Hey there MBWorld!!
Very longtime lurker, and I finally made an account to provide my knowledge on Mercedes and STAR stuff, and also to ask for any advice on this one, weird car. Strap in, because this is a long, very strange one. If nothing else, it's worth the read for entertainment of electrical gremlins.

I've owned and fixed about 20 different Mercedes-Benz cars with the years ranging from 1990-2012. Most of them being the m113 variants, AMG or not. I've fixed everything on them ranging from electronic stuff like stereo equipment and rebuilding various embedded computers, to mechanical things like motor mounts, valve cover gaskets, camshaft replacements, etc.

The car that I've come to love the most, and is most worth the struggle has been the SL55. I love the speed, the looks, the elegant engineering, the value for money, and the reliability of the important stuff (drivetrain, body, etc). Very original opinions here. I realized I'd made a huge mistake selling my last SL55, so I hastily got another one, and here we are making an SOS thread about it!

It's a 2006 with only 83,000 miles... and a rebuilt title. The car was a "flood car", but is absolutely gorgeous, and runs extremely well (when it runs), so I paid $10,000. The car is a "flood car" because it was totaled due to getting rained in. The rain caused the seat controllers to crap out, and a bunch of stuff had to be replaced inside the interior. Importantly, the engine, transmission, ABC suspension, roof unit, etc all work great... when the car runs.

The problems:

• The car does not start on key turns, or Keyless-GO starts

When I bought the car, the owner let me know that the car takes many attempts to start. You can turn the key all the way, but the engine doesn't start. It's a no crank situation, but everything else in the car turns on. His mechanic quoted him an EIS, which I didn't believe to be the problem. The number of key turns before a crank is random. Sometimes 2, sometimes 50. But when the engine starts, the car starts right up. No hesitation or long cranks. And after it starts once, you can turn it off and start it right back up no problem. If you leave the car sitting for a little while (somewhere in the 30-60 minute range) it's right back to not starting. The car has NOT turned off while driving.

• The engine's suction fan runs full blast

This seems to be indicative of the ECU not communicating. The code I receive on my scanner says there's an open circuit for the suction fan, but there's also things that make me think it may not be the ECU.

• Driver's side headlamp randomly flickers on and off

Prior owner bought a new igniter and bulb. I did all the work to replace them, to no joy (still flickers). When inside the headlamp, there were several wires flaking their coating off. I ordered an entire new (used) headlamp, because I can almost guarantee those tiny wires are shorting somewhere.

• Various stereo/electrical gremlins

These aren't really worth asking about, but just mentioning them so you don't think this car only has 3 problems.

What I've done:
Clearly, the most important issues are the inability to start the car, and the engine fan running full speed.

1. My first thought for the non starting issue is to check the batteries. The starter battery was 6 years old, and the consumer battery is almost flat. Replaced the starter battery just because it was old. Checked the power and ground cables. No joy.
2. Pulled up EIS documentation on my STAR laptop to troubleshoot if EIS was energizing circuits properly. Long story short, EIS energizes 2 different circuits in the r230 SL before starting the car. I may be remembering wrong, but I think it's 15 and 50. Either way, connected my STAR laptop and tried starting the car. EIS was energizing the circuits correctly when the key was turned.
3. Checked the starter itself. Saw no immediate problems, and then the prior owner let me know he had a new one put on not even 6 months ago.
4. I checked and replaced all prefuses in the passenger footwell (the big motherf**king fuses). All were providing correct resistance, and none were blown. But I replaced them anyways, since it's about $20 for all of them.
5. Checked for water ingress on the ECU. I disassembled the ECU and there seems to be some water damage on the back of the circuit board. No rust, and no corrosion, and no burns or exploded capacitors. Nothing that should kill the circuitry. I resoldered all pins I could, just in case it was cracked solder. No dice.
6. Took out the passenger fuse block (since it has the ECU fuses and starter relay). Checked all wires, connectors, and fuses. Nothing wrong.
7. Removed the passenger SAM from the passenger fuse block. Checked all pins and wires. Absolutely nothing wrong on this, and zero water damage signs.
8. Checked the rear fuse block, where the power regulator is. They're fine. The power regulator itself seems proper as well.
9. Checked both of the CANBUS connectors in the driver's and passenger's footwells (brown and green ones). Both have no corrosion or rust, and all pins seem to be fine.
10. Succumbed to the call of the dealership's promise of a smooth running car. I took the car to the dealer and they claimed it to be an EIS issue. Paid $1100 for mostly nothing improving. A bird also crapped on my car while it was there, and they didn't clean it. Insult to injury.

