02 Light 1986 560SL - Page 2 - PeachParts Mercedes-Benz Forum

TuumEst · #16 09-15-2009, 12:00 AM

Many thanks for your reply.

I don't have a dwell meter or similar but I can put an oscilloscope on pin 3 and measure the duty cycle. The present indication of 0.04 V (on a DVM) seems extremely low. If there are pulses present, would that not indicate a very low duty cycle?

Any idea of a valid reading for voltage and duty cycle at p 3 of the diagnostic socket?

TuumEst · #17 09-15-2009, 12:39 AM

Some more Googling found: http://www.landiss.com/mixture.htm

Seems the duty cycle is determined by the 'off' time compared to the 'on' time, rather than the other way around as one might expect. I'll stick the scope on there tomorrow and see if the 100 Hz pulse is present and, if so, determine the duty cycle. The DVM reading sure makes me wonder if there is anything on Pin 3 at all.

Will advise and thanks for getting me this far.

ctaylor738 · #18 09-15-2009, 07:07 AM

I am somewhat challenged by the concepts of duty cycle and on-off ratio. I am not sure if that tiny voltage reading means 0% or 100%. But for sure it's not right. It should be somewhere around 50% of battery voltage.

The description in the fault table is almost identical for a constant 0 or 100.

If you send me an email, I will send you the testing guide.

TuumEst · #19 09-15-2009, 07:41 PM

The scope shows no digital signal on socket pin 3, only a little noise. This matches the DVM display. The scope does show data on other pins, such as the RPM on pin 1.

I sent you the email last night.

Many thanks.

ctaylor738 · #20 09-15-2009, 08:57 PM

So we know that your O2 sensor is working, and we know that according to its voltages, that the mixture is approximately correct from start to warmed-up.

So here are a some scenarios:

- the wire or connection from the O2 sensor to the ECU is bad and the ECU is in some sort of default mode based on input from the coolant temp sensor

- the part of the ECU that turns on the light has gone south

So I would advise to check power and ground to the ECU, and the continuity of sensor wire. If those are all good, look for a used ECU.

How does the car drive? If it accelerates well, that's a sign that the ECU is at least partially alive.

Very interesting post, thanks for the good diagnostic work.

TuumEst · #21 09-16-2009, 04:50 PM

I will check out each of your last suggestions and post the results.

At this point I have a bad feeling about the ECU since there is no Lambda output at the diagnostic socket. I would guess that a simple problem with the only the O2 sensor would have given a fixed duty-cycle pulse, rather than no pulses at all.

Chuck: If you still have that troubleshooting guide, I'd sure appreciate a copy.

Thanks to all who provided assistance.

TuumEst · #22 10-04-2009, 09:36 AM

The problem has been fixed.

It turned out to be the ECU. The most incriminating symptom, besides the O2 light, was the lack of any Lambda data at the engine test socket.

Given that this is an 86 560SL with 200,000 km, changing the O2 sensor as part of the troubleshooting process likely didn't hurt.

I would like to thank Chuck for his many useful suggestions and John Roncallo for making a used ECU available at a decent price.

I hope this thread will help someone else a little further down the road.

Roncallo · #23 10-04-2009, 05:08 PM

Quote:

Originally Posted by TuumEst

The problem has been fixed.

It turned out to be the ECU. The most incriminating symptom, besides the O2 light, was the lack of any Lambda data at the engine test socket.

Given that this is an 86 560SL with 200,000 km, changing the O2 sensor as part of the troubleshooting process likely didn't hurt.

I would like to thank Chuck for his many useful suggestions and John Roncallo for making a used ECU available at a decent price.

I hope this thread will help someone else a little further down the road.

Glade to see it worked out. Thanks for the positive feedback.

Strife · #24 10-04-2009, 05:25 PM

These apparently don't fail often, but I have spares for my 86 and 85 as insurance that mine will NEVER fail!

I hope you didn't toss the old one because it may have enough value to make it worthwhile to send to a rebuilder for a few bux.

TuumEst · #25 10-16-2009, 09:18 AM

LOL. You're right. Murphy only picks on items for which no spares are available.

Anybody have an ECU rebuilder to recommend?

Kimolaoha · #26 10-16-2009, 03:13 PM

Quote:

Originally Posted by ctaylor738

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.

Hi Chuck -
Bizarre, but true. Same thing happened to me. Drivability issues solved when I replaced the OVP relay (fuse was still intact). Just for grins, swapped out the OVP relays after a few days before pitching the old one. Hmmm. Ran fine, and is still running fine a year or so later. Just a simple "reboot" did the trick.
Mike