garrymeg

Hello Everyone,
My car is manufactured in 1991, japan import, R129 500sl with 138 000km.
When i bought the car it was operating good besides, the idle was very high(2000rpm), shortly we found that the idle control valve is failed so it was changed but it did not solve completly the problem the idle was still high around 1500rpm.
After long investigations I found that one vacuum tube was not in place on the throttle body - the one which goes to the EZL.After fixing this the idle actually came down to the normal 600rpm, but the rpm jumps up by 2-300 rpm periodically on its own. Same behavior in idle, or while driving or any rpm range always the same time period every 2-3 sec.
Altough sometimes the problem dissappears and everything seems to be ok but if I switch to N position it revs up to 1500rpm.If i remove the vacuum tube from the ezl the rpm goes up to 2000 again but not doing this periodical jumps.
So we thought that something is wrong with the EZl vacuum control part. After ordering a used part from ebay and replacing it but it did not solve the issue.

Before arriving at the EZL the following components were tested or replaced:

• No vacuum leaks
• No fault codes
• New caps/rotors/plugs, Ignition leads
• Ignition coils tested
• All of the thermo sensors replaced
• Crank position sensor
• Cam position sensor
• ICV
• Cleaned throttle body
• Measured Fuel pressure
• New O2 sensor
• Replaced EZL
• Wiring loom seems to be OK

Further suspicious things:
that if any sensor is disconnected the motor runs the same, only reacting when the vacuum is disconnected from the EZL.
When the car is comletly cold started, after the start for some second it runs normal from 1100 rpm idle goes down to 700 and then like magically you would push a button the idle rpm starts the jumping 2-300 again.
I attached a short video as well.

We have run out of ideas, but we re quite certain that we are facing an electrical issue, because its periodically always repeating.Could this be in connection with ECU?Or the used EZL could be also wrong?Or any other issue? Anybody had came across this issue or has a hint where to continue the diagnosis?I d be really greatful for any hints or suggestions.The spring is coming soon and we d like to enjoy this beautiful car. Thank you!

Pazo

On your extensive checklist I do not spot the OVP (overvoltage protection) relay? It is known to crack from solders and cause goofy electric symptoms in KE Jetronic cars.

garrymeg

You are absolutely right, checked but missed from the list.
The fuse is ok on the top of the OVP, and tested the terminals of the relay, everything works fine.

darraghn

Hi. I had very similar issues last month with my 1990 500SL (non Cat). 1,500-2,00 rpm in Park.

I cleaned the ICV, which was sticky, but no change to idle. I bought a new ICV. No change.

Then I did a smoke test for the breather hoses, There were substantial leaks, so I changed all of those. Still idle at 1,500-2,000 rpm.

New OVP. No change.

Finally in desperation, I unplugged the ECU, still idle at 1,500-2,000 rpm, which seemed to be a clue. I then re-seated firmly the connector to the ECU and voila, 650 rpm at idle and now running better than before (because I changed the breather hoses). It is possible the ECU is failing and so reseating it "pushed" a connection inside the ECI back into place, or perhaps it was just a slightly loose connector on top of the ECU. I have started it a few times in the last 2 weeks and it seems to OK every time.

I have seen on YouTube a guy in Australia who fixes ECUs for W124's - Capacitors go bad and leak. So perhaps yours is going bad, perhaps if reseating the connector doesn't work you could try to borrow a know working one to test?

Good luck.

dolucasi

That gentleman makes good video's. I would have liked to see him use a properly grounded ESD pad when he was working on the ECU even though these ECU's are very well built for automotive applications. Have tested 5-6 of these for my model car from junk yards and all work perfectly in my model car. Have not come across a busted one yet. But this is in California where the weather is mild.

A closed loop electrical/mechanical system like this one, if a component is mal-functioning/out of spec can become a mono-stable multi-vibrator, or simply an oscillator.
And sometimes it is at the edge and just changing one condition throws it into an oscillator. The 2-3 second frequency is not surprising because that is the rate the EHA current is changing as well, so it is a system update fundamental frequency.

I would say if it is a regular frequency oscillation and the frequency is not random, chances of it being the OVP is near zero. I can explain that later if needed.

It is more likely that the ECU or the sensor that are input to the ECU are busted. The capacitors mentioned in the video could be the culprit if they are used to dampen these oscillations and make the system super stable and indeed those caps will be the weakest link in this board design because they do not age well.

But it could be a myriad of other reasons. For example even a vacuum leak can cause this because it will make the closed loop system unstable. You mentioned that is under control in your case.

Usually these oscillations happen when something in the system is out of range and the closed loop system starts to oscillate trying to find a stable system condition.

For example a sticky ICVA would also do this because it throws the closed loop into an extreme/out of range condition. But you have already ruled that out.

