Car wont start, Potential K40 Issue - Mercedes Benz SLK Forum

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#1 Old 12-29-2017
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Car wont start, Potential K40 Issue

I know there are many threads on this subject, but they might not all have the same root cause.

Drove my car to work yesterday no problems. I was leaving work and I attempted to unlock the car with my keyfob and it did not work. I immediately knew there was an issue. When I first got the car almost a a year ago, I had this problem intermittently a few times, but have not had issues for atleast 6 months. I have full power, lights come on, radio comes on, etc. When I attempted to start the car, there was no crank, not even a starter click, but I can hear the fan come on. NO Codes, the only error code is the Start Error that appears in the odometer display.

In the past, I have literally tapped on the control box that holds the k40, moved some wires around and got it to work.

I have read that it can be anything from the K40, DAS, PSE, to the fuel filter. I don't want to drop hundreds on something I may not need. Several months ago I had loss of power when going down the highway, but it came back within a few seconds and I have not had the issue since. I have read that this a symptom of a failing K40.

K40 looks pristine and looks to be the original. No apparent solder damage.


Symptoms:
No Start
Start Error Message
Keyfob stopped working.
Tow MODE Flashers on when opening door.



I also tried to resync the keyfob by pressing unlock twice and turning the key to ignition for 30 seconds. No Luck

My AAA guy came and checked the battery. Perfect 12v, so I doubt that is the issue. Also battery is less then 1 year old. He agreed with me on that is probably the relay. He also said to check on some sensors that have to do with the keys ignition, but I really don't know specifically what he was speaking of.


One of my coworkers(an aeromechanic) said that her husbands Ford keyfob got messed up a few times because of something magnetic in his pocket. I always carry a magnetic money clip, but I just don't know if this is it.
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#2 Old 12-29-2017
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Ok, let's start free and easy (ish).

Do you use the car regularly?
If not, do you have it on trickle charge (never boost)?

SLK battery can discharge on 3 weeks standing.
Also, if it is cold starting often that takes a lot of strain.
Cold mornings need more juice to get it going.

If you use the roof a lot and don't have the engine running
that's a major battery drain.

Now for an oft missed point.
Any signs of parking damage on the rear end.
This can activate the anti tow (and you say that's flashing).


Next...PSE
Have a look in the boot/trunk.
Any signs of damp?

Worth taking the covers off and checking the foam surrounding the pse.
Signs of damp, or staining?
Remove and dismantle. Let it dry (Don't overheat, but gentle hairdryer use can help).
Check the motherboard. Any manky solder?
You can get away with drying and resoldering, but be gentle.

K40 now.
You mention that this happened before and jiggling things fixed it.
Although the solder can look good it is prone to micro cracking.
Resoldering can be done. Again, be gentle and take care.

Many diys on the above in the 170 diy section.
See link in my signature below (under photo) if you aren't sure where.

Let us know what works as it helps the next victim.

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#3 Old 12-29-2017
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Meant to add.

I doubt the magnet theory.
It's possible but I work around magnets all day (keys in pocket)
& not had an issue.

Not totally writing it off, but doubtful.

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#4 Old 12-29-2017
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Quote:
Originally Posted by Avel Du View Post
Ok, let's start free and easy (ish).

Do you use the car regularly?
If not, do you have it on trickle charge (never boost)?

SLK battery can discharge on 3 weeks standing.
Also, if it is cold starting often that takes a lot of strain.
Cold mornings need more juice to get it going.

If you use the roof a lot and don't have the engine running
that's a major battery drain.

Now for an oft missed point.
Any signs of parking damage on the rear end.
This can activate the anti tow (and you say that's flashing).


Next...PSE
Have a look in the boot/trunk.
Any signs of damp?

Worth taking the covers off and checking the foam surrounding the pse.
Signs of damp, or staining?
Remove and dismantle. Let it dry (Don't overheat, but gentle hairdryer use can help).
Check the motherboard. Any manky solder?
You can get away with drying and resoldering, but be gentle.

K40 now.
You mention that this happened before and jiggling things fixed it.
Although the solder can look good it is prone to micro cracking.
Resoldering can be done. Again, be gentle and take care.

Many diys on the above in the 170 diy section.
See link in my signature below (under photo) if you aren't sure where.

Let us know what works as it helps the next victim.
1. Drive the car everyday
2. I think i accidently hit the tow button, and it just wont turn off.
3.Rear Bumper trim is barely cracked probably from a fender bender before I owned it. FM radio doesn't get all the stations, might be related.
4.Trunk Carpet is dry, spare tire area seemed pretty dry too, but it has been very cold and wet weather wise. Have Yet to check actually PSE
5.K40 is my main culprit, It does have a few scratches when the screwdriver accidentally hit when taking it out of the box. Solder looks okay though.

