1999 C230 K Check Engine Light becomes a cash hemorrhage

I have a 1999 C 230 Kompressor with 71K. When my check engine light came on, my NEW and highly recommended mechanic's diagnostic computer found a bad ignition coil on #2 cylinder,
and the car needing a tune up. He replaced the coil, did the tune up and replaced the stop light switch. The check engine light remained on. He discovered the compressor clutch was working erratically. Because the tab was at $700, I asked if the compressor clutch was critical. He said it wasn’t. Does all this sound okay? This shop has an excellent reputation but I have to wonder about the computer not being specific in the diagnosis process. Any input greatly appreciated.
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Which "compressor clutch"? The air conditioning compressor or The Kompressor (supercharger)?
Codes are somewhat generic, my car once had a P0170 (fuel bank trim) code which meant nothing to me but was cured by a new air mass sensor. Diagnostic codes are not simple directives like "replace mass air sensor."
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-->> T.G. Lambach <<-- wrote:

Thanks for your response. The supercharger clutch. The mechanic says it's erratic behavior won't harm the engine so it's repair isn't crucial.
Joe
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Think about this: The Kompressor clutch essentially controls the quantity of air pushed into the motor so if the clutch is flaky the motor's air/fuel ratio may suffer but even more important the motor's performance, especially when YOU expect performance, may not be available.
OTOH, perhaps the Kompressor clutch is OK and it was mis-diagnosed. Kompressors are not common so I'd personally spend some $$ at the dealer for a "diagnosis and estimate only" that may get to the root of the car's problem.
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-->> T.G. Lambach <<-- wrote:

The supercharger does not control the air/fuel mixture. It only pressurizes intake air so that more air can potentially be stuffed into the cylinders. The air mass meter measures the amount of air that is drawn (or forced) into the cylinders - a lot or a little - and commands the injectors to supply an appropriate amount of fuel. If the supercharger stops working, the engine will get less air, but the air mass meter request less fuel, too, so the mixture will be unaffected.
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-->> T.G. Lambach <<-- wrote:

They're not always very accurate, either. Instead of a separate exhaust air pump, the supercharged models bleed off air from the supercharger for exhaust air injection. If the air injection isn't working, that'll set a code and turn on the MIL.
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I am not so sure... Joe. The computer spits out codes... and the mechanic should be able to tell fromt the code what might be the cause... not just fix and the light still stays on.
If I were you, I'd run over to Autozone to borrow the free OBD II scanner and pull the code out yourself. Write the information down and return the scanner...
I'd be wary at this point... I don't like mechanic who said they fixed something and then still have to fix another thing.... heck that thing can go on and on. Sometime Dealer is not that expensive... laborwise, they charge more, but they has all the resources.
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The supercharger clutch may not be working, but it doesn't necessarily mean that the clutch is defective. There are certain other problems where one of the many sensors can produce a signal to the ECU where the ECU will then feel that things are out of whack enough that it is not a good idea for the supercharger to run, so it disables the supercharger from kicking in. If your mechanic is basing his diagnostics on a code alone, I would be sceptical. If he has tested the clutch manually, then I would have more confidence in his diagnosis.
If you read the various w202 forums and SLK 230 forums, you will see numerous posts where mechanics misdiagnose supercharger issues as being problems with the supercharger when the problem was realy elsewhere. Numerous unecessary supercharger replacements. Dealers are not immune from this mistake either. Bad sensors, undiagnosed vacuum leaks, etc., have been posted as the real root cause of the problems after further investigation.
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