EDS/ELR Idle Issues - OM603
 

This is an '86 300SDL OM603

I've been chasing an intermittent idle "miss" for some time now. The car will intermittently and randomly "shake" at idle, especially when in gear. Very similar to the behavior you get with a miss or "skip", although I haven't been able to track down where.

Imagine my total shock when I disconnect the ELR plug on the solenoid on the IP and the engine suddenly runs SMOOTH. AS. GLASS. In complete and utter disbelief, I jumped in the cab and raised the RPM up to 650ish where the ELR usually maintains and the car continued running completely smooth. No shake whatsoever!

So what on earth would be causing the ELR/EDS to make the idle shaky? I assume it is hunting? A related issue is occasional idle RPM stuck ~1000 RPM, then slowly dropping back to normal. I've caught it in the act and unplugged the ELR solenoid with the idle speed instantly dropping. Plug back in, and right back up to ~1000 RPM.

EDS computer going flaky? More bad caps/solder joints? Crankshaft position sensor? Am I just going crazy? :confused:

Believe it or not the EDS computer does have a diagnostic system. It is read with a blink code reader.

I would make a blink reader and pull codes...

Link to EDS test procedure
http://w124-zone.com/downloads/MB%20CD/W124/w124CD1/Program/Engine/602_603/07.1-1120.pdf
Your engine starts on page 22

They call for a special impulse counter but you can make one yourself.

Need a 1 k resistor, LED, pushbutton switch and an enclosure (pill bottle or other small container).
And 3 leads with banana plugs, red, yellow, black
Red lead to one side of resistor
Other side of resistor to LED anode (lead opposite the notch on the case or the longer lead)
LED cathode (lead next to the notch on the case or the short lead) to the yellow lead and one side of the pushbutton
Black lead to the other side of the switch
Solder/heat shrink it all up, install in a pill bottle.

You plug into the diagnostic socket according to the procedure, hit the button, and count the flashes, then look up the code.

Could be a lot of things. The place to start is to read the blink codes. Do you know how to do that?

I have the FSM procedure on how to pull codes from the EDS computer. I will do that in the near future. Thank you Jay Bob for the quick rundown on making a blink tester. I have all the stuff to whip one up in my office at work.

Following pressure from another forum member, I pulled the EDS computer to check for bad caps. Glad I did, not one of the electrolytic caps on the board has a reading greater than .01. Two of the "phenolic" electrolytics have cracked open and leaked on the board.

My intention is to replace the caps, then pull codes. Given my experience with this car, every single electronic module has had bad capacitors and/or bad solder joints, and replacing/resoldering them has restored functionality to every one of them. I have a feeling it will probably address the EDS computer as well. I still plan to pull codes to see if there's anything meaningful stored. With bad filter/reference caps, any codes that are stored currently are likely meaningless.

Out of curiosity, what else does the EDS do on these cars? I'm aware of it doing idle stabilization, tach output, and EGR control (emissions), but does it do anything else or is that pretty well it?

In that web location there is a whole trove of info on the engine. There is another document in that folder that explains the function of the electronic diesel system.

This site is for 124s but the 86-up 126 and the 124 share a lot of systems.

Mercedes Benz Model 124 - OM602, OM603 Maintenance Manuals

On some models, it provides the engine speed signal to Klima, so it can affect your a/c.

When I bought my '87 wagon years ago, it had that idle shake. I disconnected the ELR two-pin connector on the back of the IP, idle was very smooth but too low, so I manually adjusted the idle back up to ~750 rpm using the set screw on the back of the IP, and never looked back.

As far as I could tell, the key contribution of EDS/ELR is increasing the idle speed when the AC compressor kicks in and when the engine is cold.

I forgot about the Klima tach reference. I'm now seriously wondering if the EDS is part of the reason I get rapid cycling of the compressor at very specific engine speeds.

For Example: Doing 70 MPH I run ~2800 RPM and the compressor will cycle on and off every 2 seconds or so. If I drop to ~2700 RPM, or speed up to ~2900 RPM, the cycling stops. With the bad caps I found last night, I have to wonder if the tach signal being sent out is noisy enough that the Klima is affected by it, but not the tach.

