[mikevernon]

Potentialslkowner . PLEASE DONT REPLACE your k40 unit, or the cam position sensor or the crank position sensor or any other component simply because somebody on here suggests that it might fit your symptoms. Instead try to systematically check the problem in the same way as you would with any other car. There's no rocket science involved.

Does the engine fail to crank when turning the key, or does it crank but fail to fire up? They are two very different problems. One job of the K40 is to supply power to the fuel pump, and if the pump does not receive power than the K40 will not supply the signal to allow the engine to crank. So the first thing to test is that the pump is receiving a 12v supply when the key is turned to the ignition position.

 

[savcom]

I'd not had any starting or idling problems with my car nor any warning lights until this morning. Today I went to move her and nothing. Just a dashboard full of lights and clicks from some relays. No attempt to crank and after about 30 secs the radiator fan started going. Hmm.. Today was supposed to be a Garden Day. Silkie wasn't on the agenda.

Bonnet up, battery disconnected, K40 out and opened - all looked clean. No burning, no damp, no rust. However, looking under a magnifying glass some of the contacts looked fractured...

So after mounting the board carefully in a vice in my basement I went to work on all of the joints. Here you see the three joints above have been re-soldered.

I also sprayed the relay contacts with a non-residue contact cleaner and sprayed contact grease to the pins of the module before putting it back together - although there was no sign of any heat on the board.

When I reassembled the unit and put it back in the car I started her and she sprang into life - absolutely raring to go. I then turned her off, took the key out and repeated this starting and stopping five or six times to make sure it wasn't just a one off and after this let her idle for a while.

Then I noticed the BAS/ESP light didn't go off. I vaguely remembered seeing something about resetting this if the battery had been disconnected so, with the engine running I put my foot on the brake and steadily turned the wheel completely to the left, then completely to the right and then back to the middle. Hey presto - the lamp went out. :grin:

I then took her for a drive to re-charge the battery and make sure she was OK after a run. All well. In fact, it felt to me as if throttle response was a bit sharper and she seemed to gain speed with an eagerness I'd not noticed before, so perhaps the 30 mins without the battery had reset some of her driving parameters. >

Like many others, I'm very grateful for the tips on this thread and I hope the tip about resetting the BAS/ESP light above works for others. One final tip I'd share is that when soldering make sure your iron is hot enough and powerful enough. The amount of heat needed to properly solder these boards is rather more than many hobbyist irons can supply as these are meant for ICs and smaller tracks You should be using a 25-40W iron with a clean bit that is hot.

If you don't properly support the board and fully heat the joint and connecting pin thoroughly you will get a dry joint and you're likely to have problems six months to a year down the line as the car's vibration will take its toll on the joints. Proper soldered joints should be shiny and smooth. If they look dull and crystalline then you've got a dry joint. :nerd:

Drive safe.
Savcom

 

[Peter.s]

Hi Mike, Possibly I was not clear in my post. I was advocating further testing of the K40, not the replacement thereof. A solder joint can look perfect from the outside, and still be open circuit. Always test with ohmmeter!

 

[john hannon]

Went out in the 320 today, as usual she started first touch of the key despite sitting for 6 weeks.
Left her for two hours, put the key in, got the full array of lights, twisted further to start her and nothing, other than a relay clicking and the rad fan sounding like a 747 on the runway!!
Called the breakdown man, battery fine, no power to the starter motor, main fuses all ok, whilst he was doing this I remembered talk of the K40.
He checked the fuses above the K40 and whilst doing this the car started!!
Got her home, ran perfectly, started her 6 times since, no problem.
K40 sounds suspect to me.
I don't want to get stranded, new one being fitted on Friday.....am I wasting my money or being over cautious?

Regards all

John


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[jbanks15]

6 weeks and battery ok?
I really doubt it is ok, no offense. Get it really tested for cranking voltage, etc
who said it was ok? breakdown man? what did he test it with? his mind meld power?

 

[john hannon]

I'd done a 20 mile run, so battery had plenty of life, he tested it and said it was ok, he said he'd try his booster pack to see if it'd make a difference, it didn't [emoji34]


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[jbanks15]

John, how old is your battery anyway if I may ask?
The slk needs a great battery, not a 'good' one. 6 weeks a long time imho

 

[john hannon]

It's 18 months old.
We spend a lot of time in Spain, the SLKs kept in England, it often goes four to six weeks sat in the garage, I don't use a trickle charger, it's never let me down, first touch of the key and she starts
I really don't think it's the battery.



