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Discussion Starter #1
Hello everyone,

Before anyone says "use the search box" - I have, extensively.

My problem has been going on over a year. It started with a CEL with the code P1236 (supercharger clutch) but at the time my supercharger was engaging normally (around 2200 RPM while in Neutral) and my MB mechanic told me the car had the right amount of power so it wasn't worth doing anything at the time.

Fast-forward to now and I have to get the CEL resolved in order to pass smog and renew my registration. So I started looking into the problem.

As of this morning, I had codes P0410 and P1236 showing. I revved the engine at idle and noticed the SC clutch was not engaging at all. Google and this forum led me to believe the K40 relay might be responsible for my problems, so I addressed that first.

I removed the K40 relay and resoldered the three solder joints in the upper-left corner. I couldn't remove all of the old solder so it's possible the relay still needs soldering work done. However, upon replacing the K40 relay I noticed both trouble codes had cleared (maybe due to the battery being disconnected?)

One of the other troubleshooting steps I noticed while researching a solution was to make sure the vacuum hose leading to the secondary air injection valve has vacuum pressure. I pulled the connector off the valve and immediately the line broke, which led me to believe a cracked and leaky vacuum line was partially responsible for my CEL. I put a finger to the broken end of the vacuum line and there was indeed negative pressure on the line. At this point, the CEL was still gone and I knew the secondary air injection only occurs on a cold start so I thought I'd be able to pass smog and deal with the vacuum line later.

At this point, I went to get a smog test. The CEL light stayed off for the 10-minute drive to the smog check place. During the test, the CEL came on again but this time only P1236 is showing up.

When I got home, I called my Mercedes mechanic and they gave me a new piece of vacuum tube that I used to replace the old broken line. I thought that would resolve my problem but it doesn't seem to have made a difference.

I have been reading a lot of threads about these two trouble codes and the SC clutch not engaging, and I noticed a lot of people who were able to connect their SC clutch directly to 12V in order to test whether the magnetic clutch is functioning.

My problem: I have no idea how to do this test myself!


If you know how to test the SC clutch, please describe it in detail or direct me to a link that describes the procedure. For the life of me, I can't figure out where the wires connect to the SC assembly.

In addition, I noticed the air bypass valve that connects between the air filter box and the SC is staying open no matter the RPM. I have a feeling this is related to the SC clutch not engaging, but I can't find any useful information on how to diagnose or repair these problems.

If you can walk me through a set of troubleshooting steps, that would be really useful.

One more note: it seems the fan in the box with the K40 relay and other electronics has stopped working. So excessive heat buildup may be what caused these other problems. Because the fan isn't working, I'm leaning towards it being an electronic problem rather than a mechanical one.

Any advice is helpful! Thank you!
 

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Discussion Starter #2
Ok this is crazy... I am pretty sure I fixed it. I'll know for sure if it passes smog tomorrow.

How did I do it? First, I stumbled across a parts diagram that showed where the SC clutch connector is located. Mine was so badly caked in grease and dirt that I couldn't even see it.

Once I located the connector, I unplugged it and ran two wires directly to the battery to see if it would engage with an audible CLICK, as described elsewhere.

Much to my dismay, there was no CLICK and the clutch did not engage.

However, after plugging the SC clutch back in I decided to start the engine since it was definitely cold and I could test the vacuum line I replaced earlier.

Much to my surprise, the SC clutch engaged right away on a cold start! The CEL stayed off and I did a 15-minute test drive with no issues and more power than it did before. Hooray!

I hope this helps someone else, if not then mods feel free to delete this thread.

Guess it's not a joke when someone asks, "did you unplug it and plug it back in?"
 

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How did you run the 2 direct wires? From what I can see, the plug isn't big enough to fit 2 crocodile clips in there?


Did you get a mating connnector from somewhere? Strip insulation?
 

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Discussion Starter #4
Still broken...

It's broken again! So I do still need help, please.

How did you run the 2 direct wires? From what I can see, the plug isn't big enough to fit 2 crocodile clips in there?


Did you get a mating connnector from somewhere? Strip insulation?
I had a length of wire that is normally used to connect an inverter to a car battery. The end that connects to the inverter has two ring connectors that I was able to insert into the connector so each was touching one pin and they were not touching each other. Definitely a Jerry-rigged solution, and maybe it wasn't even making contact when I was touching the clip ends to the battery.

I still have the P1236 problem so any additional troubleshooting steps are welcome. I might try that again and have a friend help me so I know for sure it's making contact. Although the fact that it was working again last night tell me I'm maybe not too far from a solution...
 

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Premium Member 2005 SLK55 AMG; 2005 SLK350
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If you disconnect the bypass hose so that you can see the butterfly, the butterfly should snap into position as soon as you turn the key on. Does it seem to be doing this? If not, check the wiring in that area. There are some threads here an elsewhere about replacing a transistor on the ECU - that might or might not apply to you too.
 

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Discussion Starter #6
If you disconnect the bypass hose so that you can see the butterfly, the butterfly should snap into position as soon as you turn the key on. Does it seem to be doing this? If not, check the wiring in that area. There are some threads here an elsewhere about replacing a transistor on the ECU - that might or might not apply to you too.
Thank you for that tip!

I did as you instructed and the butterfly valve does indeed twitch with the key is turned without starting the engine. We did a cold start and the valve closed completely, which I believe is the correct behavior, right?

My theory is that the pressure created when the valve is closed is what was causing the SC clutch to spin, but the clutch was not engaging on its own. If that is the case, what's the next troubleshooting step? Measure the voltage coming from the ECU to the SC connector?
 

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Registered 2002 SLK320
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Hi,

Have you tried disassembly of the actual Connector plug at S/C Clutch ?

I often find that when you do this a wire with a manky green end just falls off the metal connector blade because it has fatigued over time and broken. Connector repair kits, (metal terminals and plastic parts) are normally available from MB for a few sheckles :)

HTH, Cheers Dave
 

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Registered 2002 SLK320
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