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Discussion Starter · #1 ·
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Circuit component Hardware programmer Electronic engineering Gadget Electronic component

Circuit component Hardware programmer Passive circuit component Gadget Electronic engineering

Hi all,

Hope I don't get in trouble for starting a new thread - the big K40 Relay thread seems to be closed for new replies.

My 2001 SLK230 has always run fantastically, annual services, no bother. Last week I drove 200 miles north from London and when I got back, after turning off the engine and getting out the car, I noticed the fan was still running. I thought it'd go off eventually, and it did - but only because it had drained the battery.

I couldn't recharge the battery so I had to get a new one, but as soon as I connect it the fan starts up again. I can start the engine, drive it fine, but the fan just runs on at high speed unless I disconnect the battery.

I've seen lot of people with the problem, but for the life of me I can't see any specific solution except one - replacing or re-soldering the K40 relay. I've taken the K40 out and had a look, but before buying one off ebay thought I'd try re-soldering. Thing is, it doesn't actually look (to me) like it needs one. I have never soldered a thing in my life so I might be blind to the damage, but those pins in the top right look fine to me - certainly not like the examples of burnt out / damaged pins I've seen on here.

Can somebody take a look at the images and let me know whether this does actually look like it needs soldering? I might try anyway, but given the age of the thing I can't really afford to throw away the money on a replacement unless there's at least a very good chance that this is what's causing the problem.

Thank you all,
 

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Discussion Starter · #4 ·
I can see cracking around some of the terminals. What have you got to lose by re-soldering?
I've never soldered before, so I just wanted to be sure that it was an avenue worth exploring before I do. If you think it is then I'll watch some videos and get a solder kit from Amazon.

Thanks for your quick help - really appreciate it. Any practical tips for the solder?
 

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Discussion Starter · #5 ·
Yup, several cracked joints. Re-flowing may cure the issue......
If a component has failed then a new K40 from MB is the best solution. Too many poorly manufactured copies about.
Yeah I've seen some on eBay but not sure if they're a good idea - seem to fluctuate quite wildly in price...
 

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Discussion Starter · #17 ·
Thanks for all the comments - I really appreciate it

I actually put the relay back in cause I was kinda hoping that just taking it out and putting it back would magically change something - it didn't. But I'll take it back out tomorrow and take a picture of the part number.

I've contacted a local smart phone repair place and sent them the photos. They're a bit unsure, but if they'll do it I'll take it. If not, I've got a soldering iron on the way from Amazon so I'll give it a blast myself.

I'll happily take a video and put it up here for you to check out. At the moment I can just reiterate that the fan starts literally as soon as I connect the negative clamp to the battery. It's very cold and wet in Nottingham at the moment, so no reason for it to be thinking it's hot. I did put the engine on and my obd scanner told me the engine was 76 which is fine, right? But anyway, the fan's coming on even if the engine is off, so that shouldn't be it.

The only other info that might help the mystery is that when I drove back from London on Monday, the car had been left in a Heathrow car park for 16 days. I got a meet and greet service and when they brought it to me when I returned, they told me that the battery had gone flat and they'd recharged it. The radio didn't function anymore but I didn't really think anything more about it. The drive back was about 3 hours, and the rain was really really heavy all the way. When it's like that I need to have the windows open and the hot air blower on full blast to clear the windscreen (got no a/c on this model). So the blower was on all the way back.

When I got back home and turned the engine off, I noticed the fan running on but I checked online and some people seemed to think it's was a fairly common event that should stop after 10 mins or so. Mine went on for an hour. I thought it had sorted itself out but realised the next day when I tried to start it that it had drained the battery. I'm kind of lodged in a small driveway so didn't have an easy way of jump starting the battery. I've got one of those portable battery boosters but the clamps are so so rubbish that I couldn't get a good connection. On the couple of occasions I could, the fan started immediately and the clamps disconnected before I could make it round to turn the key.

So I got a new battery on Sunday. Bigger, as it happens. I connected it and as I say, the fan just starts immediately. This is a week later, and the weather has been awful. When I looked around the fan I noticed that a decent amount of water had pooled on the tray beneath the engine and hadn't drained. I thought maybe the pool of water had something to do with the fan coming on, but I drained it at using a sponge and it made no difference. I was actually going to ask on here and the wisdom of drilling some drainage holes in the tray....

Anyway, I kept looking for potential solutions and I came across this k40 relay idea. I took it out and to me it looks in really good condition ( the full unit, if not the specific soldered pins).

So that brings us up to date and is everything I can think of. I'll happily try anything else you might suggest, but I'm in a race against the jaw cause the MOT is due on December 4th!
 

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Discussion Starter · #22 ·
Is the fan control module the block in the bottom right I hover over in the video?
That light is on because there has been a power interrupt (disconnected battery) - you need to reset the Steering Angle Sensor by turning the Steering Wheel ~ Left Full Lock, Right Full Lock and Return To Centre ~ light should then extinguish

Personally, I'd disconnect the fan control module and leave the K40 alone for now
Is the fan control module the block in the bottom right I hover over in the video?

Will the car be safe to drive if the module's disconnected?
 

