Hi all. I'm a newbie having bought a '94 SL280 (R129.058) with the M104.943 engine last Sunday, (my first Merc, I'm usually an Alfa guy, so take it easy on me), and walked headlong into a badly running car.

Symptoms are:
- Stumbles and coughs on a very light throttle after slowing down. (E.g. following slowly in heavy traffic that picks up slightly).
- Yesterday it stumbled and then died on the way to work. I nonchalantly pulled over as if that was what I was doing - turned it off and on again and drove home quick smart!

So far
So after reading multiple threads during the week about badly running M104 engined cars decided I'd better
- replace the suppressor between the coils and the spark plugs, (wouldn't want Arthur Dalton to growl ! ), and
- the plugs themselves, (I know, only F8DC4 plugs. I gapped them at 1mm. They were selling cheap because they're a deleted item so I bought 12!).

While doing that I noticed all the HT leads to the other plugs had white points where it looked like they were shorting so I bought and fitted a set of those too.

Anyway having replaced leads, suppressors and plugs, the car is running better but it still "coughs". Sounds like a pop noise that might be fuel going off on the exhaust? It's not REALLY loud, just quite noticeable...

I can now get it to happen regularly. If I slow down to about 40km/h and then gently depress the accelerator, it'll hesitate and cough and then slowly pick up. If I bury my boot it'll only stumble a little, (hardly noticeable), and then GO like a cut cat!

MAS? Built an LED diag tool.
After reading a bunch of threads about M104 MAS issues, (here and here), I thought that was possibly my problem, but decided I'd better build Arthur's simple diag tool to check the codes. (Found here for other newbies looking. Thanks Arthur! ).

Of course I should have checked my connector type first as those lovely banana plugs I soldered up were never going to fit into the 38pin sockets!

Codes.
These are the first take. I have now cleared them. Should I wait and do it again, (I'm a bit worried about getting stranded now, so I'm back driving the Camry! ).

I got the code explanations from the "Diagnostic Codes_MY 88-98.pdf" doc found here

Pin - Module - codes - my interpretation
8 - BM - 1 - No fault ???
7 - ETA - 2 - Assume my car has ASR - EA/ETA module fault.
Does that mean my EA is ?
4 - HFM - 4, 13 - intake air temp sensor sht circuit, and throttle valve pot actual value too low, (another ETA indicator?).

High pitched sound from under the intake
On top of all this, in the hour I was working on the car getting the codes, (ignition on - engine off), I noticed there was a continuous high pitched sound, (it was after 11 on a cold night!), coming from below the intake manifold. Is there a fuel pump under there, (please say yes although it's a bit high pitched for a fuel pump)? Otherwise it could have been coming from the EA. I've uploaded an MP3 of it, (smartphones are cool!) for your aural pleasure. SL_sound.zip

Where to next?
Any advice? What further diags can I do?
MAS / EA?
Either way my wallet is starting to throb...

I have a 93 300TE..our problem of the up and down idle started with checking the wire harness of the throttle actuator...from the unit to it's connection....I cut open the outside sheathing and found that "all" of the individual wire were bare....all the individual wire coatings were gone...so I have completed re-coating them from the circuit board to the pins...sliding over them heat shrinking tubing from the pins end...u have to becareful to keep track of where all the wires go back into the end...after u take it apart...since the wireing was uncovered and a rebuilt one is 400.00...nothing to loose to try.....trying it tonight...if not then will pull codes..

BINGO FRAUKE. Thanks for your input.

Last night as I was about to get the codes out of the diag ports again, it occurred to me that the MAS symptoms and the air temp sensor faults were to do with the same connector. The one on the MAS. So I had a quick look at it. I hardly moved it and it started smoking. And then the outside started to burn! I quickly ran to turn off the ignition and then cut open the harness that runs to the MAS. No insulation present in there, just corroded copper. Guess that answers the question over whether my car has the bio-degradable harness.

Guess it's time to start searching the forums for choices to do with engine wiring harnesses. DOH!

after covering the eta (throttle actuator) wires...the car is running real rich...black smoke..????? where is the control for the fuel mixture..how I miss a simple carb....

That doesn't sound like something a mixture setting would fix. The computer calculates all that with sensors etc. It sounds like you've got something else wrong.

But don't listen to me I haven't even owned a Merc for two weeks yet.

I've just called my friendly discount Merc parts and the wiring loom for my car, (R129 058) is NZ$1350. Ouch.

W

Hi I was going to replace the engine harness but it seems the MAS sensor is NOT part of that harness, but wired separately. I started a new thread about the pin outs from the MAS connector so I can attempt a home rewire job with new wires and copious heat shrink! I think this is a reasonable approach as the loom only seems affected by heat in the engine bay itself, (sitting as it does above the exhaust manifold), and not where it goes behind the "false firewall", (the firewall in front of the firewall).

Edited Insert >
Please ignore these next two paragraphs and the pic, as I think I was mistaken about later MAS wiring and don't want to mislead you. (I've only preserved it in case someone has read this all ready and is trying to find it again)
I've seen a picture from the engine bay of a '95 SL320 and the MAS looks like it could be part of the wiring harness on that car. At least the leads to the sensor are clipped to the side of the intake going across the engine to the intake side, (actually they probably found that they had less trouble with exhaust manifold heat doing it that way). However not ALL '95 SL320s are wired this way. It must have changed sometime during '95?

On my car the leads to the MAS stay on the exhaust side and go directly back to the "false firewall". (see attached pic, not my car).

Still in progress...

Attached Thumbnails

 

Hi all

Yep the rewire job did the trick. It seems that all my problems were due to the wiring to the MAS shorting out, as it is now running sweet as! Now I can actually get to enjoy my new SL!

SO I ran a new home made heat shrink covered harness, (it's only four wires), back to where the MAS wiring disappears beneath the brake booster into another very major harness coming from, (or going to?), the fuse box.

On one of these MB forums I saw an example of someone who had cut a plug open to replace the wiring inside and I used that as a model for my repair. I had to use a a razor and a hammer to gently prize open the molded plug. Downside is that the epoxy I used to close it all up isn't hardening very fast, (it's been 24 hours), so I have braced it and hope the heat above the exhaust helps the epoxy to harden otherwise I'll have to come up with plan b...

I can see why they didn't replace the MAS wiring when they did the rest of the engine bay harness, and I can see why MB changed the later MAS wiring to go across the engine bay. I slid heat shrink as far back as I could over the original wires that I connected my new harness to. The wires that far from the engine bay looked like they were in pretty good shape anyway, but I won't be putting any money on them not playing up in years to come...

These forums are a great resource for the uninitiated as a starter point to problem solving. I think I must have spent about ten to fifteen hours researching possible problems and repairs before I was able to resolve my issue, and I'm sure I'll be back to resolve a bunch more, (why does that vacuum thingy in the back buzz every few seconds? Is that normal? etc etc )

Regards

Wayne