So a month ago, had an issue with C320 not starting. 204,000 miles roughly, got codes for P0355, crankshaft sensor, but had already changed it from Bosch to Hella. At this many miles, figured the starter had given up. Tried to replace it myself, but didn't have the ability to lift the motor, and noticed a bolt sheared off from having them changed last year. Took it in, let them do the work removing mounts, they called, starter wasn't the issue. Change it anyways, you've got the mounts out and one needs the broken bolt removed. So they did it, and changed the Hella to a variant, whatever it was, and had same issue starting. It won't catch. Did say that it likely needed the electronic control module replaced or flashed. No one local could do it, so before sending it out, neighbor has about 8 Mercedes laying around the house, we used his code scanner, same thing, over and over, P0355. We'd clear it, try to start, every time, same code. So, pulled the ECM, sent it to New York to be cloned, got it back today. Put it in, wired the car back up. First crank, no go, second, and on and on. Called cloner, they said they'd send a replacement. But before I did anything else, I checked fittings on battery, and some are a little loose, tightened everything down, fired right up. Checked codes, no codes, none. Turned it off, turned it back on, no fire. I'm almost back where I was before laying money into it, except eventually it will fire, isn't rough idling now, and is far, far smoother driving, it's just getting it to light and go. Checked battery, Duralast H8 Platinum, checked out fine, too. Also replaced batteries in key fob. Yes, stock wiring was replaced about 5 years ago, because with navigation system, micro amp and subwoofer, the alternator was too weak at 120 amp to support, so I replaced it with a 160 amp, which necessitated replacing the wire with 0 gauge to not burn up anything. The last two summers here have been brutal on heat, over 100 days over 100, and this car went from El Paso to Disneyland and all around Los Angeles and back in September of 2022. The terminals are Kicker, and the positive fits on smooth as butter, the negative, however, is a 3 eyed mother I had to cut an Allen key down to even be able to tighten it, and it's also got a plastic plate under it, so only about half the terminal is actually connected. Am I up to something with the ignition switch itself, the keyfob, or keep digging?


Starter connections, line going to alternator.

Main line from positive terminal to starter.

Positive terminal, 3 way kicker with 8 gauge going to block for entertainment system, 0 gauge going to starter, other 0 gauge going to the terminal block.

 

When you say "won't catch", are you referring to "no crank no start", or "cranking but no start"?

I'm assuming your battery is still in good condition at this point if you haven't already checked. I think the starter also has a fusible link in the stock cable. I'd check for continuity in each of the wires.

When reconnecting power, did you make sure to connect the battery positive terminal first? The front SAM is known for frying if power is reconnected negative then positive.

Lastly, has the fuel pump been replaced? They are known for dying after 200k km or so.

 

Thanks for replying. Cranks but won't catch, so you can hear it turn the starter, but doesn't catch. Checked battery today, all good there. As for battery, yeah, always negative last, that's been ingrained for 40+ years working on cars even in childhood. It's been a long minute since I swapped cables in the car, but don't remember any kind of fuse on what I removed. If that wiring was over 8 gauge, I'm the head of Daimler. I did just pull the starter relay for giggles and hosed it down with electronic cleaner, and it immediately caught, turned it off, back to 2 or 3 times to start again. I want to say I replaced fuel pump, maybe 7 years ago or so, probably 100,000 miles or so ago. With the bad ECM, there was the smell of fuel, but with the cloned ECM, once car starts, idle is fine, acceleration is fine, quit as a church mouse compared to before. And fuel pump was replaced with an identical Bosch number.

 

Next thing I'd try is swapping some relays around. The front SAM often has several of the same relay model/type, so there's no need to buy a new one.

To be honest though, I'm thinking it's the fuel pump dying slowly. One thing you may try is to rock or shake the back of the car before starting and see if that fixes the problem. There might be an intermittent broken contact in the pump.

 

I've got a relay coming Thursday, Hella brand, so I'm going to try that first before looking up fuel pump part number. I'll post back as I swap stuff.

 

I just swapped the starter relay with I think the horn, just moved them from corner to corner. Starts about every other time right now, which is better than the 1 in 8-10 I was at.

 

Just an update, let car sit for a couple hours. Tried shaking it, no luck. Also checked Schrader valve, good pressure on getting gas out after turning key to ON position but not turning over. Went ahead and ordered fuel filter and fuel pump. Could it possible be sending unit as well? Replaced both, but had to have been back in like 2016 or 2017.

 

The proper way to test would be to attach a gauge to the shrader valve, but if you have pressure, the pump is probably fine. Can’t go wrong with replacing the fuel filter if it hasn’t been done in a while. Just make sure you only use OE or Mahle Knecht (the OEM for this filter). The aftermarket ones are known to have issues with the internal pressure regulator.

Another possibility would be the crankcase position sensor, but this is a wild guess.

