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Old 08-08-2017, 08:33 PM   #1
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Forest River 2014 Solera - loud clunk and brief hesitation while driving

We have had our motor home into the Mercedes dealer multiple times for two things:
1) sometimes when turning over the key nothing happens
2) sometimes when turning over the key it continues to try to start even after engine is running and sometimes while driving down the road it makes a big "clunking" noise. No error messages come on.

We are at our wits end, as Mercedes says they cannot do anything unless they have an error message.

Help!

Janell, Tacoma, WA
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Old 08-09-2017, 05:22 AM   #2
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Short

It sounds like you have an electrical short somewhere in your starter circuit. This will get very expensive if they cannot find it before a catastrophic failure. If the starter is engaging while the engine is running, it is likely to self destruct and damage other things. Intermittent electrical problems are difficult to diagnose because when they are not failing, they are undetectable. I would start at the ignition switch and work toward the starter.
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Old 08-09-2017, 06:51 AM   #3
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I had a similar ignition problem with our 2400W MBS. Sometimes I would try to start the engine and nothing would happen. During our last trip I had a check engine light, went to Mercedes dealer in Halifax Nova Scotia and the turbo sensor was replaced. After discussing the ignition problem with the technician, he mentioned that it is not really a problem, the computer checks everything when you stop, if you are not removing the key or exiting the vehicule via the driver door it will create a situation where the engine won't even crank. I now remove key from ignition and open my door and no more problem starting the engine!
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Old 08-09-2017, 08:42 AM   #4
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Quote:
Originally Posted by jjholman View Post
2) sometimes when turning over the key it continues to try to start even after engine is running and sometimes while driving down the road it makes a big "clunking" noise. No error messages come on.
I had a "clunking" noise, accompanied by a jerk. It felt as though a clutch had been disengaged, allowing the engine rpm's to vary from the vehicle's speed, and then slammed back in. This was not associated with the starter.

Eventually it did it bad enough that the engine threw a code - crank position sensor. Both the tech who worked on it initially and I thought it likely that the crank position sensor code was a result of the problem, not a cause, so I went on my way.

Months later (after no further problems) I had it in to a Mercedes dealer for regular service. I told them about the problem and they decided to test the whole thing. Yup, the crank position sensor was bad. They replaced it and I've still had no further problems.

Don't know if this is similar to what you're seeing, but maybe it helps?

Dave
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Old 08-10-2017, 03:39 PM   #5
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I am not a MB mechanic and have never owned an MB. But I have dabbled in automotive work since I was a kid. So you can take this with a grain of salt. Being a craps shoot, I will put my 2 cents in this.

It appears that the MB starter system operates as most starter systems at the starter-the solenoid actuates a solenoid which engages the starter motor into the flywheel. When the motor starts, a spring on the solenoid pulls the starter gear back. The mechanical aspects of your system seem to be functioning properly. Which leaves the electrical.

The MB ignition starter position actuates a relay that then actuates the starter solenoid. It appears the relay is configured to always have power at the relay, but requires a ground from the ignition to complete the circuit. If this proves to be the case, though the ignition switch is still a possibility, you are either looking at a grounding issue or a faulty relay. Apparently, there are several relays with the same number in your system. What other functions that relay may have is above my pay grade. Assuming it may have another function, that may cause it to trip the starter when the function activates. I would speculate that if there is another function, that function would also require a ground to actuate, thus tripping the starter. The computer is making those decisions. With this scenario, it is possible the computer would not see the malfunction.

What appears to be happening when starting is that a relay is keeping the starter engaged after your motor starts, but the solenoid spring has pulled the starter engagement gear away from the flywheel. However, when the starter engages on its own, the starter gear in being thrust against the rotating flywheel and the noise you are hearing is grinding gears which can cause flywheel damage.

Being an open discussion, this may sound off the wall. Mechanics have proven me wrong, but I have won as many along the way.
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