Secondary Air Pump - '95 E420 - PeachParts Mercedes-Benz Forum

Syntax26 · #1 05-30-2008, 10:41 PM

I've been attempting to diagnose CEL issue (and codes) for a couple weeks now. Full story - all codes - parts replaced - can be read at

http://www.peachparts.com/shopforum/showthread.php?t=220910

But, I just have a quick question or two about the secondary air injection pump. I'm trying to figure out if the pump is defective, or simply not being turned on/activated.

At cold start:

1. If pump runs...you can tell this by looking at the triangular plate in the front, correct? (the clutch engages inside the otherwise free-wheeling pulley) (is this possibly the dumbest question ever asked on here? LOL!)

2. If pump is defective and not running...wouldn't you still get voltage reading at the harness connection to the pump? And wouldn't it be 12 volts, more or less?

3. If you don't get voltage at the harness...is it possible there is a relay not operating? (in other words - the pump might be ok, but it is simply not getting energized)

4. Finally, can I test the pump directly by using another 12 volt source and connecting it to the pump?

If anyone can answer advise me on this....I'd greatly appreciate it. I'm obviously trying to figure out if it is the pump itself that is bad before I buy the replacement. Or, if it the problem is really that the pump is not getting turned.

By the way, the reason I'm focusing on this issue is that after reading codes for week, I've figured out that that first two codes that return (meaning, before any lambda or knock codes) are the following two:

Pin 4 - Code 11 = secondary air injection system, open or short circuit
Pin 19 - Code 4 - air injection system faulty

Oh...and also by the way - I've just put a new 02 sensor in.

Thanks!

Jim

Arthur Dalton · #2 05-31-2008, 04:28 PM

Well, your first 4 questions tell me that, before anything, you should check DA FUSE !!!!

#7

Is the pump running or isn't it???????????????

Syntax26 · #3 05-31-2008, 04:53 PM

Mr. Dalton,

Thanks for your reply.

I had checked fuse 7 before - it is good.

This morning I applied an alternate 12 volt source to the pump - and the pump ran. Which leads me to believe the problem is not that the pump is defective, but instead that it is not be energized (turned on).

I was trying to find a schematic, but even though I own 3 MB repair manuals, I can't find a schematic for this. I would assume there is a relay for the power to the pump.

Can you tell me if there is one?

Thanks!

Jim

Arthur Dalton · #4 05-31-2008, 05:08 PM

K17 is the relay. It is feed from fuse 7.
It is in the relay box behind the fuse box

Syntax26 · #5 06-01-2008, 07:19 AM

Arthur,

Thanks for the info on the relay.

I'll check it today to see if it is functioning.

I'll be sure to let you know if I have success or not.

Thanks!

Jim

Syntax26 · #6 06-02-2008, 10:07 PM

Arthur,

Kind of a unique problem here. I say this because I've researched this code quite a bit...and in no case have I seen the issue be the relay. Nonetheless...

It is either the relay malfunctioning, or, the relay not receiving a signal to close the contact and send 12 volts to the pump.

I know this because I pried the cover off the relay and closed the contact by hand...and the pump comes on (by which I mean the electro-mechanical clutch engages and the pump runs).

Unfortunately, I've yet to be able to obtain a replacement relay. The local MB dealership parts department insists the part number I give them (126 820 20 26 = stamped on the relay itself) is a wiper relay - so they tell me they aren't able to help me. This part number isn't recognized (let alone stocked) by the vast majority of web-based MB parts specialists. I have found 1 company in Idaho that stocks it...so I guess I'll order a unit from them.

I thought you might appreciate hearing what I've learned, since despite researching this issue on 3 separate MB technical forums, I've never seen the relay to be the issue. Always either the pump is broken, or, it's a clog issue in the head ports or other valves and piping.

But I do have a question for you - maybe theoretical at this point. If the relay is not defective...this would mean the problem is the relay is not getting a signal from the ignition control module. Which would mean either the ignition control module is not signaling the relay to engage, or, the signal is not getting to the relay (due to a wiring issue).

Does my logic seem correct to you? Or can you think of another reason for the relay contact not to close that I am not aware of.

Well, anyway - thank you very much for your help is letting me know the relay in question was K17.

I'll let you know the results when I obtain a replacement relay and test my theory.

Jim