When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Ok, Got the car home. Drive fine after clearing codes. started shaking when I idled into the alley. Same codes.
Started it up and tested voltage and got 11.5... so bad alternator. shut it off, had my wife start it and give it some gas while I tested voltage (voltmeter on positive jump start point) and it was 11.5 then jumped to 14v and held there forever while the car ran. Either way that seems erratic so I ordered a Bosch alternator and should be there Tuesday.
Pulled apart and wiped clear the ECU ports. Sprayed some cleaner in the throttle body. restarted, same codes, same shake.
I hope you will have success.... I have a very very similar situation here with my ML550. Where have you measured the 11.5V? At the post in the engine compartment, passenger site?
For me: I can clear the codes, misfire, TWC etc. and the car is running fine.
All went worse when I touched the ECU plugs to check for oil or corrosion (there was none)
Ok, Got the car home. Drive fine after clearing codes. started shaking when I idled into the alley. Same codes.
Started it up and tested voltage and got 11.5... so bad alternator. shut it off, had my wife start it and give it some gas while I tested voltage (voltmeter on positive jump start point) and it was 11.5 then jumped to 14v and held there forever while the car ran. Either way that seems erratic so I ordered a Bosch alternator and should be there Tuesday.
Pulled apart and wiped clear the ECU ports. Sprayed some cleaner in the throttle body. restarted, same codes, same shake.
Didn't you say it was at idle that you got the problems? That is when you would have low rpm and hence low voltage on the alternator.
Before you install the alternator, get yourself a battery tester and use it on your battery, with the ground disconnected. It is very possible the battery is dragging the voltage down at idle. The paperwork with the alternator will yell at you to change the battery along with the alternator. It's not a bad idea to do it as a matter of course, particularly if you have not previously changed the battery. A dying battery will take the new alternator down with it.
Just so you know, I didn't think this stuff up myself. Senior forum member alx would always insist anybody with vaguely electrical problems get the alternator-battery system to a healthy baseline before doing anything else.
While you're in there, check the voltage drop from the alternator primary terminal to the lug in the engine compartment. The cable tends to corrode at the lugs. I ended up with something like a volt across that cable. Cured it by sistering a new cable, when I installed the new alternator.
Didn't you say it was at idle that you got the problems? That is when you would have low rpm and hence low voltage on the alternator.
Before you install the alternator, get yourself a battery tester and use it on your battery, with the ground disconnected. It is very possible the battery is dragging the voltage down at idle. The paperwork with the alternator will yell at you to change the battery along with the alternator. It's not a bad idea to do it as a matter of course, particularly if you have not previously changed the battery. A dying battery will take the new alternator down with it.
Just so you know, I didn't think this stuff up myself. Senior forum member alx would always insist anybody with vaguely electrical problems get the alternator-battery system to a healthy baseline before doing anything else.
While you're in there, check the voltage drop from the alternator primary terminal to the lug in the engine compartment. The cable tends to corrode at the lugs. I ended up with something like a volt across that cable. Cured it by sistering a new cable, when I installed the new alternator.
That's not a bad idea. 1 volt is a huge voltage drop. Might be worth removing and cleaning the lugs as well.
The issue was at idle, I just wanted to see what effect the increased RPM has on the alternator.
Not a bad idea to replace the battery as well. I do not have a load style battery tester, might be worth getting one or just a new battery.
I was testing voltage and the jump start point in the engine compartment.
I would get the tester. They're cheap. And then test the battery life remaining every year or so. I'd get it first and find out how bad the battery is. You also can then test the new battery; I have encountered storage-depleted batteries.
Look at it this way: A battery may not display symptoms of age (e.g. slow cranking) until it is at 95% of its life. However, from 80% (for example) to 95% the battery is straining the alternator. When an alternator is constantly running at full power, attempting to charge the battery, high currents are running through the brushes and commutator, both of which experience increased wear. The high currents going through the diodes aren't healthy either. The failing battery really does drag down the alternator.
If you replace a battery with 20% life remaining, that is costing you about $40 (20% x $200) but you have more than saved that in terms of increased alternator life - and the alternator is quite a bit more painful to replace.
Or just replace them together, but the alternator typically lives about 2x as long as the battery, especially if it is not paired with a dying battery.
Regarding the cable: The lugs are crimped on, so they cannot be serviced in any ordinary fashion. I thought about drilling a hole in the side and melting solder into the connection, but the cable's connector at the alternator is a double-sided pass-through going to the starter, so the cable has to be removed from the engine compartment lug all the way to the starter. It ended up being enough easier to just sister a cable.