 

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Crankshaft sensor is now third one in last 6 weeks. Also changed pigtail on this current one, may need to check wiring again to it. Ordered a Mahle, can't believe it's a week or longer to get one. But yeah, hitting the valve, it came out pretty strong stream, so pressure is there. And also, no codes like before with the P0355 when it was failing.

 

For what it's worth, it's started this morning 2 out of 3 times, first time and third time. Loud smell of gasoline off it though.

 

Hold on. The gasoline smell wasn’t mentioned before. I would start poking around the engine bay.

There are very few places where gasoline would touch. Have you checked the fuel lines and injectors?

 

Haven't checked either. Fuel smell went away when ECM was replaced, but back this morning. Could be unburnt fuel through exhaust after it does catch. Garage was closed, so could be part of it. Injectors were replaced either last summer or summer before last.

 

Just checked, no gas smell under the hood, no leaks or stains either. Think it was just overpowering exhaust with garage closed. The fuel filter, it's been at least 100,000 miles since it was changed, and I'm almost 100% positive when the fuel pump gave out, it gave a code somewhere to indicate needing a new one, rather than hoping and praying. I've got a Hengst fuel filter ordered, as Mahle's were a week out, and locally, everyone wants premium pricing for crap like Microguard and Carquest.

 

The fuel pump doesn't throw any codes. Not that I know of at least. Mine just died one day (cranking but no start).

A Hengst filter should be good enough, since they're OEM grade. With that said, only 1 way to find out.

Maybe try checking for gasoline smells or leaks with the engine running as well.

 

I did run it for a bit after the garage cleared, just the usual exhaust smell, so that's a positive. When I replaced the fuel pump and sending unit, I remember I had codes, but the downside was, it didn't specify the sending unit or the pump unit, so I ordered sending by accident first, didn't fix anything, then replaced pump. I'd almost rather it be those two as they require less mess than the fuel filter, but filter is due Friday, so find out Saturday if it changes it's tune. And I definitely did not pay close to $500 for both parts, much less the sending unit alone.

 

Of course now it wants to start every time. Went on a 30 minute drive, no issues, but never turned it off. Came home, ate, turned on, went back, ran a scanner over it for everything, no issues, turned right on.

 

So, replaced sender and pump, can almost guarantee the sender was the original, because I don't remember snaking the hoses and cable through. Same issue. Made sure gas cap was tight, opened Schrader valve in the ON position. Glad the fuel filter is due today.

 

Ok, so changed fuel filter this morning, sending unit and pump yesterday. Also swapped relay from front to the rear for fuel pump. It's catching more frequently, but still the starting issue.

 

What if I told you, while thinking about the rings on the fuel pump and sending unit, I didn't think they were fully tightened? Better than that, the sealing rings weren't replaced, and I know one had a chunk missing out of it? and both were severely expanded and out of shape? Ordered 2 more and getting a loan a tool to tighten the rings correctly.

 

A bad tank ring(s) would result in fuel leaks at most. Obviously a good idea to replace those while everything else is out of the car.

I don't want to keep adding to the list of random parts to throw at the car and driving your costs up. Worst case, there's always the local indie.

In any case, keep us posted. Good luck.

 

It is driving me nuts. Along with the seal, I don't think the rings are tightened all the way, so loan a tool from autozone and make sure they're tight. Just insane, no codes, nothing showing up at all, and when it does fire, it's not stalling, losing power, nothing.

 

Finally got a new code. P0410 Secondary Air Injection System, ie smog pump. This a culprit?

 

Let's try this on for size. So the issues started when I was getting a camshaft and crankshaft sensor error. I replaced both at the same time, with Hella brand. Years ago, probably the last time I changed the camshaft sensor, I had to replace the pigtail on it because it was falling apart, and I was holding it together with zip ties. Looking tonight, the shrink tube I used on the wiring is suspect, and I'm wondering if I didn't break a connection or something in the tube, and that's causing the issue, like the middle line looks worst of the 3 as far as crimped at an odd angle.

 

So, I was close to what it was; I redid the wiring for the camshaft sensor. Go to love how the same how the exact same pigtail for ABS control modules is the same as the camshaft sensor, but the camshaft sensor is $40 more. So, $10, redid wiring. No change. Pulled pumps, changed seals, the pump side hard as a rock and looked like it fought with a pineapple and lost, sender was bad, but flexible. No change. Went and got a new crankshaft sensor, let car cool down. Bosch, 100% Bosch part. No change. I knew the camshaft was supposed to be a Hella, and was. But as a last ditch effort, went and got a Bosch, tools in hand, at Autozone. Fired on first crank. Went to 4 more destinations, 4 starts, no issues at all. Fingers crossed in the morning that this is it, because diagnosing this thing is impossible when there are literally no codes. But at least I've avoided any other major maintenance for a few years with all that's been done.