If your wiring harness was not replaced - do it. The engine harness on 600 is easy DIY job. Also check the harness to the MAF on both sides. It was realy bad on my 92 600SEL. It is available as a repair harness - just two connectors to the MAF and wires to the computer with crimped pins. Let me know if you need p/n for it.
Mike

It's interesting that this thought concept also occurred to other members. The 2nd time the issue happened, I started looking for cell sites, transmitter sites, airport and weather radar. In the area that it happened there are none of the above within a distance that might cause concern. The other consideration is that the last 2 times the issue happened, I was not in the same area, nor near any of the above. I was not using the integral phone nor my handheld digital at any one of these times. But yes that same concept flashed thru my mind. (I'm into shortwave and all that stuff).

What seems to be consistant is the amount of time that the car was running since the last startup until the issue manifests itself again and that is probably why it happened in the same area (roughly) the first three times. The amount of time is approximately 1 1/2 hours to 2 hours after startup, and with no shutdown in between the startup and the issue.

I spoke to the service rep this morning and the tech is doing the ground fault/wire harness investigation thing before doing any more substitution. There was some thought about the throttle actuator, but they want to make sure about grounds and wires first.

I had suggested the issue of the wire harness on the other occasions, but apparently this dealership hasn't come across this harness problem before and at this point doesn't see it as a likely item. Maybe that will change after the tech does his thing.

I have had a long term relationship with this dealer and have always been treated fair and professionally by all of their staff. As frustrating as this issue is for me, I do want to give the techs and service staff ample opportunity to sort this out. I know that they are unhappy that they haven't been able to solve the issue as yet.

Mike, I would appreciate the harness part number and I will give it to the service rep.

Thanx for the support and the suggestions, I will post as news comes available.

PS, the dealer GM was driving by his lot when we pulled in Sat night (1130 pm), before we could call a cab, he has opened the dealership and compound, and has given us a new demo to keep us going.. what more could you ask for?

__________________
Mithra

93 sl600
93 600sel

A mercedes is an inanimate object and therefore must respond to logic and reason.

Just got off phone with service rep... apparently they have traced as far as the (signal ?) voltage to/from the throttle actuator... apparently it is somtimes there, sometimes not. They want to make sure that everything else is right before doing the substitution.

I again mentioned the issue with the wire harness and that the MB forums were fairly unanimous on the harness being a potential cause of the problem.

However, I did indicate that I was not trying to second guess, nor tell him what to fix; only that I was trying to offer any potential information that might help solve the issue.

Till next update; thanx

__________________
Mithra

93 sl600
93 600sel

A mercedes is an inanimate object and therefore must respond to logic and reason.

Repair wire harness for Air Mass Meters is 140 440 76 37. About $100. It has two connectors and long wires with pins. It comes with instruction and special tool for extracting old pins from the connectors. Installation requires taking apart computer box, removing old wires from the bottom connectors and replacing them with new. It took me a few hours to install.
The main engine harness p/n 140 540 47 05. Very easy to install. It is on the top of the engine and goes to all 12 injectors and a few sensors at the front. It was around $400. These p/n are for 92 600SEL, make sure they are good for 93 MY. AFAIK, throttle actuators wiring harnesses are problematic too, but I don't think you can get just the harness, have to replace the actuators. Big $$$. If yours are bad, maybe try to repair or replace wires separately.
I can't see how the throttle actuator can cause running rich. It just gives air to the engine. Then it is a job of the Air Flow Meter to measure it and send the reading to the computer, and computer responds with the proper command to the injectors so the fuel can be matched up to the air measured.
Mike

Just got off phone with service rep; they slit the harness cover and peeled it back, and checked several spots, the service rep indicates that the wire harness looks like new(maybe the PO had it replaced?). No evidence of heating or any other type of insulation breakdown. At this point everything seems to point to a faulty throttle actuator on the driver's side. Apparently, the service rep has some connections and may have a line on a used throttle actuator that will do the job... tomorrow will tell.