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[jbanks15]

maybe get a charger then? luck might run out

 

[john hannon]

Hi Jeff, funny sod that I am I don't like leaving electrical apparatus turned on if I can help it, I'd rather stick the charger on if there's a problem.
The local Indie are fitting a K40 on Friday, it might not be the culprit, but from reading this thread there's a fighting chance it is!!
Better than getting stranded


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[jbanks15]

Hi Jeff, funny sod that I am I don't like leaving electrical apparatus turned on if I can help it, I'd rather stick the charger on if there's a problem.
The local Indie are fitting a K40 on Friday, it might not be the culprit, but from reading this thread there's a fighting chance it is!!
Better than getting stranded


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I don't either so i charge it once a week during the day

http://www.slkworld.com/off-topic/3...f-topic/338105-ctek-mus-4-3-12-volt-fully-automatic-8-step-battery-charger.html

 

[john hannon]

New Engine Management Relay fitted by AutoElite in Hull this morning, excellent service as usual, I was on my way home at 8.30am
The part, genuine Mercedes cost £152.40 inc VAT and the labour cost was 0.5 hours at half the Mercedes rate.
They told me that mine was the second SLK this week to have one fitted



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[6sabre]

Thanks for the advice on here. I removed my K40 and checked joints/connections. Not my issue, but boosted my confidence to dig deeper ;-)

 

[JimtotheJams]

I thought i had finally hit the k40 wall a few weeks back, car decided to not start one morning, so took the volvo, got home from work, car started, ran fine again for another week then died completely.
This was just after changing the barrel and key as it had stopped turning, £94 later and some jiggery pokery making my own tool to remove the barrel cover and hey presto, still didn't bloody turn, so £160 later i'd bought a new steering lock fitted that and all was well.
So my thoughts on this was i may have disrupted the signal wire to the starter when i changed the steering lock, some awkward multi-metering later i found i was infact getting voltage to the starter, so moved on to the fuel-pump, yep voltage there too. so out with the k40.
Everything looked joyous and decided i wasn't wasting money if i didn't have to.
Whipped the starter motor out which was surprisingly easy compared to any other car i've had.
Bench tested it, Dead. £25 for a used one of ebay, fitted and job's a good un!
Always pays to have a wee poke about and see what else might be wrong as mentioned previously!

 

[cockneyrick]

Mine (as mentioned before) has had the intermittent fail on the K40 relay. But i have since narrowed it down.
On the top of the K40 are a series of plugs, as you look at it in situ, the LH plug (black 4 pin) which has only 1 green wire going into it is now my culprit.
When it entered "Overheat" mode, fan going full tilt but not starting, i wiggled the plugs & the fan died off & car started. I then waited till it did it again & noticed LH plug was the culprit.
I disconnected this plug & tried starting & hey ho, it went into overheat! Plugged it back in after using switch cleaner & all is good.

So there you have it, a temp fix is wiggle it while fans spinning, then clean contacts for this single green wire to sort a permanent fix :wink:

 

[mge_1]

i'll have to check my k40 relay tomorrow. just got the P1235 and P1236 codes today.

I have found that this relay board is responsible for 2 failures for my mercedes



1. Check engine light error codes -

p1235, p1236, p1420, p1525 (Rightmost circle in attached image)​

2. Failure to crank - intermittently (Center circle in attached image)​

I am including the close up from another post showing the bad soldering points. Make sure that you solder all 3​

I know this is an old post. Sorry.

Got all 4 codes on my 99 (p1235, p1236, p1420, p1525)

Checking K40 relay module tomorrow... This is a good pic to reference. Thanks

 

[samzin]

Bazzle,

Just wanted to say a big thank you. I've been having problems with my SLK almost since new. Well really just one problem, an intermittent failure to crank. Most times it's fine. Then randomly, it won't fire.

Thinking I had a power drain of some kind, I installed a special device that shuts off the battery if it senses a voltage drop over a sustained period. That has really been helpful. Every once in a while that device will activate, and I'm happy to realize that I would have had another dead battery.

But I'm hoping this relay replacement will obviate the need for the device, except maybe as a backup. We'll see.

Hello there,
I have the exact same problem. Can you let me know how you fixed it?

 

[jbanks15]

Hello there,
I have the exact same problem. Can you let me know how you fixed it?

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[mge_1]

I know this is an old post. Sorry.

Got all 4 codes on my 99 (p1235, p1236, p1420, p1525)

Checking K40 relay module tomorrow... This is a good pic to reference. Thanks

My K40 board looks good to me. All of my error codes are active. This sucks lol

 

[First_timer]

trouble starting my 2000 SLK 230

Hey Bazzle thanks for the post.
I had trouble starting my 2000 SLK 230 yesterday. When I turned the key to start, I got all the idiot lights and the radiator cooling fan was running in high speed. The engine was warm but not hot. The car did not crank the starting motor at all. I checked and it did not set the check engine lightl. A couple hours later I tryed to start the car and it started right up normally. I thought that the problem might be in the ignition switch or the starting motor/soleniod.
I found your post and decided that I better check it to see if that was the problem. Did you figure out what the cold solder joint controlled?
I took pictures of mine and they look identical to the pics that you posted. SO this problem is not unique to your car. If that is not the problem I will pull the starter/soleniod and then check it. There was no noise except for the radiator fan when I tryed to start it.

I seem to have the same problem. It all started after I received a shock from starting the car. It left a burn mark the size of a pin on my fingernail. Ever since, I have had the issue with the 230 not starting right away and or cutting off completely after putting it in gear.


I am going to look at the K40 to see if there are any obvious cracks to solder.