Our Cars : One SLK, no charge

Richard Kilpatrick’s SLK decides that wet weather, rubbish tyres and motorways are far too great a risk, and becomes a little lethargic…

Not long after getting the SLK, and throwing back into work despite a barely functional leg, a quick blast up the M1 on one of the few sunny days during this ‘summers’ constant rain seemed like just the ticket. Getting to experience the little car’s performance was a useful bonding exercise, but it seemed a little unpredictable.

Overtaking which had been a joy, became worrying. And when it felt like the car needed all the throttle just to keep up with motorway traffic, it was clear something was not right. Given that Leicestershire had clearly decided an hour of sun was too much, and opened the heavens on the little yellow car, I assumed water must be the problem. But solving it would take, on and off, most of four months. Not, I hasten to add, due to any inherent difficulty, but prioritising costs and trying to balance repairs with the inevitability that the car was a toy, an optional thing, and the Chrysler was doing all the hard work.

Research began with checking how the supercharger works. On this era of SLK, the blower is engaged with an electromagnetic clutch, and then boost is controlled with a bypass flap – so two elements can conspire here. The engine management uses the usual mix of sensors and computers, with the critical elements on the SLK being the crank sensor, cam sensor and cam adjuster solenoid.

The SLK’s electromagnetic clutch and supercharger pulley.

Given the same job to do again, I’d have it sorted in no time, but that’s no fun. So here’s where the route took me:

First, there was no evidence of the supercharger engaging. Except when it did, of course, which seemed mostly random – sometimes, it was working when the car was warm. Sometimes it wasn’t. Open the bonnet and move the throttle cable, and it should cut in around 2,000rpm with a distinctive noise and of course, visual confirmation that the pulley has engaged.

Most forums point you at a substantial relay box in the ECU compartment, the ‘K40” relay. It suffers from dry joints and can be a source of woe for many users – if nothing else, relays for things like the fuel injection system live in here, and are hidden from view when doing a normal ‘Blown Relay” check.  Mine was okay, but whilst investigating I found that a small blower fan had burnt out, so that needed replacing.

The ECU box showing the K40 module (with fuses) and 38-pin diagnostic port.

The blower is £133 from Mercedes, for the module. The motor is £7 from eBay. Obviously, the motor was replaced, a 10 minute job made into 40 due to the bulky set of computers and looms that need to be moved.

Most research implicated the camshaft magnet as the likely culprit. Sure enough, when I checked my car I found that the solenoid was leaking oil down the front of the head, and more critically out through the electrical connector, onto the pins and into the loom. Early cars have the connector pointing up and it’s less of an issue than it is for the later models, for which Mercedes-Benz has produced an additional loom to protect the wiring from oil contamination.

The Camshaft Solenoid/Actuator is an electromagnet that pulls a collar to adjust the cam timing. And it can leak, externally and internally.

In the worst cases, the oil in the loom can damage the ECU. Again, my early car was merely a little grubby, but enough for me to be confident that replacing this adjuster would fix the issue of no boost – if not, then at least it wouldn’t have a leak.

At £64 this seemed reasonable, so I bought that, and the correct black sealant, and the plastic part of the wiring connector (a pin bushing in MB parlance) to repair a broken clip. The new part fitted, the car was tested, and the supercharger worked.

With the images taken, it’s probably easier to flick through the gallery to see the oil contamination on the plugs, and repairs.

Except, the next time I drove it… it stopped after a while.

At this point, I did what everyone running a modern car should do, and bought a code reader. I found an Autel MaxiScan for £40 or so on Amazon, but the range of OBD-II scanners is quite wide and prices range from £20 to £2000 – the Autel has engine codes for Mercedes, and with a 38-pin adaptor I was on my way. For more sophisticated and body functions, the readers are far more expensive – these are just for engine codes on most cars.

A code reader is almost essential for modern cars – and thankfully, no more expensive than a decent set of screwdrivers.

