1997 SLK 230 Kompressor

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mercedade
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1997 SLK 230 Kompressor

Post by mercedade » Thu Aug 20, 2020 8:50 am

A couple of weeks ago, I picked up a second car. This is quite an unusual thing for me, but the combination of decent bodywork and novelty of folding roof, and being surprised at how cheap they are, had me hooked in.

Image

It's got a few issues that need to be resolved, so I'm spending quite a lot of time reading and learning on various MB fora.
Adrian
1998 Rover Mk2 820 Vitesse Hatchback (white gold)
Gone...1998 Rover Mk2 820 Vitesse Hatchback (green)
Gone...1992 Rover Mk1 827 SLi Manual (green)
Gone...1998 Rover Mk2 825 Diesel (green)

mercedade
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Re: 1997 SLK 230 Kompressor

Post by mercedade » Thu Aug 20, 2020 9:01 am

First and foremost - the supercharger.....doesn't.

This seems to be a fault with many potential causes. The supercharger is driven by a pulley on an ECU-controlled electromagnetic clutch which engages at around 2000rpm. There are many reasons why the ECU may not engage the clutch, and finding them is something of a battle!

MAF - without getting a clean MAF readings, the ECU will refuse to play ball. There seemed to be quite a lot of oil around when I pulled the air intake system apart, but after I cleaned the MAF with contact cleaner, I managed to get the following readings which look pretty sensible to me

ImageImageImageImage

The Merc is, like the Vitesse, too old for ODB2. However, it's possible with a cheap adaptor, to connect a standard bluetooth ODB2 module to the 38 pin Mercedes connector under the bonnet.
Adrian
1998 Rover Mk2 820 Vitesse Hatchback (white gold)
Gone...1998 Rover Mk2 820 Vitesse Hatchback (green)
Gone...1992 Rover Mk1 827 SLi Manual (green)
Gone...1998 Rover Mk2 825 Diesel (green)

mercedade
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Re: 1997 SLK 230 Kompressor

Post by mercedade » Thu Aug 20, 2020 9:09 am

So I'm pretty confident that the MAF is OK actually.

It could be the magical K40 relay module (what is it about Mercs? Each model seems to have its own Achilles heel relay - for the W124 it was the fuel pump relay and the 'OVP' relay), but I would expect some other running problems, and I don't have any. The car itself runs well and pulls very quickly until about 40mph. What it has is a deadzone above that, which should be plugged by the supercharger. I might get it out today and check soldered connections, but I'm not confident there.

There's also talk of replacing a MOSFET on the ECU, which sounds eminently more achievable. I've just got to find someone with decent soldering skills!
Adrian
1998 Rover Mk2 820 Vitesse Hatchback (white gold)
Gone...1998 Rover Mk2 820 Vitesse Hatchback (green)
Gone...1992 Rover Mk1 827 SLi Manual (green)
Gone...1998 Rover Mk2 825 Diesel (green)

mercedade
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Re: 1997 SLK 230 Kompressor

Post by mercedade » Thu Aug 20, 2020 9:13 am

ImageOne last thing - it runs rich for quite some time after a cold start. Anyone here understand lamba sensor values?

Image
Image

Basically, sensor 2 reports a constant 0.48v irrespective of pretty much anything, whereas sensor 1 oscillates between those two values, either at idle or with revs increased. Do either of those sound wrong to you?
Adrian
1998 Rover Mk2 820 Vitesse Hatchback (white gold)
Gone...1998 Rover Mk2 820 Vitesse Hatchback (green)
Gone...1992 Rover Mk1 827 SLi Manual (green)
Gone...1998 Rover Mk2 825 Diesel (green)

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scoobyh123
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Re: 1997 SLK 230 Kompressor

Post by scoobyh123 » Thu Aug 20, 2020 9:57 am

How soon after a cold start is the reading available for the Lambda sensors Adrian?

Given it's not a Vee engine, i'd suggest the "sensor 2" value of a constant 0.48V is the post-cat sensor. However, that's guesswork. The sensor 1 value that oscillates between 0.18 - 0.82V is correct for a hot engine under closed-loop control.