Mike, thanx for the post with the part numbers, I have them saved for possible future use.

__________________
Mithra

93 sl600
93 600sel

A mercedes is an inanimate object and therefore must respond to logic and reason.

No used throttle actuator. New one will apparently arrive in the morning and be installed.

The service rep indicates that the original ecu will be reinstalled, and the car will be checked for codes. Then the onroad testing to ascertain that the actuator is the remedy.

The service rep is still puzzled over the fact that the error codes followed the original ecu when it was swapped from side to side in earlier diagnosis.

__________________
Mithra

93 sl600
93 600sel

A mercedes is an inanimate object and therefore must respond to logic and reason.

Mike, (myarmar)... the dealer did the fuel pressure thing and apparently it is in spec. And, as you suspected, the new throttle actuator was not the answer either.

The current thinking is that a coil may be breaking down under heat. If that it is the case, it would account for the problem manifesting itself after 1 to 2 hours of continued driving, and for the fact that the car runs fine after being shut off for a period of time. According to the feedback from the service rep, if the coil breaks down and there is no spark in the associated cylinder bank; that bank would essentially be pumping raw fuel thru those cylinders causing the extremely rich exhaust and smell.

In retrospect, the symptoms felt like the car had lost power, and then regained it, lost power, regained it, and so on. (surge, settle down, surge, settle down)

The service rep has a couple of coils on their way in, should be here tomorrow.

I am really starting to question the usefulness of the MB diagnostic devices, but as SteveBfl indicates, the diagnostic computer is just a starting point....not necessarily the answer.

Let's see what tomorrow brings.

__________________
Mithra

93 sl600
93 600sel

A mercedes is an inanimate object and therefore must respond to logic and reason.

Well, the new coils are in, picked up car from dealership last evening and have already got 300 miles (500 km) on her. So far, seems to be ok after a 3 1/2 hour cruise at 120 kmh (70mph) in 80 deg F weather.

I hope this solves the issue (so does the dealer...he's pulling his hair out).

__________________
Mithra

93 sl600
93 600sel

A mercedes is an inanimate object and therefore must respond to logic and reason.

Well, to this point, there are 2 MAF sensors, 1 ECU, 2 coils.

Last evening after approx 1000 miles on the new coils, the car did it again. Same routine. No MIL light.

Floated the car to the dealer. When I went in this morning, the car started and ran fine, no MIL. Service people are getting exasperated. I left to tend to business.

Cancelled my appointments, then went back, got the car out of the shop and drove it until it started to act up again (about an hour), called the dealership and let them know the car was coming in "hot".

We got the car in, on the diagnostics while it was acting up... NO fault codes. The car had a lumpy idle, and had an extremely rich exhaust while it was on the diagnostics.. and a misfire.

The actual running values for the LHSFI units ... approx 30kg/hr fuel(or is this air?) flow at idle for LH1SFI, and approx 300 kg/hr (not a misprint)for LH2SFI at idle.

THe readings for 1 O2 sensor fluctuated all over the map, the other O2 sensor (related to LH2SFI) fluctuated very little and was very slow to change values.

There was NO check engine light.

This is the first time that the car has actually been acting up while the techs were able to diagnose in real time.

Would a faulty O2 sensor have been causing me all this grief?

__________________
Mithra

93 sl600
93 600sel

A mercedes is an inanimate object and therefore must respond to logic and reason.

Tech took car for ride, brought it in "hot", hooked up diagnostics which pointed to the LH2SFI side of things. The tech then switched the connectors side to side for the O2 sensors, .....no change in values. The tech then pulled the O2 sensors and physically switched the sensors side to side. You could see that the O2 sensor from the driver's side was black and sooty. After the switch, the diagnostics still indicated values that were way out of wack for the LH2SFI side. The tech then disconnected the MAF for the LH2SFI side and the readings dropped to the values that they should when the MAF is disconnected. Reconnect the MAF and the readings go way out of wack again (all this happpening at idle).