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Discussion Starter · #24 ·
Yes that I've just disconnected mine by squeezing the tabs together and pulling the connector off, it took me 10 seconds but if I was planning on leaving it like that I'd probs put it in a bag or something to protect it, or go looking for the 50A fuse View attachment 621373 As I see it... The fan won't come on, but late-November UK it won't be needed anyway - probably/maybe 🙃 The fan is designed to come on at 100c so unless you have a severe coolant loss or we get a sudden heatwave and you happen to find yourself sat in heavy traffic for an extended period it should be fine as a (very) short-term solution. Maybe there's an issue with the controller, maybe there's an issue with the wiring, maybe there's an issue with the K40 after all but at least you won't be draining the battery every time you connect it up.
My solder kit arrived and it came with a multimeter, so I tested the pins on the relay board and it seems to be telling me that there is good connection of those spins.... So I'll put it back and try disconnecting the fan module as you've suggested - I'll report back. btw, somebody has commented on my video to "replace the temperature sensor and clean the contacts of the fan unit" Do you think there could be anything in that? I've had the engine coolant light on my dash for a while but the local MB specialist said it was most likely just the sensor. I bought a replacement but didn't fit it because it sounded difficult (involved draining a tank, if I'm remembering correctly), and because it didn't seem to be having any negative effect at the time. Does anyone think that could be to blame?
 

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Discussion Starter · #25 ·
Ok - I've put the relay unit back and unplugged the fan module. As you say, I can now connect the battery, the engine starts fine, but obviously the fan isn't working. I plugged the module back in and it immediately starts again.

I looked again at the coolant temperature sensor replacement and I must've been looking at something else last time cause it was incredibly simple - I've replaced that sensor now. No change, but then it wasn't likely to be a sensor problem if it happens before the engine is started, right? Or am I misunderstanding how these things work?
 

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Discussion Starter · #29 ·
At least the float looks it's actually still floating in something - Yes? :unsure:
Err.. I'm not sure it was actually. I topped it up, and it took about a litre to get it looking like in your picture, floating in the coolant. I left it for a bit, and.....

the fan isn't coming on when I connect the battery! I started the engine, left it running for a bit, and when I turn it off the fan doesn't keep going. Does that make sense? Could there be a leak in the coolant tank that it's somehow detecting and running the fan to compensate?

I've unplugged the battery again for now as I didn't want it just starting up again overnight, but fingers crossed we might have stumbled on an answer or if it's totally illogical then something else might have just fell into place somehow.

I'll let you know....
 

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Discussion Starter · #37 ·
Hi everyone,

This orning I attached the battery and again, the fan didn't come on. Amazing! I fixed the battery clamps, started the engine, and went for a drive. Everything seemed fine - the engine coolant light was gone, there were no codes on my scanner, and the car drove great. I drive it for about an hour in total, with stops, and then went to the supermarket.

I went to the supermarket, and next door there was an in-'n'-out MOT centre with a sign up saying they had slots available. So I thought, why not?!

I booked it in for later in the day, left it with them, walked home and crossed my fingers...

Unfortunately, it was too good to be true. I got a call from the centre not long ago. When they drove it in, before they even started the MOT, they'd found engine coolant had leaked out. They don't know where from. They've asked if I want them to do a diagnostic to find out where, and I've agreed, but that's another 60 quid on top of the 45 for the MOT that I'm worried might not be well used. I don't know what they'll find, but I'm not hopeful. For context, the value of the car is about 600 quid max. I could get about 320 in scrap. So even though I really want to get it through its MOT I've already stacked up about 200 quid this week and I can't really afford much more unless I know it'll get through the MOT. I'm dreading the cost just to fix the coolant leak, never mind the fan module. I haven't mentioned it here, but there is a persistent problem with the rear lights too - I've replaced individual bulbs and both full light clusters, but lights on both sides work for a bit, then stop, then work again, then stop. I took it to an MB specialist who checked the connections and ran diagnostics but they couldn't find a reason. So that was another 120 down the pan.

Anyway, just filling you all in cause you've been loads of help and I'm grateful for you all taking an interest. We'll see what happens with this coolant tomorrow....
 

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Discussion Starter · #41 ·
Not being funny just asking, coolant leak is or is not a MOT failure?
Knowing issue with pipe, expansion tank, etc....is not making the car "dangerous" is it?
I haven't seen it for myself yet, they just said "we're not able to run the car". - I asked if that meant the engine doesn't start, and they said it starts, but essentially they don't want to risk running the car without coolant (I'm assuming that means it's all gone. Considering I filled it up 24 hours earlier, that's not a good sign...
 

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Discussion Starter · #43 ·
it is also possible that you have a head gasket issue though bearing in mind how quick the coolant has dropped.
Bingo - they've just called and said the head gasket needs replacing. Not something they can do so I'll have to bring it back.

In terms of putting it through its MOT anyway - I'd love to, but because of the problem with the lights, I'm not confident that even with a new head gasket and heater matrix it would pass. If I was, I'd consider it. The outlay just seems too steep without reward. Plus, I don't have a garage to do the work in myself, so even if I looked at it as a project (which I'd love to), I can't realistically be doing that work on a steep cramped driveway in the rain.

I'm guessing the cost to replace head gasket would be hundreds (thousands?), so it's unfortunately a non-starter (pardon the pun).

Thanks again everyone. If there's anybody local to Nottingham that fancies taking it off my hands, let me know!
 

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Discussion Starter · #48 ·
I picked it up and it drove home beautifully. Such a shame, but it's getting picked up for scrap tomorrow. They're giving me 410 for it so all in all I shouldn't be too disheartened. If I had a garage to put it in I'd keep it and learn to fix it as a project, but alas....

Thanks again for all your help. If I ever get another SLK, I'm sure I'll be back here!
 
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