Until the sensors are hot enough (i presume they're heated on the Merc as they are on your Rover) they shouldn't give any voltage at all. More worrying is the fact that the sensor 2 is showing 0.48V constantly. If it is indeed the post-cat sensor it should follow what sensor 1 does but with a lower value. Have you actually confirmed it has 2 sensors?

If it has, i think you may have found the problem. It could be sensor 2 is unplugged or some other workaround has been used, assuming it has two sensors of course.

With the mileage at 133k on the last MoT in March, i'd suggest renewing the Lambda sensors and resetting the ECU and also running some Cataclean through the fuel system. The generally accepted service life of a Lambda sensor is 10 years or 100k miles so the chances are renewing them and resetting the ECU will return things to normal operation.

Just another thought - have you tried disconnecting the +ve feed from the compressor clutch and feeding the clutch direct from the battery? That will prove the operation of the clutch at least.

Just FYI, basic Lambda sensor operation :

0V - lean
1V - rich
~0.45V - stoichiometric

During idle, the ECU will constantly run a rich/lean cycle on closed-loop control, usually under WOT conditions it will go open loop and enrich the mixture, during cruise it is once again closed-loop and will oscillate up/down.

A cold sensor won't give any output voltage and nor will a dead one - this causes the ECU to try and enrich the mixture causing rich running.

During the MoT on my Coupe one year, one of the Lambdas failed during the emissions test. The CO rapidly climbed from 0.1% to 9.5% at which point the tester pulled the probe out of the tailpipes to protect his analyzer.

It took another 1/2 hour of idling at the test station trying to figure out what was wrong followed by about 15 minutes and ~9 miles of driving before the engine management light came on and i could read the codes on the ECU telling me the rear Lambda was FUBAR.

Ordered two new sensors and rebooked the test. Removed the cat and soaked it overnight in hot water and about 6 dishwasher tablets (lemon fresh jobbies! :P ) then rinsed and dried it the next morning and gave it a coat of VHT paint followed by Satin Black and refitted once dry.

Next day the sensors arrived so fitted them, reset the ECU and road tested it.

All good so it was back to the test station on the Monday (it was Saturday at this point) and it passed with flying colours.

During the retest, the tester kept muttering "Why can i smell lemons?" :lol: :lol: :lol:
Cheers,
Dave

'02 Honda CR-V SE Executive
'99 Jag S Type 3.0 V6 SE


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mercedade
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Re: 1997 SLK 230 Kompressor

Post by mercedade » Thu Aug 20, 2020 2:00 pm

Fantastic info, thanks Dave. I have to say, information is pretty hard to come by on MB fora. There's definitely an underlying attitude amongst the wider membership that the way to fix things is to take it the dealers and fit a new *thing*.
scoobyh123 wrote:
Thu Aug 20, 2020 9:57 am
How soon after a cold start is the reading available for the Lambda sensors Adrian?
Literally from start up - 2 never changes, whilst within seconds 1 is oscillating.
scoobyh123 wrote:
Thu Aug 20, 2020 9:57 am
Have you actually confirmed it has 2 sensors?
Not yet! There appears to be some disagreement online about an SLK this early, so I'll have to get it up in the air to check
scoobyh123 wrote:
Thu Aug 20, 2020 9:57 am

Just another thought - have you tried disconnecting the +ve feed from the compressor clutch and feeding the clutch direct from the battery? That will prove the operation of the clutch at least.
So, interestingly it turns out I'm not the first person to start looking into the supercharger clutch wiring...

Image

More concerning is that I get continuity across the two wires, which turns into 13v+ on both when you turn the ignition on. So a short in the clutch?
Adrian
1998 Rover Mk2 820 Vitesse Hatchback (white gold)
Gone...1998 Rover Mk2 820 Vitesse Hatchback (green)
Gone...1992 Rover Mk1 827 SLi Manual (green)
Gone...1998 Rover Mk2 825 Diesel (green)

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scoobyh123
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Re: 1997 SLK 230 Kompressor

Post by scoobyh123 » Thu Aug 20, 2020 9:45 pm

mercedade wrote:
Thu Aug 20, 2020 2:00 pm

So, interestingly it turns out I'm not the first person to start looking into the supercharger clutch wiring...

