SLK R172 - Hazard fuse keeps blowing plus smoke!

[azibux1]

Hi all,

Hoping somebody has seen similar before or can point me in the right direction. Will try to explain as best as I can.

So my mum got in the car to drive to town and returned quite quickly saying there's a message on the dash saying along the lines of "hazard light system malfunctioning" - and sure enough the hazards were not working but the indicators were.

When she got back from town later I took a look and fuse 18 for the hazards was blown. I thought this was strange as the bulbs etc were all OK so changed the fuse (engine bay) and opened the door to test the hazards. Upon opening the door there was smoke quite visibly wafting out of the car and it was coming from the trim around the rear view mirror / 'behind' where the rain sensor is. At this point mum said there was also a smokey electrical burning smell she noticed earlier - so I assume the initial fuse blew at some point overnight or over the past couple of days (she hasn't driven the car for a few days).

I took all this trim apart and unplugged all the connections there (rain sensor, alarm switches panel, etc) and thought I'd plug everything back in one at a time and test (taking the fuse out each time I was plugging something in, then replacing it after).

All these connections are in a line in the 'module' by the headliner that is above the rearview mirror. It all works fine until I plug in what I'm going to refer to as the fifth cable (goes in the fifth connector from the drivers side). This cable seems to lead down the pillar and behind the glovebox somewhere. Note, it does NOT go into the unit I've put the blue star on in the picture, it goes behind it and downwards.

So anyway, it all works fine without this fifth cable, apart from the auto lights as you get a message saying auto lights not available - so it seems related to the auto light system somehow. When plugging this cable in, the HAZARD fuse pops again (I don't see how they're related) but the auto lights DO actually work fine with the cable in. So overall, with cables in / out the hazards do work (at one point) and the auto lights also work (at another point).

Since that time there was a lot of smoke there has not been any more. My plan was to take the trim off and see which part is smoking but that can't be done since it isn't smoking anymore - just popping the fuses straight away instead when I put that fifth cable in.

Autoglass changed the windscreen about 2 weeks ago - possibly unrelated though I thought I'd mention it as they had that area of the car apart for the rain sensor etc.

Anyone got any thoughts on this please?? I am a bit stuck on what to check next.

Any help will be appreciated.

Thanks!!

 

[optimusprime]

You need an auto electrician to sort that out, some one that specializes in auto wiring problems

 

[malbry]

There are a lot of knowledgeable SLK owners in the forums at www.slkworld.com who might be able to help.

 

[richard]

Eek. When the car catches fire and burns out, then your insurance company is going to say "what did you do" !!

** You must find the damaged section of wire and replace it !!! Its probably lost its insulation and may well go up in flames **

Assuming this is a R172 SLK

Fuse 18 is the fuse for the Upper Control Panel and the Overhead Control Panel and the backup relay if you have Start/Stop function

UCP is the one with the Hazard button, which is why its noted as being related to the Hazards.
OCP is the one you've been dismantling and thus you are looking in the correct sort of area

The lead you are looking at is the power feed to the OCP !
RD = permanent power
Brown = Ground
GY/YE is the control link (LIN) to the front SAM
RD/WH or RD/BU (age dependant) is the permanent power that turns off a few hours after car is off or if battery goes flat

I'd guess the red one has been smoking as thats the fuse 18 one.

From your description I can't tell if its the OCP failing, or if its one of those things plugged into it failing. I.e did it smoke with just that connector plugged in

*HOWEVER* you must find the damaged section of wire and replace it !!! Its probably lost its insulation and may well go up in flames


Richard

 

[azibux1]

Eek. When the car catches fire and burns out, then your insurance company is going to say "what did you do" !!

** You must find the damaged section of wire and replace it !!! Its probably lost its insulation and may well go up in flames **

Assuming this is a R172 SLK

Fuse 18 is the fuse for the Upper Control Panel and the Overhead Control Panel and the backup relay if you have Start/Stop function

UCP is the one with the Hazard button, which is why its noted as being related to the Hazards.
OCP is the one you've been dismantling and thus you are looking in the correct sort of area

The lead you are looking at is the power feed to the OCP !
RD = permanent power
Brown = Ground
GY/YE is the control link (LIN) to the front SAM
RD/WH or RD/BU (age dependant) is the permanent power that turns off a few hours after car is off or if battery goes flat

I'd guess the red one has been smoking as thats the fuse 18 one.

From your description I can't tell if its the OCP failing, or if its one of those things plugged into it failing. I.e did it smoke with just that connector plugged in

*HOWEVER* you must find the damaged section of wire and replace it !!! Its probably lost its insulation and may well go up in flames


Richard

Thanks for the very detailed response!

Logged a warranty inspection with AutoGlass but not sure when they are going to be able to come out - they're looking into it as urgent.

Do you know how the OCP is removed and roughly where the power feed for it runs?? Is it near the windscreen and therefore the insulation likely to have been nicked into by autoglass? I assume (as I can't see it) that the power feed into it is on the 'other' side that is further into the headliner / lighting unit that I can't seem to figure out how to remove.

If I can fix it sooner than AutoGlass (for safety reasons) I'll be happy and will come to some arrangement with them for the costs to be covered if it turns out they've obviously pinched a wire somewhere.