A few codes remained from past repairs – the MAF in particular caught my eye. Upon removing the pipework, I found an unmarked third-party clone MAF, rather than the Bosch or Mercedes part, and these are generally a cause of poor running. However, at £180 (or £230 for the genuine one) there was no rush to replace it – the inlet and breather pipes were given a clean and the car checked over a few days, having cleared all the codes.

One came back – P0341, camshaft sensor. This can be thrown by a faulty adjuster magnet, as well as wiring, so after checking the loom condition I took the plunge and bought a Febi-Bilstein part from eBay.

The old sensor had a date code of 1998, so was clearly the original or at least, a secondhand one fitted during the car’s previous ownership, but at £133 for a new one the Febi unit was tempting for only £39. When it arrived, it looked like an exact copy of the Mercedes one, except the logo had been ground off.

In fact, it was so exact, even the moulding lines and texture of plastic matched.

And the old sensor has a Febi logo…

Turns out that various OEMs supplying German manufacturers quite often grind off the genuine part number and manufacturer logos. Whether there are quality control or other factors, I don’t know, but what I got in the box was to all intents and purposes the same item I’d have paid almost £100 more for from a dealer.

The good news is that with that part replaced, the car works exactly as it should. Lively, smoother and generally ‘better’. Following the vague techniques of ‘suck it and see’ without the code reader, I’d probably have changed the sensor anyway, but would also have ended up going down all manner of dead ends to get there.

The code reader isn’t an absolute indication, of course. It narrows down the process. For the cost, it proves to be a very useful tool regardless. And the SLK was repaired for under £200, when a couple of trips to a specialist or dealer would probably have resulted in that cost on diagnostics alone…

The original sensor has a Febi logo on it. All the moulding lines and details are a perfect match; the OEM unit for £100 less appears to be the exact same component with the part numbers ground off.
Richard Kilpatrick
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  1. Good story and excellent pics. Well done for fixing your car quite easily and keeping ££££ away from Benz dealers.

  2. Oh, I’m entirely happy with this one DaveH – it’s a 14 year old car and I cured a running problem with £120 or so of parts – I also found that many irritating little bits are quite cheap from Mercedes – for example my Chrysler Voyager’s lower windscreen seal was part of the scuttle panel moulding and £140+ – it had shrunk and kept on escaping from the car in crosswinds.

    The SLK’s had also shrunk but was a whole £5. as a separate part which also was a better design which wraps around the base of the windscreen glass, so is always retained. If I wanted to replace the plastic scuttle (sun fading, etc) it’s a whopping £35. And there are two rear bumper rubber seals – one each side – which had gone very strange, rusting inside the metal sprung section and swelling up with this terrifying grey crystalline goo that erupted out when I poked one. Took a while to work out the correct part on the EPC, but again, a fiver each to replace.

    And I got lucky with the wheels, too. The 17″ Boss wheels didn’t look bad but they were too small (width and overall diameter) for the car, and the wrong offset. I found a set of original, average condition (corroded centres, but not the worst I’ve seen) Pictor alloys on eBay with brand new Avons on the back, got ’em for the cost of the tyres pretty much, and sold my Boss wheels for the cost of the replacements.

    So the SLK is working fine – and none of these issues were a show-stopper – it got sluggish, but it didn’t stop working; I chose not to drive it because I knew it was only a simple fix that was otherwise affecting the car’s performance and fuel economy, and figured “a stitch in time” applies here.

    Compared to the XM…

  3. Good story of progress and a successful outcome. Good to hear that an identical sensor was sourced at a much cheaper cost and restored the car’s performance. Like the old adage says “why spend £1 when 50p will do the job”

  4. richard,

    are congratulations in order for you coming out of the closet ?

    or make a mistake & buy the custard cabriolet from a monochrome ad in the trader ?

