[agreenroberts]

Long-time lurker, first time poster! Just bought my second r170 yesterday, which unfortunately won't start today...

Upon turning the key into position 2 (the "run" position), the whole dashboard illuminates and engine cooling fan comes on at max speed. I am fairly sure I can hear the fuel pump prime. Position 3 will not engage the starter. The battery is charged btw.

In my testing I have sent 12v power direct to the starter motor which turns over fine. All the pins in the engine relay module (part no. A1705450105) appear to have the correct voltage at the respective key positions, except for the send to the starter motor, which doesn't get 12v when the key is turned to pos. 3. Am I right in assuming that putting in a used engine relay module, A1705450105, from ebay should rectify the problem?

Is it worth taking the engine relay module apart to have a look at/solder? Should I have a look at/replace the K40 while I'm at it - just seen a lot of talk about it on these forums!

Thanks Guys!

 

[Dave2302]

K40 Module, don't mess with it, just buy a brand new one from MB, which is not a fortune and usually cheaper than the Chinesium junk on various sites

Then it will last another 20 years, repairs depend exactly what is wrong and are a lottery as to how long before it leaves you stranded somewhere on a dark rainy night

 

[agreenroberts]

Thanks, Dave - I was under the impression that it was actually the engine relay module (part number A1705450105), which is located on the drivers side which was at fault. Why would you recommend changing the K-40 instead of the engine relay module?

 

[LeonardoDaBenz]

☝ wot @Dave2302 said.

Soooooo many stories about folks trying to re-solder and ......re-solder and..... Well, success stories are few and far between.

Just buy new MB (and not ebay, etc)

 

[agreenroberts]

☝ wot @Dave2302 said.

Soooooo many stories about folks trying to re-solder and ......re-solder and..... Well, success stories are few and far between.

Just buy new MB (and not ebay, etc)

Thanks Leo,

But should I be buying part a1705450105 (the engine relay module) or a1705450305 (the K-40 module)

 

[LeonardoDaBenz]

As Dave said--K40

Many have tried to re-solder but even using a magnifying glass, it may prove difficult. And don't bother trying to eyeball the solder joints. You may see cold solders and try to re-flow but not many are successful at it.

 

[SLK32Rick]

Genuine MB replacement K40.

For those who are truly skilled at soldering, I have a success report, fixed ours and been working fine for many years. Reflow and cleanup any questionable solder areas AND inspect the relay contacts for pits, polishing with 2000 grit paper to new surface and check contact resistance with a meter. It will get you down the road, then order a new one and keep this for a spare if ever discontinued.

Happy top down spirited motoring, reliably.
R

 

[Australian]

I reflowed some joints on my K40, in the secondary air pump circuit. They've lasted 10 years so far, and I'm not particularly skilled in soldering. At least if they go bad again they won't leave me stranded.

 

[M4rCu5]

Buy a new K40 from MB, they were @ £176.40 inc. for the V6 last year..

And IF you can successfully repair your original you'll have a working spare - never a bad thing

 

[Dave2302]

Why would you recommend changing the K-40 instead of the engine relay module?

Because ....

1) The Module on the Drivers side is the Pulse Module and does not have Engine related Relays inside it aside from the Starter Relay, which won't work when the K40 is not supplying voltage to the Motor ECU !!

2) You have the classic symptoms of K40 not supplying power to the Motor ECU.

3) I have been known to fix the odd few Mercedes.

should I be buying part a1705450105 (the engine relay module) or a1705450305 (the K-40 module)

What you should do, especially as this car is new to you (might have a wrong part fitted) is to call Mercedes Parts Dept and give them your Reg Number and they will supply you a correct and decent part, don't cheap out and buy from internet etc they seldom work for long, if at all

HTH