Thanks for your thorough clarification, information, and references. One of Buster's later posts seems to indicate that his pump was running at this voltage - which contradicts his earlier post.

The article on MB Secondary Air Injections Systems is excellent and could easily replace most of the threads and posts here that are related to this topic.

There seems to me to be one major irony to the entire system. I understand that the Secondary Air System adds air during a cold start to assist in burning the cold rich exhaust stream mixture at the cats, and thereby warming up the cats much faster than they would warm up otherwise. Another result is that the secondary added air is detected by the oxygen sensors and interpreted by the brain to be a too lean fuel mixture, which the brain then corrects by adjusting to a richer mixture. What a snowballing of cause, effect, and solutions! It all seems like a disconnect in engineering coordination - like the motor, emission, and control system engineers not communicating with each other during development. I am no automotive engineer, and my 120,000 mile ML-320 demonstrated excellent low emissions output when smog tested in the warm condition. However, this cold start Secondary Air System, to my current understanding of it seems almost comedic. Can you help me on this as it must surely be my misunderstanding again.

Do you subscribe to Star Tek Info? Looks like a great resource, but at $3000 per year it is much too costly for my non-professional use. Too bad it is not an integral part of the service manual purchase. I shudder to think what that will cost, but will need to look into this purchase.

I don't have an update on my pump other than I removed it and it is not seized or otherwise looking burned up. It's cold here (15 degrees F). Without a garage, my car repairs go very slowly in these conditions. Fortunately I found a supplier of new identical OEM pumps for $167. That's way more digestible than the dealer price if I indeed need a replacement. Hopefully I don't have clogged cylinder head exhaust passages.

Regarding the relay test - the simplest diagnostic solution frequently escapes me!

Thanks again for your expert advice and information.
 

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What author of that article fails to mention is that during air injection operation automatic correction of mixture is blocked. See paragraph 8 in pdf. What is evaluated during test is drop in front oxygen sensor voltage. Voltage needs to drop to <100mV within 20 seconds.

Below is graph recorded during air injection test performed by ME every day or once per trip.



On X axis time since engine start in seconds. On Y axis:

V front oxygen sensor
V rear oxygen sensor
C engine coolant temperature

.
 

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Discussion Starter #19
Thanks for your thorough clarification, information, and references. One of Buster's later posts seems to indicate that his pump was running at this voltage - which contradicts his earlier post.

QUOTE]

Hi Jolson,

Just to clarify, I had bought a new pump and when installed it would not operate. My thread was basically me figuring out that the new pump I bought was faulty, just a bad coincidence. It turned out my old pump would actually operate when installed on the vehicle, it just wouldnt operate when it had the plastic cap at the top attached. The apparent contradiction is due to the fact that I was checking two different pumps, one that I thought was NEW and my original. You can imagine the problem you would have when you buy a NEW pump and it fails to operate, you believe the pump is good so you start assuming it must be something else. In my case I was getting spec voltage at the harness and it turned out my NEW pump was bad.

When I wrote about a pump operating I was referring to my original pump with the plastic cap removed.
 

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Witek - Blocking the automatic mixture correction certainly makes sense of all the cacophony going on!

1. What does acronym ME actually stand for (I've assumed it means the computer control of the auto systems or what I loosely call the brain)?

2. Is the front oxygen sensor before the cat, and the rear after the cat?

3. Why does the voltage of the front oxygen sensor jump around so much before and after the test (normal operation I assume)?

4. Apparently this test took place at about 10-11 minutes after a cold start. What is the blue line (%) on the graph indicating?

5. Understanding a system's components and what it is supposed to do sure helps diagnose trouble codes and problems. Is there an inexpensive way for us home mechanics to get Star Tek information?

Thanks for all your help! I wish you were closer to Lake Tahoe because I would have no trouble paying you to work on my car. A good car mechanic is really a sharp guy these days, and worthy of a high salary. A bad one can be worse than throwing money into a paper shredder (the paper shredder is fast and doesn't cause more problems)! The closest MB dealer to us is in Reno (about 100 miles from us) and my experience with dealerships is that it's always a crap shoot - and Reno is a gambling city!

Best Regards, John
 
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