It is currently parked at my job and I desperately need to get it home pretty soon. I called to get it towed home but AAA never sent a tow truck, which is what I initially asked for, they sent light guy instead.
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#5 Old 12-29-2017
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We've had lots of folk saying the K40 looked fine.

Problem with micro crack is you don't see them with the naked eye.
Same folk have often tried the resolder and got it working again.

Is the anti tow switch stuck?
Might need some electrical spray to get any gunk free.

I'll give the R170 Bat signal a go:

@Mn car guy
@efair
@stu32
@savcom
@Turdo2
@Padgett
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#6 Old 12-29-2017
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Quote:
Originally Posted by Avel Du View Post
We've had lots of folk saying the K40 looked fine.

Problem with micro crack is you don't see them with the naked eye.
Same folk have often tried the resolder and got it working again.

Is the anti tow switch stuck?
Might need some electrical spray to get any gunk free.

I'll give the R170 Bat signal a go:

@Mn car guy
@efair
@stu32
@savcom
@Turdo2
@Padgett
Yeah I am leaning towards a K40 issue, but haven't ruled out the PSE. Even though I have not checked the PSE, I don't think its that being that the trunk is always dry and I believe my rear drains to be flowing fine. My Front right drain got clogged and it killed my blower resistor/regulator. Isn't the air vent that the water leaks into is in close proximity to the K40 Control box? LHD US SLK


See what everyone else has to say.

I understand that K40 controls the fuel pump and load etc, so I understand why that would prevent a start, but why would the PSE prevent starting the car? I thought it controlled locks, dimer lights and such.

Anti tow switch/button only presses up, no down. Is it supposed to be both ways?
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#7 Old 12-29-2017
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Can the check the fuse and bridge the relay for the Engine Control and see what happens.
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#8 Old 12-29-2017
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Quote:
Originally Posted by Padgett View Post
Can the check the fuse and bridge the relay for the Engine Control and see what happens.
#3 on my car has a 15amp(blue) instead of a 20amp(yellow)

How would I go about bridging the relay? Is that directly running power to the relay? What equipment would I need to do this.

EDIT: 15 amp vs 20 is 230 vs 320.
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#9 Old 12-30-2017
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The previous owner had this issue. Apparently Mercedes thought it was a Keyfob/immobilizer issue and they replaced the fob and programmed. The report states they checked the K40 and said everything was running, but he insisted that they keep the car for a few more days so that they could see that it intermittently would not start. Maybe the whole time it was the K40 having issues.

Would dead batteries in the fob prevent the transponder from getting its signal?
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#10 Old 12-30-2017
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Friend of mine had this problem with a 2003 SLK320. The symptoms are exactly the same and the fix was to clean fuses 35 and 37 in the under bonnet fuse box. That's all he did and it still works ok now about a year on.
I think this is to do with the power to the DAS and as it appears the car is not recognising you key fob it is a good starting point.
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#11 Old 12-30-2017
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Quote:
Originally Posted by stu32 View Post
Friend of mine had this problem with a 2003 SLK320. The symptoms are exactly the same and the fix was to clean fuses 35 and 37 in the under bonnet fuse box. That's all he did and it still works ok now about a year on.
I think this is to do with the power to the DAS and as it appears the car is not recognising you key fob it is a good starting point.


Gonna go check those fuses when I get to work. Also will reset battery too. Gonna get new batteries for the fob as well.

If this doesn't work, then on to the DAS Module/Immobilizer ring and K40. Will let you all know results when I get home tonight.
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#12 Old 12-31-2017
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Well what do you know. I got to work and pulled the fuses. I cleaned one of them and I happened to have a fresh 5A fuse which I replaced. Fuses 35 and 37. My keyfob immediately worked. I then pulled my negative terminal battery and left it unplugged while I was working. When I got off I reconnected it and cranked the car first try. Everything started up normally, actually the start and idle were extremely clean, more so then usual. I let the car warm up and left.

About half way home, I lost throttle for about 2 seconds and it came right back. No issues since then. I still think my K40 might be intermittently working but I drove the car home perfectly other then that.

How might I prevent this in the future?

My alarm also went off a few times, I turned it off with the keyfob, haven't heard it since.
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#13 Old 12-31-2017
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Quote:
Originally Posted by stu32 View Post
Friend of mine had this problem with a 2003 SLK320. The symptoms are exactly the same and the fix was to clean fuses 35 and 37 in the under bonnet fuse box. That's all he did and it still works ok now about a year on.
I think this is to do with the power to the DAS and as it appears the car is not recognising you key fob it is a good starting point.
Yes yes yes. This happened to me once 3 years ago and this was the exact fix. I wouldn't swear on a stack of bibles but I think that K40 issues are generally on pre-facelift cars.
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#14 Old 01-01-2018
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Having had the battery disconnected for a long period it could help if you do a re-set on things like throttle position, gearbox adaption and steering angle.
The K40 can still cause problems but it won't break the bank if you have to buy a new one.
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