What's... interesting:

1. When you unlock the car and get in, if you don't attempt to start the car, the gauge cluster may flicker slightly between bright and dim. Like day/night settings.
2. If you sit there for about 30 seconds, a red BRAKE warning comes on on the dash
3. There are a couple of CAN communication errors that come and go. This could just be from the car thinking it's on, but the engine not running.
4. I've also had times where I try to start the car once, and it fails. But then I sit there for 3 minutes and try a second time, it starts right up.
5. About 50% of the time, the gauge cluster is not accessible via OBD2 or STAR. Sometimes it has absolutely no issues.

I am at my wit's end (not really). I would just like to get the car consistently running. But most importantly, I'd like to figure out the problems. It's not so much about being cheap with fixing the problems, as much as I'd like to be able to say "I fixed it".

I currently have the central gateway and the ECU in my scopes.
Central Gateway: I think the CGW is not waking up components correctly when it's time to start the car. I feel this may be the case because of flickering on the gauge cluster, and being able to eventually start the car after enough waiting/attempts
ECU: I'm also stuck thinking it could also possibly be the ECU, because of the water damage indicators, and the fan seemingly not receiving a signal.

Any help or advice is appreciated. I'm attaching a picture of the car because it's currently a nice parking garage ornament.

 

Here is a video I just recorded, if you want to see this weirdness in action.
[video deleted. post #4 describes new problem]

 

Alternator, voltage regulator, brake light switch? Have you checked those?

 

Brake light switch, I didn't think of. What problems can that cause? Battery drain?
But I did check the alternator and voltage regulator. I've tested the voltage at the starter relay when the key is turned. When the car starts, the full voltage is sent to the starter motor and it cranks. When it doesn't start, absolutely no activity on that relay/wire.

Fun thing that just happened when I went to get coffee. The car had sat overnight again. I unlocked the car, got in, tried to start it (no crank), and while the car was "on", I pushed the unlock button on the dash to run the PSE pump to unlock all locks. I then turned the car "off" right after I heard the PSE pump finish running the locks. I immediately pushed the starter button again and the car immediately cranked. Less than 10 seconds to cold start from start to finish, with the main difference being that I fully ran the unlock on the PSE. I've also noticed that even though I get in the car, usually the lock switches in the doors aren't actually raised.

Any ideas? PSE pump potentially hogging the CAN wires? Anyone seen this before?

 

So, sorry to bump this thread again, but there's a development! A mostly good one!

So I've found a method to start the car every time... Manually unlocking the door! This reels back the problem areas from being a bunch of stuff, to just a few things (ECU is still suspect though).

When I grab the door handle, the door clicks and can open, even if fully locked and left overnight. It has Keyless-GO. However, the PSE pump does not fully run the door lock nub (the thing that comes up when unlocked). The car does not start immediately, and the symptoms in post #1 happen.
I should note that the PSE pump works fully when the car is on, whether the engine is fully on or not.

The weirdness happens when I manually unlock the door with the key shank. The car will always start when I open it this way.
Again, the PSE pump works correctly with no leaks. It just doesn't seem to run correctly with the car off. I don't think it runs at all when I press unlock or grab the handle. Ever seen anything like this?

 

You better be super buff with that username haha!

 

Leg press (on a machine) is 640 lbs, bench presses at only about 230 lbs currently.
The username is mostly a joke.

 

THREAD REVIVAL
If you experience intake fan issues like I described, it is sadly likely to be your ECU. A new ECU fixed this.

The weird part was the car would still not start the engine. Same symptoms even with a new key, EIS, and ECU. Fuel pump works, all SAMs work, all relays replaced.

The solution? Believe it or not, disabling secondary air injection solved the inability to start. There could likely be a short circuit in the secondary air pump. My problem was SO specific, I doubt anyone will find this helpful.