K40 relay module. What exactly does it do?

mikevernon · Oct 10, 2012

My SLK320 does not crank when I turn the key, and there is also no power reaching the fuel pump. The battery is absolutely fine.

I suspect that the K40 module may be the cause of the problem but I'm not cetain about how it works so can anybody explain?

I think that the K40 should send power to the fuel pump for a few seconds when the ignition is turned on. Then, and only then, the K40 should send power to the starter when the key is turned. Is this correct? If it is correct then the no cranking symptom is a result of the k40 not supplying power to the fuel pump?

I could just go out and buy a new K40 but they're not cheap so I want to convince myself that this is the cause of the problem Can anybody tell me the best test procedure? Or failing that, where can I go for some good technical advice?

And failing that, does anybody want to buy a 2002 SLK320 with a genuine 20,000 miles from new, but currently DEAD?

Kobus · Oct 14, 2012

Nobody close-by from who you can borrow a K40? It may help if you update your profile with your location.
But it does sound like the K40 - although failures were rare on the facelift models.

mikevernon · Oct 15, 2012

The fault was the K40 relay module, as suspected. I replaced the whole module, but it was probably a failure of the relay controlling the fuel pump (2nd from left when viewed from the side with the fuses).

If my interpretation is correct, the no cranking symptom is a consequence of the failure of the K40 module to supply power to the fuel pump.

theptyboy · Oct 1, 2021

Hmmmmmm, so we got a SLK320, no crank, but dash lights - intermittent starting over the last few months (but running fine otherwise)...

I've gone over the RCM and K40 (3 times), refloated all joints, with special attention to the prime suspects...
Cleaned all the contacts, (very) slight residue, but nothing of the slightest concern...
I've seen 2 of the bank operate with the key at ignition (2&3 or 3&4 🤔) but STILL no crank...

So what do I rule out now? Starter solenoid or starter motor...? Does the ignition switch ever fail or gearbox Park switch? Fuel pressure switch?

I'll try to get 12V onto the starter solenoid to give it a jag, but a right pain without a lift - is there an easier way of jumping 12V onto the starter solenoid on any of the fuse boards?

Hedging my bets on a new starter motor atm, but just want to rule out all other possibilities...

Perplexed Pete

tyreej4151 · Jan 12, 2022

hi im tyree from san francisco this is my slk 230 and i have a problem with startin my car it cranks but wont start.i hit a pothole after that a few secound later my dash lights started to blenk on and off also my meter not the speedemeter went down than back up the one that shows the car heating up goes down than up erverythings blanking than stops than starts again... i turn the car off for sec.it started up than starts over again..turned the car off than tryed to start it and it doesnt start at all for like two weeks noe .new battery still no start.i than checked my fues and my num.36 fues blown.dont know why 2001 slk230.help........

20210905_192745.jpg

hollerbanger
Dec 2, 2021

Our 99 SLK230 Kompressor over 200,000 miles and just had it's first limp in mode happen, car was...

NCPanzerWagon · Jul 15, 2022

So where in the car is the K40 relay???

Subby_ · Jul 16, 2022

Its in the engine bay inside this black box

once inside the box, it looks like this

K40 relay module. What exactly does it do?

mikevernon

Kobus

mikevernon

theptyboy

tyreej4151

20210905_192745.jpg

NCPanzerWagon

Subby_

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