Here's where the fun starts........ We retrieve the old MAF from the trunk and connect it to the circuit "BUT" the MAF is NOT installed in the intake plumbing.... the diagnostic values are perfect. So, out comes the new replacement MAF and in goes the old MAF and the values go thru the roof.(at idle) We remove the old MAF and replace the new MAF (which showed proper values when not in the intake plumbing) and the values are back thru the roof again.

The whole time this process was underway(about 4 hours), the tech was on the phone with MB technical support going step by step with the diagnostics in conjunction with a pinout box verifying actual values and comparing them on both engine banks.

Values were checked and verified correct for throttle actuators, idle and WOT switches and on and on (I can't remember them all). Values at connections for the MAF's were checked and verified.

At some point late in the process, MB tech support suggested pulling the diagnostic module (remember...no check engine light, no codes). After the diagnostic module was pulled from the box, and the engine restarted, all the values immediately went to spec, and the car idled normally. All the values for the LH2SFI were where they should be.

With the diagnostic module out of the loop, we haven't been able (so far) to get the car to go to the intermittent extreme rich mixture that has been plaguing me. The next few days should tell wether or not this is the root of the problem.

This particular module has a sticker on it that says "CARB", the tech indicates that I can drive the car without it until we verify that this module is causing the heartache.

If I have understood the information from JimF's auto page properly, does this module trigger the air injection system on startup? ( No.....I reread JimF's pages...this triggers the diagnostics)

I guess I'm trying to figure out how a "diagnostic module" could affect the engine's performance. (If, this is the root of the problem)

__________________
Mithra

93 sl600
93 600sel

A mercedes is an inanimate object and therefore must respond to logic and reason.

Part of the aggravation and frustration of this particular issue is the fact that, once the car leaves the dealership, it will run for about 1600km (1000mi) before the symptoms reappear.

The symptoms have not reappeared yet (about 400mi since leaving the shop) and the car appears to be operating flawlessly.

Some questions for the membership; theories, suggestions, any answer is appreciated.

a) I am wondering if the distance (or time) from the shop is actually the number of "drive cycles" that the DM module monitors. Is this possible?

b) Does it sound possible that the diagnostic module could be generating an intermittent voltage or current that is throwing the engine management system into a tizzy instead of triggering the MIL light?

c) Should the Diagnostic module trip the MIL light when the engine is misfiring, running so rich that the O2 sensor is sooty, and the exhaust looks like that of a Mack truck?

c) Does it sound reasonable that once the Diagnostic module is pulled, the values for the LH2SFI MAF sensor went to normal?

d) Is there a method to test the MAF sensor to determine if it is in spec or faulty?

I guess that I'm grasping, trying to understand what else could conceivably resolve this issue.

__________________
Mithra

93 sl600
93 600sel

A mercedes is an inanimate object and therefore must respond to logic and reason.

[QUOTE]Originally posted by mithra
[B]
a) I am wondering if the distance (or time) from the shop is actually the number of "drive cycles" that the DM module monitors. Is this possible?

No, a drive cycle is mainly determined by engine temp. There has to be a change of 40 degrees. So once hot, all the small trips or otherwise aren't counted till it cools 40 deg.

b) Does it sound possible that the diagnostic module could be generating an intermittent voltage or current that is throwing the engine management system into a tizzy instead of triggering the MIL light?

Its hard to say how much command comes from the DM to the LH. Much of the contact between them is just information and I haven't heard of the DM affecting performance, but it wouldn't surprise me.

c) Should the Diagnostic module trip the MIL light when the engine is misfiring, running so rich that the O2 sensor is sooty, and the exhaust looks like that of a Mack truck?