Image

More concerning is that I get continuity across the two wires, which turns into 13v+ on both when you turn the ignition on. So a short in the clutch?
Lack of earth on the clutch and looking at the wiring you've shown, one is a +ve feed, the other is earth switched via the ECU. If you disconnect those horrible blue crimp terminals and switch the ignition on, you should only get +ve voltage on one of those lines. If you reconnect that line to the compressor and take the other compressor line to earth, it should operate the clutch.

Also check all your fuses, there may be one that protects that wiring that has presumably got hot (hence the nylon sleeve to insulate it in the pic) and shorted to earth at some point, hence the "repair". It's also possible the ECU has been damaged by it and when it was found the supercharger wouldn't engage when it should after repairing the wiring, they sold it.
Cheers,
Dave

'02 Honda CR-V SE Executive
'99 Jag S Type 3.0 V6 SE


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'94 827 Sterling saloon
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mercedade
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Re: 1997 SLK 230 Kompressor

Post by mercedade » Thu Sep 17, 2020 1:57 pm

Clutch is behaving fine with 12v applied directly (thanks Dave) but no amount of ECU and wiring check can get it operating.

So, rather like the Vitesse now has a non-ECU controlled cooling fan, I'm going to try the same with the supercharger clutch.

Can anyone knock me up an idiot's wiring diagram please?!

My sketches are getting less and less comprehensible and my brain gets tangled.

I'm going to have:

Electromagnetic clutch (application of 12v engages it, driving the compressor)
An illuminated three-pin latching switch (lit to show 'On')
Relay to power the clutch circuit

So I need the clutch (with a fused power supply), the relay, an illuminated 3 pin switch to activate the relay (running from an circuit that's only live when ignition on - let's say the radio for now).

I know this is cheeky, but I keep falling down trying to get the circuit straight in my head!
Adrian
1998 Rover Mk2 820 Vitesse Hatchback (white gold)
Gone...1998 Rover Mk2 820 Vitesse Hatchback (green)
Gone...1992 Rover Mk1 827 SLi Manual (green)
Gone...1998 Rover Mk2 825 Diesel (green)

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scoobyh123
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Re: 1997 SLK 230 Kompressor

Post by scoobyh123 » Thu Sep 17, 2020 3:24 pm

I'll see what i can knock up later for you Adrian, silly thought though, have you checked the clutch output on the ECU at all?

I seem to recall you said there were two wires going to the clutch and one was +12V with the ignition on, the other one would be connected to the ECU. If you can test continuity to the ECU on the switch line (that will be the 0V line) that would narrow it down to either wiring or the ECU as being the prime suspect.

If it's the wring, there's no reason you couldn't repair it and return normal functions, if it's the ECU and it's not providing an no output it might be an idea to send the ECU off for repair, i can't remember what year yours is so i picked 1997 at random but it will give you an idea and get you onto the site :

https://www.atpelectronics.co.uk/p/merc ... -21-1.html

I think (i've not read it properly so might be wrong) that's for either an exchange unit or a repair to yours. They also do testing and reporting whether it's faulty or not, if it is and you choose for them to repair it, the cost of testing (£43+VAT???) is discounted from the final invoice.
Cheers,
Dave

'02 Honda CR-V SE Executive
'99 Jag S Type 3.0 V6 SE


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mercedade
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Re: 1997 SLK 230 Kompressor

Post by mercedade » Thu Sep 17, 2020 7:48 pm

Cheers Dave - the truth is, you've largely answered your own suggestion. I don't have £300 to throw at it, especially when even that wouldn't guarantee operation.

https://forums.mercedesclub.org.uk/inde ... 806.89215/

https://www.benzworld.org/threads/super ... e.1684378/

I'm only posting these links to highlight how out of my depth I am with this side of things!
Adrian
1998 Rover Mk2 820 Vitesse Hatchback (white gold)
Gone...1998 Rover Mk2 820 Vitesse Hatchback (green)
Gone...1992 Rover Mk1 827 SLi Manual (green)
Gone...1998 Rover Mk2 825 Diesel (green)

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Re: 1997 SLK 230 Kompressor

Post by scoobyh123 » Thu Sep 17, 2020 9:09 pm

I should have asked sooner, have you checked your codes at all?

