Hi All,

I have a 93 300CE with about 187,000 miles. I’ve owned it for 16 years still love it like the day I bought it.

It’s been running fine except for a tiny roughness in the idle, but otherwise great.

Yesterday, drove about 20 miles on highway about 65 mph, stopped at a light and it stumbled and shut off. It started right up, pulled into a parking lot and it was idling very slow. Drove back home at highway speeds and was OK along as I was on the throttle. if I let off the gas, tach still showed 2000 rpm, but engine RPM was dropping. If I gave it throttle, there would be a big stumble and jerk and then pick up again.

Today, it started fine cold but idle was rough at about 500rpm. If I work the throttle with my ear near intake, I can hear a slight hissing/vacuum leak sound.

I ran the codes with my homemade tester and it is confusing:

using the button and light on the 16 pin diagnostic port I get: 6 (idle speed control faulty, 12 (Headed Oxy sensor circuit open or short, 19 (Injector circuit open or short), 22 (crankshaft position signal faulty). I can’t imagine it would run if all of this was going on!

Using tester button and light, yellow in PIN 3, I get code 6 and 22.

using PIN 14, I get code 2 (cruise control/ idle speed module)
and code 5 (stop lamp switch)

PIN 8 gives the No Fault Found code.

The CE light is not on.

Two years ago It dropped a cylinder. I replaced the plugs, connectors and coils that solved that problem. I do have a little trouble with the neutral safety switch where sometimes it has to be in neutral to start.

Other issue that has to be addressed is Catalytic Converter and pipes are original and have to be replaced now, but all I see is replacements in plain steel with one inlet/outlet converter, instead of stainless with dual converter.

I have opened up the engine harness in different places and wiring looks OK. Previous owner had throttle body replaced, head gasket and timing chain at about 120K.

Is it worth cleaning the throttle body, EGR tube and resonance valve just as maintenance at this point.

Sorry if I’ve gone overboard with long post.

Thanks in advance for any help!

bk, when did Mercedes begin using the eco wiring harness? I thought it was MY93. If so that may explain so many fault codes.

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Handyjay

I know, I'm worried about that, but I've opened parts of the harness and insulation seams not brittle. But it can be bad in other places

There are two wiring harnesses I believe. Upper and lower. Not out of range cost wise and from my past experience not to difficult to do it yourself. If you go this route take pictures for reference first and then do one connection at a time. If you did not see deterioration then you may be OK. But someone in this forum will know when the eco wiring started. Good Luck

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Handyjay

I had similar symptoms once on my 1987 W124. It turned out to be a bad coil.

Yes, we did have a similar problem, only the symptoms where much worse. I replaced all three coils, wires, connectors and plugs. That solved that problem.

Is it possible that circuit in the stop light switch is bad which is giving code 5 at pin 14 even if the brake lights are still working?

I'm beginning to think more about pulling the harness and repairing it if needed. I will check the date tag first.

I opened some areas of the harness today. I find no indication of the crumbling insulation but it could be someplace else I guess.

Also quickly checked visible vac hoses and everything looks soft and like new.

I looked at harnesses on ebay and see the date tag, but can't find it on mine. Where should I look?

I know people complain about MB quality and failure to act on this issue, but the wiring on our 97 jeep Cherokee is a constant issue of too small of gauge wire that is corroding inside of the insulation and breaking off of connectors. I've rewired most of the front of the Jeep.

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just an update...

I did find the date tag on the harness and it is F.D. 13.10.99.

The green label date code on the ETA is worn off, but has the identical printed part numbers shown on another site for a 94. Mines a 93 so?

I was worried that cute little critters might have nibbled on the wiring underneath like the oxygen sensor wire, but all is OK there too.

There was no gas or gas smell in the Vacuum port on the fuel pressure regulator.

I put a mitivac tester on the EGR valve and the idle does slow down if with vac applied.

I have ordered a new stop light switch just to see if it will clear the code.

It starts and idles fine but "bogs" if you open the throttle quickly and the CE light still isn't on.

Thanks

Well, today I replaced the brake light switch with a NAPA replacement and the car runs great. The idle still has a very slight unevenness that it had before.

After looking at the NAPA switch, I'm going to check the dealer.

I cleared all the codes and made 7 "trips" stopping and restarting (to reset the diagnostics as I read somewhere?) and ran at highway speeds and all is good. I don't know if I actually fixed anything or if it's a fluke that will show up again.

But NOW, the heater blower dosn't work!? What could I have done to affect that? I just removed the plastic panel and replaced the brake switch.

Thanks

I forgot to mention, right in front of the fuse box mounted separately, is a small black plastic box about the size of a regular relay only a little taller. The cover comes off and there are two fuse circuits, one white fuse and one empty.

Any ideas what that is?

Thanks again.

Maybe I answered my own question... maybe trailer coupling/heated seats? I don't have heated seats, so maybe that's the empty spot.

the brake light switch seams to have cured the drive ability issues. Drove it more yesterday and it runs great. It was colder and it starts quickly, idles around 1000rpm and settles in at about 5-600rpm. Maybe a little slow, but lubricating the linkage has helped this in the past. No more codes!

Now the issue is the blower motor is dead as a doornail. I'm trying to retrace what I did that may have been connected to it.

I replaced all the fuses with copper and OEM ones. I had already replaced the 30 amp strip fuse with a 30amp inline fuse holder.

Reading the factory CD, it mentions using the code blinker on pin7. that showed code 13 temp sensor "interrupted" I did move those wires checking the harness,
but it is the newer harness. After checking the wires, code cleared.

I've been searching the forum but haven't come up with anything yet.

Any suggestions please?