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Discussion Starter #1
My '98 SLK with 80K miles just started running rough (surging and fading, but not quite stalling). One weird symptom is that the instrument cluster LCD display windows (odometer and trip odometer) go blank at the same time that the rough running starts. If I then turn the key off to stop the engine, then turn the key back to the on position the LCD displays return (until the rough running begins again).

The first time it happened I got a CEL, but I don't have a code reader so I don't know what the stored codes were. I then read up on the K40 relay board problems, so I went ahead and disconnected the battery to do the K40 check and after that the CEL hasn't yet come back on (though I expect it will eventually).

The K40 board solder points looked good, but I went ahead and resoldered the upper left three points just in case.

I also cleaned the MAF since that was also a reported cause of rough running. I think this is the original MAF, so it wouldn't be surprising if it had failed. I just can't figure out how a MAF problem could result in the LCD display issue.

After those two changes I started up again and thought everything was good, but after just a couple of minutes of idling the rough running returned, though without a CEL this time.

Crankshaft position sensor? It is reported to fail in a way that gives rough running without throwing codes. But the LCD display symptom is bothering me - how would a CPS failure account for that? That feels like a computer problem - an intermittent power supply problem? Bad alternator regulator? That's why I liked the K40 theory - except I don't see any mention of this LCD symptom in any of the K40 or MAF writeups.

Fuel problem? I put in a new fuel filter last spring, but I suppose the fuel could somehow have been contaminated. Fuel pump? But if so, how would that explain the LCD issue?

Spark plugs? I put a new set in about a month ago, but maybe an early failure? If it was just a rough running issue then I'd like the plugs theory, but again, how would this result in the LCD display issue?

This is my current plan:

1. Check out the spark plugs. Seems unlikely but it's an easy check and I've still got the old plugs so I could put them back in and see if things get better.

2. Wait for the CEL light and go get the codes read. If they point to the MAF then replace it. If not consider crankshaft position sensor swap.

3. If those don't yield then take it to my indy mechanic.


Anybody see a better approach? Or had any experience with this LCD display issue?

Thanks in advance for any advice.
 

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'01-E320 & 02-ST2
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Just a thought but I'd check the ground connections under the hood, including the ground strap to the engine.

But I'm with you re: CPS or MAF; that would not explain the simultaneous cluster issues.

Also, I would not wait for a CEL. Codes can store even without the CEL coming on, it has an algorithm based on the severity of the code, etc. The pending/stored codes can often prove instructive when you're having a problem. Have 'em read, some parts stores will do that free, Aamco is advertising that too. You can get a basic code reader on Amazon for about $35 and if you want to jump into the DIY realm it's really a necessity on the MB lineup.

Good luck.
 

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2001 SLK 320(217K Miles), 2002 E320 Special Edition(183K Miles)
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CAN Bus problems? The CAN bus is carrying the signals to the instrument cluster.
ECU packing up? There is a small fan in the ECU box. If that dies, the ECU will overheat after some time. Look for a 'pipe-like' device in the ECU box. Inside it is a fan. If the car is powered up, the fan should run.
 

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Discussion Starter #4
Update:

gregs210 - good tip on the grounding strap, thanks. I checked it out and it looks and feels solid and with the neg battery lead lifted I get 0 ohms from the terminal to the chassis so I don't think this is the problem. I agree with your reader comments. I actually already have a low end OBD-II reader which works fine with my other vehicles, but as I understand it the '98 SLK only supports emission features on it's ODB-II port - for engine and other diagnostics it needs a 38 pin reader.

I've called around to my local auto parts stores (Autozone, Kragen and Midas) but at least in my area they don't do scans on CEL.

Kobus - also a good tip on the ECU box fan, thanks. I found the fan, and it does seem to be turning just fine with no sign of an air blockage.

I went ahead and changed back to the old spark plugs and cleaned the CPS and at first I thought it had fixed the problem. But after a 10 minute test drive I got the same problem as before: loss of LCD displays (Odometer, trip odometer, time and temp) accompanied by engine hesitation.

I'm wondering if it now makes sense to try reseating the ECU connectors. Is this a safe thing to do? ie lift the neg battery cable, reseat the connectors and reconnect the battery and try again? I'm a little hesitant to lift the batter terminal just because of the reports of instrument cluster failures - on the other hand ya gotta do what ya gotta do.

I'm still curious to know if anyone has ever seen an LCD display symptom like mine.
 

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2001 SLK 320(217K Miles), 2002 E320 Special Edition(183K Miles)
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I actually already have a low end OBD-II reader which works fine with my other vehicles, but as I understand it the '98 SLK only supports emission features on it's ODB-II port - for engine and other diagnostics it needs a 38 pin reader.
I would guess then that the OBD2 port capabilities was expanded as part of the face-lift and the 38-pin port dropped. Mine doesn't have a 38-pin port anymore and I can read all the computers with Carsoft using the OBD2 port.
 

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Discussion Starter #6
Update: bad coil

Update: the problem seems to have been a bad ignition coil.

My mechanic hooked it up and found the error codes:

P0300-080 Misfiring Damages TWC
P0304-083 Misfiring of cylinder 4 : 83

He swapped coils and the misfire moved with the coil, so replaced them both (there are two in the '98 SLK) and I've been running for a week now without any futher misfires.

I'm still not quite sure how a bad coil could lead to a temporary loss of the LCD instrument cluster displays - maybe some kind of short or noise that sends the ECM into limp mode? Or maybe just the misfire itself? In any case I'm back in business.

I'd also like to put in a plug for my mechanic - Tony at Mercedes Exclusively in San Jose. They get top marks on Yelp for a reason.
 
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