If you regularly store the vehicle for more than about a day or so, put a good battery maintainer on it to offset the standby drain, which depletes the battery.
11.5 v with engine on? What about with everything off? You need to determine whether its battery or alternator or even both. 11.5v engine on and I'd say you've got an alternator problem first and foremost. Small drops in voltage can trigger all sorts of codes. There's no such thing as proper diagnosis unless your battery and charging system are working 100%.
Ok. Got a $50 battery tester from Amazon and it's garbage... readings are so inconsistent on other batteries i had lying around. Going to return and get an old fashioned analog load tester.
That being said, installed new bosch alternator and new AGM battery, charged it up and still the same codes... battery voltage and charging seem fine, no voltage drop.
Idle seems OK... slight stutter, but seems to run fine. Based on before I'm assuming once i drive it and it warms up it will get worse.
So... buy used ECU and send mine out to get flashed? Take it to Mercedes dealership and eat the diagnostic fee? What's my next step?
Strange, the $44 (why did I think it was $10bux, sorry) tester I got seems quite legit. Maybe your batteries lying around are in various states of death.
Anyway: The electrical system had to be ruled out, and it sounds like you did so. I take it you are now getting a steady >14V at idle. It's too bad that didn't fix your problem. You still get the same set of issues, in the same order? Alternator and throttle body errors at idle, followed by oxy sensor errors at rpm above idle?
It is very possible the crummy voltage supply fried something in the ECU. I wouldn't be surprised if the alternator sent all kinds of spikes while in its death throes.
I liked the guys at Pressertech (pressertech.us). They helped me, even though ultimately the ECU was not the problem. They can diagnose and repair the Bosch ME (yours is ME9.7) ECU's. You can pull the ECU and send it to them; they'll either repair it or clone it to a replacement. Replacementt isn't cheap (somewhere around $1000) but it's much cheaper than going the dealer route.
You can diy (buy a used ECU and get it SCN coded, either by yourself or with assistance). Personally, I'll do lots of stuff, but I avoid doing things that I'll likely never do again, and coding an ECU is something I hope I never have to do again. But you seem like you work fast and enjoy challenges, so ... you do you.
Last edited by eric_in_sd; Jun 18, 2025 at 08:39 PM.
Anyway: The electrical system had to be ruled out, and it sounds like you did so. I take it you are now getting a steady >14V at idle. It's too bad that didn't fix your problem. You still get the same set of issues, in the same order? Alternator and throttle body errors at idle, followed by oxy sensor errors at rpm above idle?
It is very possible the crummy voltage supply fried something in the ECU. I wouldn't be surprised if the alternator sent all kinds of spikes while in its death throes.
I liked the guys at Pressertech (pressertech.us). They helped me, even though ultimately the ECU was not the problem. They can diagnose and repair the Bosch ME (yours is ME9.7) ECU's. You can pull the ECU and send it to them; they'll either repair it or clone it to a replacement. Replacementt isn't cheap (somewhere around $1000) but it's much cheaper than going the dealer route.
You can diy (buy a used ECU and get it SCN coded, either by yourself or with assistance). Personally, I'll do lots of stuff, but I avoid doing things that I'll likely never do again, and coding an ECU is something I hope I never have to do again. But you seem like you work fast and enjoy challenges, so ... you do you.
voltage is steady above 14v at idle. The alternator and throttle codes are always present, o2 have not come back yet, but i have not had a chance to test drive, only idle.
I'll look at getting a used ECU and then try to find a place to reprogram it.
Just to confirm: You cleared all DTCs, then re-started, and the alternator and throttle codes came right back?
The ECU might be repairable. If I were you, I would contact a repair and service place and explore that option. It should be considerably cheaper than outright replacement.
I'm also curious what you come up with for replacement - what a shop charges for the coding, which has two parts, one the configuration and the other the VIN. It is a risky process, as the ECU can become bricked. See https://oemtools.com/wp-content/uplo...ing_230512.pdf for some related info.
This is a job I'd leave to trustworthy professionals, but that's me. I'm less inclined to risk these days.
Just to confirm: You cleared all DTCs, then re-started, and the alternator and throttle codes came right back?
The ECU might be repairable. If I were you, I would contact a repair and service place and explore that option. It should be considerably cheaper than outright replacement.