We'll see if new caps helps out any of these issues. I fully expect to find fault codes for the EGR system when I pull codes. The diaphragm in the EGR valve is bad, so I have vacuum disabled to the whole system. I don't intend to hook the EGR back up either, so let's not go there. Other than that, the engine controls are factory-stock.

Max - Interesting to hear of another person with the idle shake from the ELR/EDS. I did quite a bit of searching before posting this thread and didn't find anything from anyone else. Hopefully this thread has a happy resolution and helps someone else in the future. If not, my plan is to do exactly what you did - set the idle mechanically and forget about the ELR functionality.

New capacitors came in today so I recapped the EDS computer this evening. Only 1 of the capacitors was any good, the bulk of them didn't even return a capacitance reading when checked with a Philips capacitor tester.

The difference of before/after is PROFOUND. With the EDS solenoid plugged in, the car idles just as smooth as it does with it unplugged now. More interesting than that is the throttle response when returning to idle. Prior to the recap job, the RPM would typically drop to ~800 RPM, then slowly reduce back to 650 or so where the idle speed was set. Occasionally the RPM would stick ~900-1000 RPM and nothing you did would get it back. Unplugging the EDS solenoid would drop it immediately. Post-recap, the idle speed drops immediately to the idle setpoint. None of this weird coasting business, and so far, no hangups at 1000ish RPM either.

The lack of idle shake is a major accomplishment in my opinion. I have ~25 PSI compression difference between cylinder balance, so the engine will never be 100% smooth, but without the EDS enabled, the idle was smooth enough that you didn't notice the car idling. With EDS enabled, on a good day there was a very slight wobble. On a bad day, it would rock the car enough to spill your coffee (not exaggerating, ask my lap). I can only assume it was reacting to that very slight imbalance and buildup of tolerances sending it wildly out of control. Sitting at a stop light would usually have the car rocking by the time the light turned green. It doesn't do that any longer.

I still plan to pull codes and will report back with longer term results. I'm interested to see what the A/C does on the highway on the next warm day.

I don't suppose you could share a list of caps?

There are 5 Electrolytic capacitors on the EDS computer boards. On the 126 the EDS computer is located in the passenger kick compartment and requires no tools to remove. Not sure about the 201 and 124.

4 phillips screws in the lid removes the boards which are clipped together. Looking inside the 2 boards, you can see 2 heatsinks that are clamped to some transistors on one of the boards. You will want to remove the 2 phillips screws holding the OTHER board to the main heatsink/plug assembly. You can then lift it off and pivot backwards on the ribbon cable. There are plastic clips that hold the 2 cards together, they simply unsnap.

Once you have the cards opened up like a book, the locations of the capacitors are obvious and the capacitors are clearly marked. On the board you didn't unscrew from the main heatsink/plug assembly, there are 4 capacitors: 100µF @ 50V Radial, 100µF @ 10V Axial, 22µF @ 40V Radial, 7µF @ 63V Radial.

The board you unscrewed and pivoted out of the way has a single electrolytic capacitor 22µF @ 16V Axial.

The circuit cards are double-sided, so be careful desoldering and resoldering. I used 63V parts for all my replacements. 7µF is also not a standard value, so I substituted a 6.8µF capacitor for it.

Out of all of my capacitors, only the 22µF radial was good (I replaced it anyway). The next highest reading I got out of any of mine was .006µF In Germany, they'd refer to that as "kaput".

I'll keep this post updated on my experience with lack of "jiggliness" at idle. Hopefully this solves that ANNOYING issue.

Disconnected mine, it ran at 500rpm, maybe 490+ something. The idle improved a lot, besides from the low cylinder. It isn't randomly twitching and tweaking constantly at idle.

I wonder how a could start will it affect it.

Thanks, that's really helpful. I'm sometimes getting stuck at 1000RPM with a slow descent to idle speed. I may be able to tackle this before winter sets in.

It happens to me as well on occasions as well.