I would first find a replacement known good used ECU and replace yours to see if the oscillations go away. Rule that out first. Not sure if you did that already.

- Cheers!

garrymeg

First thank you both for the hints! I dont have a chance to get a replacement to try but i continued the investigations.
I checked the ECU connectors, seems to be OK. I also took took a look into it, no visible sign of damage. I have a friend into electronics who measured the capacitors, also seemed to be no sign of failure. But the problem still exists.

How could i basically exclude completly the ECU failure? Could you suggest any method?

Further hints:
- engine is not reacting:

• Disconnected ICV - nothing
• Disconnected cold start injector - nothing
• Disconnected tenperature sensor - nothing
• Disconnected EHA - nothing

- the only reaction when i diaconnect vacuum tube from EZL then the engine surges up, but the whole phenomena of the periodical jumps stops and the engine runs normal. (Besides the high idle...)
- for every periodic jump the ICV closes for a brief moment( i took a look into it)and recieves measureable more voltage.

Any further hints to the ECU or direction where to continue would be appriciated.
thank you,
G

dolucasi

Can you please re-phrase the following?

Further hints:
- engine is not reacting:

• Disconnected ICV - nothing
• Disconnected cold start injector - nothing
• Disconnected tenperature sensor - nothing
• Disconnected EHA - nothing

So "nothing" here means nothing changes, the idle rpms do not change and the rpm cycling remains the same?
This is a bit odd if so because in a working system every of those connector disconnections will cause a jump in idle.

This aside, your model year is past the point in time where Mercedes partitioned the idle controller from the ECU.
Can you please verify if you have a MAS unit? I have to say I am less familiar with the post partition design as mine is pre ('89)

So it seems your problem is related to just the idle controller loop being busted and cycling which is why the EZL is effecting it.
If you have a MAS unit, you may need to start looking at the idle controller (I think it is a combined idle control and fuel pump relay control if my memory serves me right)

However the "no change" to all the other emission control sensor and actuators maybe an additional problem.
Are you sure your lambda control loop is even working?

So before make further assumptions, please clarify the statements in italics above. Particularly the ICV disconnect causes nothing is hard to believe because you just disrupted the idle loop control and I need to know what exactly happened then.

- Cheers!

garrymeg

Yes, from the listed components if i disconnect any of it(even in the same time), the idle rpm is not changing. And the periodically repeating jumps as in the video i attached to the first post remains the same.
The only thing which increases the idle rpm is the disconnection of vacuum tube on the EZL, and with this the periodic jumps dissappear as well.

Yes i have MAS but i dont exactly know what is the exact task of it.
I also attach a picture of the ECU when i took out.

I don t know if the lambda controller works or not. How could I check?
When i recieved the car i got recognized that the connector of the O2 sensor ia broken so i changed it and also the O2 sensors to new.
I did not noticed any change tho.

For me its also hard to understand why the listed components disconnection does not have effect on the engine operation.

Tomorrow I ll try to make some more specific videos.

dolucasi

Curious when you disconnect the vacuum tube to the EZL are you plugging the intake vacuum side or are you letting more air in?
If so, try to plug it up to stop airflow to see cause and effect (does that make the oscillations come back an the rpm drop as it was not even.

Like I said in my previous post, the MAS has some of the functionality instead of the earlier ECU models. Idle control and Fuel pump relay control are some of them.
So the idle is not even controlled by your ECU directly but rather MAS as far as I know but take that with a grain of salt, I do not have a MAS unit so I'm not 100%.

I think you have bigger issues than just idle oscillations. I doubt your lambda control is doing anything useful.

You need to take duty cycle measurements from the X11 diagnostic port before doing anything else.
Read up on that or consult your friend who knows electronics to help you. Likely the duty cycle measurements will tell you a fault code instead of Duty Cycle and that's were you start looking.

Report on the results of the vacuum plug and duty cycle please.

garrymeg

Thank you for the ideas.
I will need a little more time to do the tests, but I ll get back and share the result after the weekend.
Also with some specific videos.

G

dolucasi

Great! Also you forgot to answer the question about the EZL vacuum unplugging method. Can you please address that?, or show in the video how you are unplugging it?

garrymeg

(MAS was taken out and reseated in its place, no sign of visual damage.)

Duty Cycle:
So I measured the duty cycle with warmed up engine and it was fluctuating between 51-58%. If I understand correctly, the lamda circuit should be working based on this?

Video description:
In the attached video you can see that nothing happens during engine operation, if I disconnect ICV, EHA, cold start valve(neither with cold or warm engine), the only thing the engine is reacting if I remove the vacuum tube from EZL( 0:28 in video) at the end of the video.
By the way you can also hear theese periodic jumps throughout the whole video.