All I've been able to successfully remove so far is the pillars / trim around the visors and the bit at the back of the rearview mirror. Couldn't figure out how to remove the actual mirror from the mounting, seems it is on a ball joint but didn't want to pull too hard in case there was some technique.

Thanks again for the info! Very helpful and helps focus the investigation. Just need to get the OCP out somehow to hopefully find the culprit cable!

Also on the R172, can the battery be disconnected / left disconnected for an extended period without causing all the electrical systems to go haywire / lose all their memory?

 

[azibux1]

From your description I can't tell if its the OCP failing, or if its one of those things plugged into it failing. I.e did it smoke with just that connector plugged in
Richard

To help explain see below.

So there's 5 connections (one empty) from drivers side to passenger side.

If number 5 (seems to have some relation to the auto lights as they become unavailable when this is unplugged) is unplugged, the fuse does not blow. As soon as this is plugged in and a fuse is inserted, then the fuse pops straight away. The pics of the connector I uploaded are this connector in question.

 

[azibux1]

I should add that the overhead interior lights now no longer work. So I assume it is the OCP itself that has fried.

 

[azibux1]

Sorry I misunderstood your post when I read it. I took it as I need to find the power lead for the OCP...now I understand you mean that IS what I've found and the one that's causing it to go pop.

So obviously me testing with the other connections plugged in and saying it didn't pop was a useless test - as none of it was powered!!

 

[richard]

The rain/light sensor plugs into the OCP. So if you disconnect the power to the OCP the rain/light sensor doesn't work any more.

Will be videos on Youtube to remove the OCP

Its either the OCP or something else plugged in. Just plug in that connector you identified and nothing else and see if the fuse still blows etc, then you know its the OCP.

Richard

 

[azibux1]

I think you responded while I was outside messing about with it.

Update! The OCP is fried for sure - that's the cause of the smoke.

Only issue is, I don't know if that just happened (if that's even possible just with age / heat maybe?) or is there some underlying cause that has made the OCP go pop. I don't know how much they cost, but would hate to change it for a new one only for it to blow up again! Based on the fact that we haven't touched anything, the windscreen repair was a whole month ago and all wires (that I can see) look OK... I'm leaning towards it just being bad luck / aged and that's the cause.

But your thoughts would be appreciated

Thanks for the help so far!

 

[Ted]

That looks like a component has failed.
I can’t see because of the damage, but I would hazard (pun intended) a guess that they are power transistors - there will probably be three pads.
A short might damage the components but unlikely to be that much devastation on the pcb

 

[azibux1]

That looks like a component has failed.
I can’t see because of the damage, but I would hazard (pun intended) a guess that they are power transistors - there will probably be three pads.
A short might damage the components but unlikely to be that much devastation on the pcb

That's what I'm leaning towards - simple failure on the PCB not caused by anything 'external' but my electrical knowledge is lacking a bit to confirm that for sure.

 

[Ted]

Yes, I understand. Do you replace and risk the new one being burnt out from an external short, or spend the time and money on a proper strip down and diagnosis.
Personally it it were mine, I would replace the unit and keep a close eye on it for a few days, but as it is your mothers that might not be the best option.
Could also be an external conponent failing and frawing too much current.
Difficult one.

 

[azibux1]

Yes, I understand. Do you replace and risk the new one being burnt out from an external short, or spend the time and money on a proper strip down and diagnosis.
Personally it it were mine, I would replace the unit and keep a close eye on it for a few days, but as it is your mothers that might not be the best option.
Could also be an external conponent failing and frawing too much current.
Difficult one.

It isn't too much hassle to strip it down and check after a few days to be fair.

Just not sure on the cost of the unit.

Getting one from a scrappy would be ideal as it would be cheaper and very little concern if it were to go pop again

 

[Codger49]

You might wish to post up your VIN to allow someone with EPC to identify the particular version of OCP required for the car.

There are about a dozen different part numbers depending on the year of manufacture and the options fitted. That's before any colour specs.

 

[azibux1]

Thanks! Assumed there would only be one or two variations.

Don't have vin to hand but was planning on calling the dealer tomorrow for a price. I expect they will be expensive but not sure where else to try?

 

[sjmaxwell]

For second hand parts Dronsfields are worth a call Mercedes-Benz Parts - Dronsfields, a Mercedes Breaker UK

 

[azibux1]

Mercedes wanted about £340.
Another local place we know and are friends with said £310 but they weren't 100% if it would need programming or not... Does anyone know?

And waiting on a call back from Dronsfields... Thanks! Hadn't heard of them

 

[AMGeed]

If the part # matches, this might be an alternative.

MERCEDES R172 2012 SLK AMG INTERIOR REAR VIEW MIRROR A1728100417 [CY910] | eBay

 

[azibux1]

Thanks. That's a different part though. I'm after the unit installed in the roof with the light etc.

Big thanks to sjmaxwell for the Dronsfields recommendation! They have one and it should be on its way to me for about £60!

Also spoken with a Mercedes technician who says if I can replace the fuse and it doesn't pop with the unit unplugged, then the cable cannot be shorted "externally" anywhere and it must be an internal fault on the board. Says he's never seen anything like it in 5 years. So new one should fix the issue!

Fingers crossed