1999 SLK230 Failed to Crank / Engine Cut Out

Several months ago I had a panic call from the wife saying her SLK wouldn't start (apparently the fan was running flat out when she turned the key and all the lights on the dashboard were on). She called Green Flag and when they arrived it started without any problems. Varta battery is relatively new and alternator is charging fine. The car has been fine until last week when it cut out on her on the way home from work. No fan running this time and it started after the 3rd attempt. I again checked the battery and alternator which were fine and then checked to see if there was any fault codes of which there were none.

After a Google search the K40 relay module seems to be mentioned quite a lot. Apparently it sinks quite a bit of current (there's a 40A fuse on board so a few hundred watts potentially) and some of the solder joints can deteriorate as a result. Has anyone had similar problems either with an SLK or other models fitted with the same module?

 

Thats the issue, k40 relay block.

 

Many thanks. Is it just a question of redoing the solder joints on the offending connector? I'm fairly confident with a soldering so would rather give this a go in the first instance.

 

Well you have nothing to lose by trying!

 

I hadn't got round to looking at the K40 relay module until this morning after the wife complained about the engine cutting out on her again during the week (dash lights on, she turned the ignition off removed they key, key back in and it started immediately and ran fine). This is the 3rd time it's happened although the first time the engine failed to crank but started after about an hour. There was no engine light and no fault codes (I realise my reader is no substitute for STAR).

Module came out relatively easily although a bit of a fiddle. It was easier to disconnect connectors once it had slid out. The offending solder joints (as seen on YouTube videos) didn't seem too bad although they were not shiny like all the other joints. PCB was not blackened in any way around these joints. I carefully re-soldered, pulled out all the fuses and cleaned with contact cleaner. Re-fitted module and re-connected battery and it started / ran fine. I guess it's now a question of seeing if it behaves itself. I'm not 100% convinced it was these joints but let's wait and see.

Are there any other sensors that could be on their way out that would not produce a fault code / engine light?

 

The only sensor that regularly causes non starting issues is the crank position sensor. Usually these fail when hot then need to cool down. If you try to start when it's in a failed state the engine will crank but not start. They usually start fine after cooling down.

 

Each time it's happened the wife has been in traffic and a bit flustered so she can't quite remember the precise sequence. If it happens again I've asked her to note down the exact sequence of events. Would a crank position sensor put the engine light on or give a fault code?

I guess it's one step at a time and see of the re-soldering makes any difference. Failing that I will have to get the indie to put it on STAR. An intermittent fault with no code it's probably the worst scenario to try and fix.

 

On my 210 it logged a fault whilst it was failed but it cleared after it started.

 

The CPS never failed on my W211 until it was changed under recall.. I don’t remember any warning lights, just not starting.

 

I've just had a look to see where the CPS is and it looks fairly accessible on the SLK so possibly worth popping one in if the re-solder job doesn't work (a Bosch one is only £30 or so). The vehicle's almost 20 years old (70K miles) and I know it hasn't been replaced as we've had it from almost new so I guess it could be on the way out. That said the last two times it's cut out the vehicle has re-started immediately. As my wife is a midwife who frequently gets called out in the night I'm thinking it's probably best to get it on STAR if it does it again.

 

fit the CPS to save another breakdown and help see where u r with current fun