If your codes bear any relevance to the first link where replacing the BUK101 transistor sorted it (which is what i was thinking of but didn't know the component designation) then it might be worth investing in one (and a tube of heatsink compound) and changing it yourself.

https://www.ebay.co.uk/itm/BUK101-50DL- ... 3819403887

If the wiring has continuity to pin 21 on the ECU from the -ve terminal on the compressor clutch then it's a fair bet it's the ECU - as it seems to have a weak point for that transistor that's where i'd start.
Cheers,
Dave

'02 Honda CR-V SE Executive
'99 Jag S Type 3.0 V6 SE


Image
'94 827 Sterling saloon
'88 Volvo 760 V6 Estate
'95 827 Coupé LPG gone but not forgotten!

mercedade
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Re: 1997 SLK 230 Kompressor

Post by mercedade » Fri Sep 18, 2020 9:10 am

Hi Dave - yes, I have that exact code - p0806.

I've actually already replaced the MOSFET (BUK101) component, and although it had the car running differently (it seems to hang on to the revs for longer, and at full throttle now gives a little 'cough' at the gearchange), it hasn't sprung the supercharger clutch into life.

However, I'd also like to find a fault-finding process that would at least allow me to test whether or not that component is working correctly, and if the K40 relay is behaving as it should because (rather like when I do domestic plumbing), I find it hard to trust the quality of my work.
Adrian
1998 Rover Mk2 820 Vitesse Hatchback (white gold)
Gone...1998 Rover Mk2 820 Vitesse Hatchback (green)
Gone...1992 Rover Mk1 827 SLi Manual (green)
Gone...1998 Rover Mk2 825 Diesel (green)

mercedade
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Re: 1997 SLK 230 Kompressor

Post by mercedade » Fri Sep 18, 2020 9:14 am

Also worthy of note - it was clear when I opened the ECU, that the MOSFET had already been replaced, so there's been a history of fault finding
Adrian
1998 Rover Mk2 820 Vitesse Hatchback (white gold)
Gone...1998 Rover Mk2 820 Vitesse Hatchback (green)
Gone...1992 Rover Mk1 827 SLi Manual (green)
Gone...1998 Rover Mk2 825 Diesel (green)

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scoobyh123
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Re: 1997 SLK 230 Kompressor

Post by scoobyh123 » Fri Sep 18, 2020 10:21 am

Hmmm, interesting! Confirms my original suggestion when you first found the supercharger wasn't working - someone else had the same problem and couldn't fix it so sold it on.

Apparently those K40 relays are cheap according to one post you linked to, could be worth just throwing one of those at it on the off chance.

However, reading up on code P0806, i think the trouble may lie elsewhere :

https://www.yourmechanic.com/article/p0 ... e-johnston

Worth a read! Is the clutch cable or hydraulic on yours? If it's cable, adjust the cable a little and reset the codes and see what you've got - may have simply stretched a bit causing the problem! :wink: :D
Cheers,
Dave

'02 Honda CR-V SE Executive
'99 Jag S Type 3.0 V6 SE


Image
'94 827 Sterling saloon
'88 Volvo 760 V6 Estate
'95 827 Coupé LPG gone but not forgotten!

mercedade
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Re: 1997 SLK 230 Kompressor

Post by mercedade » Fri Sep 18, 2020 11:08 am

Aha - no, mine's an auto
Adrian
1998 Rover Mk2 820 Vitesse Hatchback (white gold)
Gone...1998 Rover Mk2 820 Vitesse Hatchback (green)
Gone...1992 Rover Mk1 827 SLi Manual (green)
Gone...1998 Rover Mk2 825 Diesel (green)

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