02 Light 1986 560SL - PeachParts Mercedes-Benz Forum

TuumEst · #1 06-12-2009, 05:16 PM

Gents:
I am a newbie to this forum. In reading most of the previous posts, I am truly impressed with the quality and friendliness of the responses.

My vehicle has 220,000 km on the clock. Last week, the O2 light came on and has remained on, despite a battery disconnect.

The car is now very hard to start. When it does light up, it runs rough for a few minutes and then appears normal. The next day, the problem repeats except it is becoming increasingly difficult to start.

My question is:
Can I determine the cause with an OBD reader or do I replace the O2 sensor on spec? Or, God forbid, am I looking at catalytic converter replacement?

Paratroop · #2 06-12-2009, 06:38 PM

Just change the O2 sensor,you will have to pull the floor mats up,
liner,remove the front plastic kick panel,and a nut or two on the
hold down clamp,undo it plug up the the new one.Make sure
you use some PB blaster on the old 02 a couple of days
before you try to change it.Takes longer to jack up the car
and slide under it than it does to change it!!

jplinville · #3 06-12-2009, 07:02 PM

I agree with the man from above...just replace the O2 sensor and the issue should go away.

TuumEst · #4 06-12-2009, 07:55 PM

Thanks guys. I'll do just that.

TuumEst · #5 09-08-2009, 06:23 PM

Well gents, after much PB Blaster, O2 sockets and breaker bar, I could not remove the sensor. Finally gave up and had a pro remove it by heating the threads with a torch. It took him almost an hour.

Unfortunately, with the new sensor installed and the wiring checked, my O2 light remains on. I disconnected the battery for a while, hoping for a reset but still have the problem (light on, car almost impossible to start).

Does anyone have an idea on what to check next?

Really appreciate your help. Thanks.

Strife · #6 09-08-2009, 09:26 PM

When they are stuck, they are really stuck. I used both kinds of oxygen sensor nut-rounding tools before bringing it to a shop and the burned it out with an acetylene torch. I was told that this flame gets things so hot so fast, it doesn't damage the surroundings. They didn't even have to weld in a new bung.

Strife · #7 09-08-2009, 09:26 PM

Did you get an exact replacement and connect it via the connector, or did you splice a new (4-wire, I hope) unit to the old wiring?

TuumEst · #8 09-09-2009, 05:22 PM

I replaced it with a Bosch 13930 'Premium' Oxygen Sensor, ordered from the local MB dealer for my model car. It is a 3 wire unit and came complete with the wiring harness to connect into the receptacle in front of the passenger foot rest. At this point, I have to assume they gave me the correct part but stranger things have happened...!

Is it necessary to accomplish some kind of reset or long-term battery disconnect when changing these sensors?

Appreciate your assistance and previous emails. Thanks.

Strife · #9 09-09-2009, 07:46 PM

Hmmm...the dealer very likely gave you a good part. Assuming that the connector is tight, I wonder if the ECU itself is working. Unlike the earlier cars, the O2 lamp is really controlled by it.

I find it VERY suspect that it's coming on immediately. MAYBE this is an OVP relay problem?

TuumEst · #10 09-10-2009, 01:05 AM

Thanks for the reply.

Just to clarify: the light comes on when the ignition key is turned on and it stays on IF the engine starts.

If I understand that OVP is over-voltage protection, how does that relate to the O2 light? I'm fairly sure that the alternator (changed a year ago) is putting out the correct voltage.

Can you point me to the ECU location? Perhaps I should try cleaning any connectors involved there. Otherwise, I really don't know what my next step would be to resolve this darn thing.

ctaylor738 · #11 09-10-2009, 12:56 PM

The ECU is under a cover under the carpeting in the passenger footwell.

But just for grins, remove the under-dash cover on the passenger side and look at the relay farm above the fuse box. You should see a relay with a fuse on its top under a plastic cover. Make sure that the fuse is good.

TuumEst · #12 09-11-2009, 04:06 PM

Thanks for the suggestion. I pulled the OVP relay and found the fuse was good. I also located my long-lost MB Electrical Troubleshooting Manual and now understand the significance of the OVP relay in all this.

Prior to pulling the relay, the car was almost impossible to start. After replacing it, the engine started immediately but the O2 light remains on. The voltage at the battery while idling is 13.84 V.

The sensor has 3 wires. Two go to a dual connector and I presume they are the heater supply lines. At idle, they have 13.5 V across them, which seems reasonable. I couldn't measure the third wire which I assume is the sensor output today but it's the next step.

I understand these sensor cells should be putting out roughly 0.40 V with a stoichiometric fuel / air mixture. Anyone know if this is about right?

Thanks much.

ctaylor738 · #13 09-11-2009, 06:36 PM

You can best measure the sensor output by putting a little jumper in the connector on the sensor output wire, and measuring the voltage between it and ground. I always thought .45v was the magic number.

I think the problem with starting was that the ECU was not getting power, therefore no enrichment. Pulling the relay must have caused it to do a "reset," and now the ECU is doing its thing. Bizarre that pulling the relay had a different effect than disonnecting the battery.

Can you measure the voltage between pins 2 and 3 of the diagnostic socket with the engine at hot idle? What is it? Is it constant or fluctuating? This will give us an approximation of duty cycle.

TuumEst · #14 09-14-2009, 06:35 PM

I tapped into the sensor line as you suggested. At initial engine start, the sensor outputs 4.00 V. As the engine warms up, the reading diminishes and at hot idle outputs 0.405 V with a little bounce on the last digit.

The reading between pins 2 and 3 of the diagnostic socket shows a steady 0.041 V.

The engine starts immediately. That's quite a change from before pulling the OVP relay. However, that O2 light just stays on.

ctaylor738 · #15 09-14-2009, 08:51 PM

Well, the good news is that the oxygen sensor and ECU seem to be working. Starts out rich and then brings the mixture down to where it should be at hot idle. I am not sure what to make of your reading between pins 2 and 3 of the diagnostic socket.

If you look at the wiring diagram, you see that the warning lamp gets + from the OVP relay, and the ECU supplies the ground if it thinks there is a problem. So I would say that either you have a ground on the grey/green wire from the lamp to the ECU (unlikely), or the ECU has detected a fault.

What we need is a duty cycle reading from pin 3 of the diagnostic socket. If your meter has a duty cycle function, that would work, as would an analog dwell meter. Lots of posts on doing this.