  5. Nope, I’d never have been in the closet in the first place, and I like yellow. I wanted a cheerful car, and generally don’t base my car choices on sexuality or vice-versa 🙂

  6. Sounds like what a certain French vehicle lighting manufacturer would do – charge the car manufacturer for tooling bespoke headlamps, etc, with their logo moulded in and then sell the very same parts via their own aftermarket division with the logos crudely ground off…

  7. hi 1998 slk 230 my sc stays on all the time could changing my rh camshaft timing solenoid help it to work properly its code is p0808 and p0805 and p1525


  8. hi 1998 slk 230 my sc stays on all the time could changing my rh camshaft timing solenoid help it to work properly its code is p0808 and p0805 and p1525 id love an answer please

  9. Diagnosing a fault over a single comment is going to be sketchy at best, but Google suggests P0808 is Clutch Sensor – P0805 is supercharger bypass issue, and P1525 is indicating a fault with the solenoid or sensor. P0808 would probably apply if it’s a manual car, otherwise I’m stumped on that one, but P1525 and P0805 I would go in this order.

    ECU box out, check K40 relay for dry joints, solder lose/cracks or burned connections.

    Fit working or repaired K40, check. If fault persists or no fault is found on the K40, check the Camshaft solenoid (not sensor) for oil in the connection. That’s the one up front under the plastic cover. Look for oil loss around it, and in the connection and loom itself. If no oil loss, and the car sounds okay, then check operation of the bypass on the supercharger.

    If your car is manual, and that P0808 code does indeed relate to the clutch sensor, that would explain why your supercharger is always engaged and would also explain the P0805 code, which generally seems to go with boost issues – the car should, in idle, disengage the supercharger and engage at 2,000; in gear/drive, it will engage at lower engine speeds to provide boost. The bypass is always open when in N/P/neutral – or should be.

    Finally later models don’t have a clutch, so if your car has been fitted with a later supercharger/replacement engine it may not be intended to disengage, though I’m guessing you’ve eliminated that by looking at the supercharger pulley and identifying that it’s the one with electromagnetic clutch.

  10. hi my s/c clutch use to stay on all the time now it does,nt come on at all, it came on if you feed direct a direct current to it but that wont even work now,i swapped the maf with my friends car same as mine and the clutch worked 2000rpm came on great took it for a short spin and it stopped again back to square one. thinking ive knackered my mates maf put it back on is car and it worked fine.so i bought a maf fitted it started working again after a short spin packed up again im baffled

  11. Again, with the caveat that diagnosing a car over the internet with a couple of comments on a post is a somewhat inexact science, if swapping the MAF restored correct clutch operation for a time and then it failed, it’s probably the case that your breather system is clogged and crankcase vapour is filling your intake with oil. There are two sides to the crankcase ventilation system (CCV/PCV) – you can get to the main set of hoses easily, the oil separator on the top of the engine and pipework going to the inlet, the obvious breather hose on the rocker cover.

    The set that is going to be the most likely cause of a fouled MAF, however, is much more involved task. The inlet manifold has to come off, and two brass nozzles in the ports for the inlet manifold on the cylinder head have to be cleaned. The PCV valve and associated pipework would also need replacement and cleaning.

    There’s more confidence in this if you cleaned the MAFs between swapping them.

    You’ve checked the K40 relay for dry joints/cracked solder/bad contacts? There’s also a transistor that fails in the ECU, though swapping the MAF and having the car work should eliminate that.

    How much oil is present in the intake and intercooler pipework when you remove the MAF?

  12. hi all the pipes are free and clean, air filter clean, oil seperater clean, put new plug seals in tried my mates k40 same as mine no joy. just put everything back together as i speak now

  13. PROBLEM WITH THE K40 RELAY (SLK 230 Kompressor)
    In yor article I can read:

    “Mine was okay, but whilst investigating I found that a small blower fan had burnt out, so that needed replacing”.

    My question is: Where can I find the blower fan in the car? or What is its technical name in order to buy one?

    I would appreciate you help. Thank you very much.

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