the dreaded "Check Engine" light strikes again

[tower]

Hello you all.

Just picked up a high milage 95 S420 for a good price. Had it brought over and the dreaded "check engine" light illuminated. Need some help to start.

I took the car previously to the local MB shop for a pre inspection. Only thing showing up was a need for new front brakes (still waiting for Phil). As soon as it was delivered, the CE light flared. Okay...is this thing OBDI or OBDII? I can't find the diagnostic terminal for the OBDI situation anywhere, but the service manager at MB says its OBDI. Maybe someone can clarify where the OBDI terminal block is suppose to be for me.

Looking for assistance...

[LV202]

I'm pretty sure it's OBD II (since mine is a year younger than yours and is OBDII), check right under the steering wheel for a port. You can pull codes at most auto stores for free.

[wbain5280]

You would do well to invest in a stand alone code reader, available at most par parts places, or get a program to run on your PC or Laptop.

You will be able to read the generic codes and reset the MIL (malfunction indicator lamp).

Do a google search on OBDII OBD II etc.

[stevebfl]

Your car is most likely a OBDI car. All cars were OBDII by 1996. The "C" car because it was designed in these times had OBDII in 94, but the rest got them as production schedules allowed and by 1996.

The OBDI connector, the "38 pin" connector, is under a cap something like a coolant cap (black) situated at the forward side of the CAN box in the right rear of the engine compartment (passengers side). There may be a LED for doing DM testing (diagnostic module - where the light is turned on from) outside the CAN box. If not you will need a connector diagram and a impulse counting method. (the general description of which can be read in the evaluating engine controls DIY article and in many threads).

[tower]

Well found the area, it is definately an OBDI configuration. Got 4 blinks. I am now looking for the definitions of this error and will proceed to look through out the forum first before asking the question.

[tower]

Well. Code #4 for a 95 E420 shows to be an inoperative air injection system. Is it the same for the 95 S420? I have pm'd arthur d and MBDoc but figured I would post to the masses as things progress.

Cheers

Ron

[tower]

I have since checked the air pump on start up. Turns on for about two minutes on every start.

Arthur, in your conversation last february for and E420, are you saying that the SOV and the "bad/stuck air valve" are the same part? I have not checked vaccum yet, but will.

Ron

[JimF]

the DTCs from my page, MENU #6 for the Diag Module. CLick on the 'www' icon below.

DTC #4 could be a broken vacuum line, a bad actuator or, the worst case, the AIR pump itself.

[stevebfl]

Actually this is really the worse case code.

In the worse case the air injection passages in the head are plugged and head removal is the answer.

The code is tested real simply with a proper scanner. In activation mode, air injection is selected, the O2 sensor voltage is monitored and close loop is waited for, the air pump is engaged (which also triggers the switch-over valve to pass vacuum to the gatevalve (whatever they call the valve that keeps air from being aspirated into the exhaust when the system isn't workin), and finally the O2 sensor is verified to drop below 80mv.

By dropping to less than 80mv it is proven that a significant amount of air entered the exhaust. This test is done when conditions are correct to evaluate the system. The system actually works before the O2 sensor is heated (and operational in its normal activity).

If on the V8 both sides don't drop, possible causes are the O2 sensors them selves, the gate valve doesn't open (usually a vacuum problem to the valve), and restricted passages and of course the pump.

If the voltage drop is marginal on the O2 sensors one might crack the vacuum line to the brake booster loose. If that air source does drop the voltage easily below 80mv (on later cars with ME the volatge is 40mv) then some restriction or deficiency of the secondary air does exist.

[sixto]

My 95 S420 has a button and LED to read CEL codes. Does a reader tell you more?

Thanks,
Sixto
95 S420
87 300SDL
83 300SD ... $950, see cars forum

[engatwork]

Are there test "ports" beside that pushbutton and LED sixto? If there is then yes you can find out more than just the information that lights up the CEL. Do a search on code reader/scanner. Authur Dalton posted a link to a homemade reader one time if it is applicable to this car. They are easy to make and then you can open up a whole new chapter on obtaining data about your car.

[sixto]

Sorry, I wasn't clear. I understand that there are multiple systems and the LED only displays codes for the engine system or something like that. In Ron's specific case, what more will a scanner say about the inoperative air injection system fault? Will it give a code more specific to the nature of the fault such as one of the scenarios presented by JimF or Steve?

Thanks,
Sixto
95 S420 ... reason this topic matters to me
87 300SDL
83 300SD ... $950, see cars forum

[JimF]

is terrific! My tech's scanner doesn't do that as far as I know. He just got the newest 'head' for his AST Retreiver and it does do live data on almost all functions but I did not see that feature.

Gentlemen, what Steve said is that his scanner can force the AIR to engergize forcing extra O2 into the system causing the O2 sensor to go below 80 mv. That's pretty terrific diagnostic power.

If it O2 sensor doesn't 'move' then that would confirm the blocked passages or maybe the actuator not working/plugged, etc.

Steve, what scanner is that, model and manufacturer ??

[tower]

Hopefully our investigations will clear the trouble code and this thread. Thanks one and all for your helpful insite. After tracing through the vacuum lines, we found one of the lines dangling behind the passenger side head light. It would appear that the head light unit was replaced with new hardware and someone forgot to reconnect the vacuum line. I have cleared the codes, now we will wait to see exactly what is what.

Thanks again to all.

Ron