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Old 07-11-2014, 12:16 PM
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Join Date: Dec 2001
Posts: 577
M103 - CEL Diagnosis - Where is the red LED light?

CEL comes on after 4-5 mins off a cold start. Found instructions for reading this vehicles OBD-I codes.

- Lift small cover to part 126 545 9740 just to right of battery
- Turn Ign. switch to 1st position - dash lights on
- Depress stalk type button on 126 545 9740 and hold for 3-4 seconds. Release and count blinks from LED light.

There is no LED light. I see 8 ports under the cover once lifted. Thought perhaps I'd have to read blinks from an LED on dash as some OBD-I vehicles work this way. Did not see anything going on there.

Any idea where this LED is located?

Update - Guess I needed to dig a bit more first. Turns out not all M103s have an LED. The following URL shows a homemade light used to diagnose an SRS issue. That is port 6. CIS(what I suspect is my problem) is port 3. The layout in this document lines up with my 8 pin setup. I'll make this gadget and report back my findings. MBDoc reported that most often CEL on this car is O2 Sensor. I want to verify and I need a way to reset code once fixed. Might be awhile as this is a non-critical issue at this point. Car not being driven right now.

Last edited by Mike Richards; 07-11-2014 at 01:04 PM.
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Old 07-14-2014, 10:48 AM
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Join Date: Oct 2011
Location: Columbia, SC
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You're on the right track, only the Cali 124s had the LED and button built in to comply with their regulations in place at that time. For the 49-state models, you will need the impulse counter. This can be simply built with an LED, pushbutton, resistor, fuse and 3 test leads. Or if you are not handy with a soldering iron, eBay is awash in these devices.

You are basically emulating what is in the Cali test box that was not included in the 49-state models. MB saved a whole whopping 50 cents on that deal not going ahead and including that on all models.
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Old 07-16-2014, 07:28 AM
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Join Date: Dec 2001
Posts: 577
For those who like nice and pretty:

Mercedes Diagnostic Code Reader Tool W124 OBD1 300E 320E 400E 190E and Others | eBay

And for those who like do-it-yourself-quickly - extracted from a post by stevebfl:

The impulse method of code retrieval uses a box that counts and sums electric inputs. The input that is used is a switched ground. One proper method (which I have used) is to wire a led (light emitting diode) between batt positive and the diagnostic socket in question. . Once the diode is wired in place, turn on ignition (the diode might blink once but it shouldn't be
lit) and with a second jumper wire short the diagnostic terminal (which you are checking) to ground (the led will of course light during this event. Do this activation for
2-3 seconds. After a brief pause the ECU being addressed will send its ground signal output. One pulse means you have no faults, any other number describes a particular
fault code. Do you have data to decipher the code?

After the last pulse of the string and you have written the code number down, initiate the sequence again. If you get
the same code that was the only code in memory. If you get a higher number (the lower numbered codes will come out
first) write it down and repeat until you get the first code to repeat.

You now should have all your codes recorded. Now you should probably erase them and reread them to determine which faults are permanent (exist at the moment). To erase the code you start where we left off; having read all the codes and then re-read the first code. At that point short the diagnostic pin to ground for 6-8 seconds. Next do the retrieval initiation (the two second grounding), this will pull the
next code. Do the 6-8 sec erase procedure and then reread again and erase. Do this until all the codes you recorded have been erased. You will know this has happened when you again reread the code and you get one blink for "no codes set".

One should not do this unless you have appropriate service info to use the results and understand the system.
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Old 11-30-2014, 12:47 AM
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Join Date: Dec 2001
Posts: 577
Got thru my summer projects - back to the Benz.

Pulled 3 codes:

Code 13 - intake air temp reading illogical
Code 22 - O2 sensor heater voltage illogical
Code 31 - Intermittent contact engine coolant temp sensor

Hard to believe all 3 of these devices went south at the same time.

Will check wiring on devices for codes 13 and 31 and clean int air temp sensor with some MAF cleaner as per a comment made by Arthur Dalton in a previous thread regarding that device.

The O2 sensor being shot is not at all hard to believe. As mentioned above, MBDoc stated in an old thread it's the most common cause of a CEL in my vehicle and likely others with an M103 engine.

Will update this thread when the CEL is cleared.

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