Purpose of diagnostic module (DM) on 95 E420?

[sytruong]

Hey everybody, so I'm chasing down a check engine light. This is a 1995 E420.

I'm new to this platform, so I immediately noticed an 8 pin diagnostic module on my firewall. I ordered a OBD reader from eBay from OBD kits with the larger pins to read from this port.

I was getting 4 blinks. So I cross check it with the literature that came with the reader and I cannot find any information on my platform using the 8-pin port. I also notice that the only holes that actually have metal contact is 1 and 3, #1 being ground. According to my literature, #3 should be CIS-E? Again, I'm getting four blinks at this pin, but I can't decipher what it means according to the literature I have because my car isn't listed with this 8 pin DM, but rather the 38 pin, for which I just ordered a breakout box for.

I guess my main questions is: what is the 4 blinks at pin #3 mean? My car isnt listed in the "Master code file" that came with the reader as having an 8-pin DM socket.

And also, what is the point of this DM if there is only pin number three available?

 

[gsxr]

That small plastic block in front of the CAN box allows you to read fault codes from the DM only, using the button to the left of the red LED. No hand-held blinker box needed. This ONLY shows codes from the DM (which is what triggers the CEL on the dash).

To read fault codes from the other 5 powertrain modules with your large-pin (4mm diameter) reader, you'll need a "breakout box" that plugs into the 38-pin diagnostic port.

Toss whatever "master code file" came with the reader, it's junk. Use the PDF's you'll find in the "De-Coding" subforum below. Everything you need to know is in the "Sticky" threads there.

4 blinks from the DM (which is pin #19 in the 38-pin port) is "Air injection inoperative".

De-Coding

Information pertaining to diagnostic codes, how to obtain and dechiper them, and how to remedy them.

www.500eboard.co

 

[sytruong]

Ah, OK. Thanks. Yeah, I was reading all the work and documentation on decoding. I'm slowly piecing it together. Thank you.

 

[sytruong]

I don't know any way to fool the O2 sensor readings.

Why not remove the CEL bulb if it bothers you? The CEL only indicates emissions faults anyway. If the car is driving ok, check codes on the other modules periodically if you're worried about some other issue popping up. IMO, the W124 CEL is nearly useless, I can count on one hand the number of times it's illuminated and identified an actual problem with my cars in 12+ years of owning .034/.036 chassis. The vast majority of the time, important codes never trigger the CEL as they are not emission-related.

@gsxr I just found this old post you made almost 10 years ago. It's pretty clear what you mean, but just double checking if this applies to my situation as well. Because if so, I'm going to stop chasing down the CEL's. Car runs phenomenal

 

[gsxr]

@gsxr I just found this old post you made almost 10 years ago. It's pretty clear what you mean, but just double checking if this applies to my situation as well. Because if so, I'm going to stop chasing down the CEL's. Car runs phenomenal

First check what fault codes you have on the other powertrain modules. And, see if you can figure out if the air pump is running or not. If the air pump is running at a cold start, you can likely ignore the CEL.

 

[nocfn]

The DS headlight BACK BLACK COVER has a air temp sensor on it. If that thing is in operative or you start the car and it isnt plugged in you get a CEL. It is common and its a small plug on the back of the black plastic back cover. it is not existing on the PS of the car, you will see it if you look, the wired plug to the AATS. ...added the caps for emphasis to a change.

 

[sytruong]

First check what fault codes you have on the other powertrain modules. And, see if you can figure out if the air pump is running or not. If the air pump is running at a cold start, you can likely ignore the CEL.

Ok, the breakout box is coming in from eBay, I'll run diagnostics on the rest of the modules.

I just checked the smog air pump, it doesn't engage at all from a cold start.

I'm also not getting power at one of the valves on the passenger side fender. I believe it is the switchover valve? It has the vaccum tube that goes to the smog air pump.

I'm getting power at one of the other valves. I believe it is the shutoff valve? It has the vaccum tube that goes to a diaphram looking thing on the intake manifold.

I'm also not getting 12V at the electrical connector to the smog pump. Could this be the relay? I've already pulled the relay in the "E" spot.

I have not ran 12V directly to the smog pump yet, as I cannot tell which connector pin was + and -, so I didn't want to blow anything up yet.

The DS headlight has a air temp sensor on it. If that thing is in operative or you start the car and it isnt plugged in you get a CEL. It is common and its a small plug on the back of the black plastic back cover. it is not existing on the PS of the car, you will see it if you look, the wired plug to the AATS.

Thank you, it was plugged in and I tested it and it was voltage responsive.

 

[gsxr]

The smog pump clutch is powered directly by the LH module. Try sending 12V directly to the pump and see if the clutch engages. If not, the clutch or wiring is bad. If it does, something is up between the LH module and the connector at the passenger fender area. The clutch is a simple electromagnet so polarity shouldn't matter, but if one wire is brown, that is negative/ground.

 

[sytruong]

Thanks. I was just browsing around Google for possible solutions and found this. I pulled the E relay

If it matters, I just tested the relay and it doesnt click, so I assume its bad anyway.

Going to apply 12v to that pump now.

 

[sytruong]

Smog air pump runs with 12v directly applied.

 

[nocfn]

Thats good now the LH needs to be checked out, bet some caps are blown... Do you have a spare.... time to buy and have them both reworked by Al!

 

[sytruong]

Right on, I don't have a spare. I have a picture from a former thread that @gsxr said was a newer replacement.

I found my relay (pictured in the above thread) wasn't working. Its a green one. According to the diagram, it's a relay for the secondary air module.



Funny story is that I found a local guy who had 2 of these, and none of them worked either. Neither did mine. None of them responded to 12V at pin 85 and 86. I measured voltage across the power pin where the relay is supposed to sit, and I get 12-14V. Which further points to this relay being the culprit.

Anyway, I have a used, supposedly working one coming in from eBay. We'll see if it's functional.

I opened up the relay itself, and its not a mechanical relay. I has capacitors and chips in it. I assume it's because its protecting the computer circuits, and I assume why it has a higher failure rate. Who knows.

I also just got my car *smogged* (I'm in CA)... and regardless of whether I get my smog air pump resolved, I will finally be putting this thing on the road.

 

[gsxr]

The MB systems tend to provide +12V power and then switch the ground, i.e. connect the negative side of the coil to ground on demand. So, applying +12v to the relay may not do anything because there's no ground on the other side (which may go to the LH module).

Schematics are here, K17 is the smog / air pump relay:

https://www.500eboard.co/forums/misc/Manuals/Servicev3.2/W124/w124CD1/Program/ETM/24075001.pdf

 

[sytruong]

Ah, I see thank you very much. I will reference this during my troubleshooting