ECU:
I have found a method, how the ECU can be checked. (see attached pic) According to this measurement I should measure 75mA on the EHA with ignation ON, but I measured 0.
Could this be an evidence that this is an ECU failure?
Previously I also attached the picture of the ECU, but I m not even sure if it is a correct one, I do not find any info about of this PN on the web.

dolucasi

There is a lot to cover here, so here is my feedback.

Your rpm oscillations are somewhat different than what I'm used to. It looks like your idle controller loop is toast.
But yet when you disconnect the IACV nothing happens. So your IACV is not even in the loop anyway.

My guess is that because of the idle controller loop being busted the idle is in some kind of a limp mode anyway.

For this I would measure the IACV voltage during idling. Have we done this already? Stick a couple of thin wires into the IACV connector, or better yet wire wrap around the male terminal and leave it connected. Let's see if that voltage is already "zero" as I suspect. Generally this condition manifests itself as a high idle in 2.6 liter engines but on a 5 liter engine that needs double the volume of air maybe the idle is not that high.

If this all checks out, this all will leave us with the fact that your EZL loop is causing all the oscillations. The reason why the oscillations stop is because when you unplug it and there is no vacuum in that EZL port, the EZL loop must be broken or you have thrown it into high RPM state in which the oscillations have stopped. Either way there is an issue. BTW when you do that even the ignition timing will be shifter a little. This is just a side note. For the EZL we will have to chase that down. I have to review your earlier posts with a different eye now.
But all and all your oscillations have nothing to do with air, it is all in the ignition in IMHO.

As for your emissions loop, everything seems to be fine there. I think you made a mistake in measuring that 75mA current. BTW that procedure is what I observe every morning when I start my car. Except in a 2.6L engine that 75mA is actually 20mA. Again, things need to be multiplied for a large engine as the EHA valves are common (just like the IACV valve). I think you should re-take that measurement though. I think the second wire going back to the ECU was not making connection. But again this is a disruptive procedure and if not done properly will throw off a CEL. It is best to use a breakout harness for this like the one I used to permanently connect my EHA current Gauge. Just to repeat, the 0mA ignition on Engine off measurement in your case is a red Herring in my opinion. Your EHA loop is working just fine in idle condition as evidenced from the duty cycle measurement you just conducted. Remember that duty cycle number is just your EHA current translated to voltage on the X11.

For your viewing pleasure on that procedure in a 2.6L engine:

garrymeg

I m not sure if maybe I highlighted enough but this periodic jumps also occours at all rpm. Higher rpm as well, e.g. while driving it does the same periodical jumps.

Back to the measurements,

ICV:
I measured the ICV. From cold start the voltage is increasing to 4V until around 10 seconds. After 10 seconds, then it is not recieving voltage and the engine starts theese rpm jumps.
When it jumps for a short moment the ICV moves. I attached the video while ICV dissassembed and you can look into it and hear how the moves/jumps are.

EHA again:
I measured the EHA again and still the result is 0 mA.
Altough the voltage I measured 4,5V, if we could believe the hint from above this would indicate an ECU failure.

Another clue:
I found on the web a topic, whereas if the MPG device is not working properly it is a good chance that I have an ECU problem.
My MPG is continously shows 0 (Fully to the right), even if going full throttle acceleration. Sometimes it goes to the left when I release the has pedal, then goes back to 0 again.
Could this be an ECU problem as well? What is your opinion?


What I did not try yet is to disconnect ECU and try to start. I m not sure what could happen or If I d damage anything or is there any info where I can find?
That would be my final desparation.

I m highly suspecting ECU, can't really think of anything else.
(Its quite hard to get one, I ve started to look around already)

Strange thing is if you check the picture previous post the part number is somewhat very hard to find. There is almost nothing found on this partnumber on the web. I only found like 1 ebay match for sale, and 1 website of elektronics in germany. (Not too much info just a list that it was assembled into SL500)
But really I m starting to doubt whats going on with this part number and ECU.
Also I did not find too much info if there is any other PN compatible.

dolucasi

I'm not sure how you took that video but that is interesting. Did you have a second IACV that was installed but unplugged and this one was connected to the ECU?
Assuming the set up itself replicated what happens when it it installed in the car, indeed the rpm blimps are coming from the MAS periodically giving a jolt to the IACV.
I am not aware of a malfunction that will cause that. Have you replaced the MAS before?

darraghn

I am no expert, but based on my car experience, I agree with you that is is likely to be the ECU. Switch off the car and unplug the ECU and see if anything changes (make sure when you plug it back in that the car is off).

The following Part Numbers should all work for the early CIS-E / KE 5.1 (1989-early 1993 models) A009 545 57 32 replaced by A011 545 41 32 replaced by A012 545 25 32

Also can use A008 545 76 32 replaced by A011 545 22 32

Also can use A011 545 22 32 replaced by A012 545 22 32

dolucasi

I agree with darraghn that you should do the following. Start the car an let it warm up. Then turn the ignition off and unplug the ECU.
Then start the car and report on the idle. What is the rpm and does it still surge periodically?