I'm also curious what you come up with for replacement - what a shop charges for the coding, which has two parts, one the configuration and the other the VIN. It is a risky process, as the ECU can become bricked. See https://oemtools.com/wp-content/uplo...ing_230512.pdf for some related info.
This is a job I'd leave to trustworthy professionals, but that's me. I'm less inclined to risk these days.
Correct, both are always on regardless of the engine temp/drive time. The o2 sensor ones come after driving.
I did drive it for 15-20 minutes today in 97 degree weather and it drove fine, no stumble at idle, possibly due to AC demand keeping the idle high. Checked the codes after driving and I am down to just the sticking throttle body, with the o2 sensor codes coming in later. The serial alternator code seems to have subsided, possibly due to the new alternator lol. Just odd that it remained there for a while after replacing the alternator.
I messaged the company you mentioned the can repair, but they are $990 if it need to be replaced, much higher than a used ECU being cloned ($300ish), but that is also a gamble as well...
I am still inclined to believe its the ECU, the amount of random electrical issues popping up does not seem possible on components that I have not touched.
Yeah that's about what I paid, for an ECU that wasn't actually the fault. Money and a great deal of heartache down the tubes.
It is maddening how the errors come and go. I admire your persistence in all this.
I'd send the ECU to Presser for diagnostics, and if they cannot repair, then decide whether to get a cloned ECU from them or go the DIY route. Something buggered about the I/O might be a relatively easy fix.
I'm pretty curious at this point what the ultimate root cause of all this is. Did the alternator fry your bank 1 exhaust cam magnet?!?
Yeah that's about what I paid, for an ECU that wasn't actually the fault. Money and a great deal of heartache down the tubes.
It is maddening how the errors come and go. I admire your persistence in all this.
I'd send the ECU to Presser for diagnostics, and if they cannot repair, then decide whether to get a cloned ECU from them or go the DIY route. Something buggered about the I/O might be a relatively easy fix.
I'm pretty curious at this point what the ultimate root cause of all this is. Did the alternator fry your bank 1 exhaust cam magnet?!?
Yeah I am not opposed to sending it in. Just need to find a place I can park it for a few weeks where I wont have to move it...
My gut feeling is a bunch of aging electronics being the weakest link and once I fix a weak link, the next weak link fails... ending with the ECU.. I hope.
One thing you might want to check is the catalytic converter. When they block up everything goes crazy. If you have a temperature gun, they run at about 400 degrees celcius when operating normally. both should be the same. Im not a parts canon sort if guy, but is the cats check out, changing out the 02 sensors is a bit like making sure your battery and alternator are working properly so your diagnosis is sound. Might be a cheap way of removing one more variable.
I am traveling in July so my game plan is to pull the ECU and send it in for inspection then so the car can be parked for a bit.
Up to a few hours in 90 degree heat and no running issues. Have issues with my scanner so cannot see codes, but will report back when I get it working.
So during the process of driving around both of my Autel AP200's stopped working... tried them on a different vehicle (ECU being suspect...) and they also did not work. So looks like I need a new scanner to proceed on this mission. Any suggestions on a good one to get to diagnose the electrical issues I am dealing with?
It would seem to me a hardware device like the Autel MP808, not using OBD power, would be more rugged. It is possible that prior voltage issues caused the internal power supplies on the AP200s to degrade.
I own an MP808 and think it's pretty good, but don't have any other experience to compare it to.
That is an interesting thought... one of the AP200s was never pugged into the ML until the other one failed and I tried to test them. It feels to me more like a Bluetooth connection thing...
Either way I need to upgrade anyways... Might have a line on a used SnapOn Solas and might go through route, if not the MP808 seems to be recommended by most people.
I had a very similar problem, and it was the ECU at the end.
Capacitor leaked on the PCB, electrolyte destroyed the PCB, caused arcing and burnt traces.
I got a used ECU and cloned the data from the old to the new. Car is running fine now.
I had a very similar problem, and it was the ECU at the end.
Capacitor leaked on the PCB, electrolyte destroyed the PCB, caused arcing and burnt traces.
I got a used ECU and cloned the data from the old to the new. Car is running fine now.
What process did you take to clone the ECU, Did you do it yourself?
Mercedes SLR McLaren 722 S Is Extremely Rare Example Modified by McLaren
Slideshow: A one-of-one U.S.-spec Mercedes-Benz SLR McLaren Roadster became even rarer after a factory-backed transformation at McLaren's headquarters.