I doubt the DM monitors misfires on that early car. It only monitored some testing early on.

c) Does it sound reasonable that once the Diagnostic module is pulled, the values for the LH2SFI MAF sensor went to normal?

If it happened I can believe it. could be just a conincedence. If it hasn't happened again in 400 miles and then does happen I pity your poor tech. If those old onboard diagnostics don't catch it the only way is a bunch of substitution.

d) Is there a method to test the MAF sensor to determine if it is in spec or faulty?

MB says NO. There are many things you can test. You can see if there is NO load signal but quantifying a signal is not possible do to the dynamics. A 10% wrong signal would be bad, 20 % unacceptable to pre 95 cars and yet the spec is about 50% wide.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

As you probably realize, the techs and service reps are ready to weep when they see the car in their lot.

I am trying to remember anything that may have a cause or effect, that may help these gentlemen.

I asked about the "drive cycles" trying to relate them to the number of times the car is started and shut off after the car is released from the shop. The tech indicates that each time, he clears the DM module before I get the car.

The thought that I had in mind relative to the "drive cycles", was that the symptoms "appear" to occur roughly the same number of miles/days into my work week. I guess I was trying to translate that into some sort of a relationship to "drive cycles" after the DM module is cleared.

What has everyone stymied is the fact that the car will go just about the same number of days/miles when it leaves the shop before the symptoms appear.

Weather and ambient outside temperature don't seem to have a determining effect.

Thanks to all, suggestions welcome

__________________
Mithra

93 sl600
93 600sel

A mercedes is an inanimate object and therefore must respond to logic and reason.

You guessed it.........the wiring harness. After considerable consternation, parts substitution, wacky readings and other associated grief; the sun, moon and planets aligned to give us a break.

After about 1000 miles, the car did it again.

I'm really getting to know the local float driver for the dealership.

Once we arrived at the shop and moved the car (ran normally at this point again) hooked the car to the diagnostics.....no fault codes.....values normal.

Ran the car for about 20 minutes on the highway, returned to the dealership (car was symptomatic) and hooked diagnostics again while the car was running. Actual values for the LH1SFI was 1.2v and 1.8v for LH2SFI (MAF readings). First time we had seen this.

Swapped MAF's side to side....still the same, swapped ECU's.... still the same.

AT that point the razor came out and all the sheathing, tape, and whatever from the wire harness between computer box and the LH2SFI MAF came off (except for about 1" at the connector to the MAF). Nothing. Grounds were rechecked. Nothing.

For whatever reason, the tech looked inside the remaining 1" of sheath at the MAF connector.....Bingo! Bare wires. Right at the connector's hard case.

Other than the 1" or less at the MAF connector, the rest of the harness looks new. Every other spot that had been checked, the wires were pliable with no signs of hardening or brittleness.

Mike (myarmar), thanks for the part number for the replacement harness. I don't know if it has been updated, but the number that they are using (here anyways) is 140 540 00 36. According to the instructions that are associated with this repair kit, the harness is cut and repaired in the cable tray, rather than in the computer box.

Thanks to all for the suggestions and support. I know that in the shop, as well as for myself, that there is a certain sense of relief for finding something tangible that could account for all this grief.

__________________
Mithra

93 sl600
93 600sel

A mercedes is an inanimate object and therefore must respond to logic and reason.

Hi Mithra, I'm glad you found it! I'll double check the harness p/n tonight and let you know. I don't thing it is a good idea to splice the harness in the cable tray. You still have the remaining wires with defective insulation. It was not bad doing it all the way to the bottom of the box. If I had it done, sure dealer tech would be able to do it just fine. I would insist on the full harness replacement.
Regards, Mike

Confirmed:
I ordered mine from the PartsShop here. This is a part number:
140 440 76 37
Copy/paste it to the PartsShop search field and the wire harness comes up $99. This is a harness with two connectors for MAS on one end and 10 crimped pins for the ECU box end. It comes with the instruction and special pin extractor tool.
Mike