I still believe this is a MAS problem but we will find out soon.

darraghn

If is the MAS module. The part number is 011 545 85 32, however, I know the M104 version works also 011 545 82 32, which widens the source cars you could borrow from for testing purposes.

garrymeg

Hello together,
Thank you for the comments!
So I did the unplug ECU test with warmed up engine.
With unplugged ECU the engine started and the periodic jumps dissappeared, the engine runs smooth, reacts to gas. No sign of periodic jumps at any rpm(only thing is that idle is higher around 1100rpm)

I think all of theese evidences finaly came together to the suspected ECU, so my next move is to get one and replace it.

Thank you also for the part numbers, after a short search I see that my part number and some of that you mention its ia quite rare. The one which seems to be more common is 0280800408 - 0125452232 so this is going to be my primary target.
However I saw also 0280800410-0115452332 quite common, but I did not see any your list before and anywhere in the web if it is interchangeable or not.
Do you happen to know anything abou this PN?

I m really hoping that after the replacement I can come back with good news.

darraghn

It looks like that 0115452332 will work - see attached screenshot. But again - I am no expert.

dolucasi

I think we have run out of experiments to run and it is indeed time to swap the ECU and hope for the best. At this point I can not think of an input to the ECU that can cause your IACV to receive periodic surges and there is evidence of other foul output from the ECU (zero EHA current at ignition on, engine off, etc)

garrymeg

Hello Guys,So I managed to get an ECU from ebay, used working condition 0280800408 pn.I changed it and good news the periodic jumps dissappeared. 🙂🙂🙂The car has very good reaction to acceleration. The idle is Ok 6-700 while cold and in Drive. So the old ECU deffinetly had troubles. Also the ICV I can constantly measure 4,5 V. While before ECU change it was 0, only gettinh it for a moment. Measured duty cycle: it oscillate between 50-60%.

Only I have one remainig trouble, if I put it into P or N the idle goes up to around 1100-1500rpm. In D everthying seems to be perfect, idle is 6-700rpm and everything seems to be fine while driving as well.
The MPG device seems to be better but still I suspect it is not working always.

I ll start to re check some of the old threads because it seems I still have some issue to fix. It should be something in connection when I shift P and N, i read maybe there is a wheel sensor. I should find something soon.Cheers

dolucasi

Great news garry! On the massive idle jump, sorry but I am of no help because I have a manual transmission car. But as far as I know it should only go up 50-100 rpm from park to neutral.
Hopefully some one here knows the answer.

Can you take a picture of the "MPG" device, I would like to know exactly what it is doing now.

-Cheers!

garrymeg

Hello together,
Issue FIXED.

So the ECU was definetly failed, it was a good choice to change it, but after there was still a long way to go with this high idle issue in P and N, but at the end it was resolved.
Basically after, reading endless forums and trying out various things I decided to take it to a high reputation mechanic specilazied for old cars, with experience in KE jet since the begininngs.

According to his explanation various reasons led to the behavior:

• Someone messed with mixture screw --> mixture was lean
• After changing the ICV, it needed to be fine tuned. It happens a lot in his experience. Even sometimes with MB parts.
• Added 3dl transmission oil
• Transmission cable needed to be fine tuned with some centimeters, was a little loose.
• There was one tube stucked, but I did not catch which one unfortunetly.

He fixed the issue in one afternoon, and no parts needed to be replaced. He had diagnostic cabinets- the old ones from the 90' plus specialized in the topic.

So for us there was one big takeaway from the story, we were really suspecting the mixture screw from the begining and also there was at the end(after changes) no more failed component left in the system. But we were still unable to resolve the issue without proper equipment and knowing the values, it is very very difficult.

Now the car has a very good idle 600 rpm and just sightly increases while in P and N (~700rpm). Drivin, testing very good behaviour. Also the mechanic told it is basically perfect as new. Hope he wont be wromg on the long run
I m still greatful for the answers here we proceeded a lot.

I saw a lot of other threads around and I believe this can be a good hint, to look out for a diagnostics specialist because the issues might be just fine tuning, but still quite hard to resolve without proper equipments.

Hope it helps someone in the future, and I m looking forward for the summer to drive the car.
G

dolucasi

Great news. So the primary culprit was the ECU. IMHO the other fine tune adjustments (duty cycle screw and IACV) probably was super secondary and did not amount to much.
Both the IACV and Duty cycle adjustments are closed loop systems that do not need adjustments but only in the rare occasion they are way out of range (because someone tempered with them)

The idle jumps were probably related to your Auto transmission issues that the specialist fixed which was the last nail it seems.

Happy motoring!

- Cheers!