Mercedes electrical issues

[Acegolfman]

Hi, I have a 2006 mercedes vito, after some heavy rain the battery light stays on and runs the battery flat over a few weeks, the fuel gauge just keeps going up and down, can't hear the indicators working, sometimes the wipers don't work, if I press the horn it may stay on for a while also the lights come on soon as ignition is on and never used to, even if I disconnect the light switch!? This started after a down pour last July and thought it would dry out as the following week was in the 30s. Ive had fuse box out checked all connection even took the back of the ecu and was nice and dry? Would appreciate any help if I can't sort it will have to scrap as think garage bill would be more then it worth

 

[00slk]

It is possible that some corrosion may have started with the electrical plugs that got wet during the heavy rain. If you can check points where the water car get in trace any water stains and see where that takes you. Most likely to some electrical component.
Water and Mercedes are worse enemies
Yes it may have dried out, but then corrosion starts.

 

[LostKiwi]

Check the fusebox under the bonnet (aka SAM).
If the lid doesn't fit correctly they can fill with water. Check inside to make sure it's dry..

 

[Acegolfman]

It is possible that some corrosion may have started with the electrical plugs that got wet during the heavy rain. If you can check points where the water car get in trace any water stains and see where that takes you. Most likely to some electrical component.
Water and Mercedes are worse enemies
Yes it may have dried out, but then corrosion starts.

Hi OOslk
I've take the fuse box out and remove ecu checked all connections all look ok‍

 

[Acegolfman]

Check the fusebox under the bonnet (aka SAM).
If the lid doesn't fit correctly they can fill with water. Check inside to make sure it's dry..

Hi lostKiwi:
I've checked all that and looks fine! Any chance it could be an alternator issue?

 

[LostKiwi]

Unlikely.

Check the earth from the front Sam. Unbolt it from the body, Emery paper everything and reassemble. Cover with a little grease to keep moisture out.

 

[M80]

Unless cable lugs are already in a poor state it's better not to remove the protective coating, mild abrasion with a kitchen cleaner is ok.
Wire brush rusted threaded studs, new nut, and a little copper grease is good.

I would expect you're SAM to be goosed though. Source an identical one off eBay, £100 ish. As long as the replacement is good it will be plug and play.
It plugs into the underside of the fuse tray.

'After heavy rain' sounds like no coincidence. The drains are poor design and if not in good order, with all foam protectors, water will deluge onto the fuse enclosure.

 

[Tony Dyson]

Unless cable lugs are already in a poor state it's better not to remove the protective coating, mild abrasion with a kitchen cleaner is ok.
Wire brush rusted threaded studs, new nut, and a little copper grease is good.

I would expect you're SAM to be goosed though. Source an identical one off eBay, £100 ish. As long as the replacement is good it will be plug and play.
It plugs into the underside of the fuse tray.

'After heavy rain' sounds like no coincidence. The drains are poor design and if not in good order, with all foam protectors, water will deluge onto the fuse enclosure.

Contrary as the name suggests, Copper Grease is a good Heat Conductor, manufactured as an Anti-Seize compound but has very poor Electrical Conducting properties, Petroleum Jelly is a good alternative to a Dielectric Grease.

 

[Taffy7hfa]

Classic symptoms of a water damaged/corroded SAM (ECU is a different module)

 

[M80]

Contrary as the name suggests, Copper Grease is a good Heat Conductor, manufactured as an Anti-Seize compound but has very poor Electrical Conducting properties, Petroleum Jelly is a good alternative to a Dielectric Grease.

I'm thinking more for protection against corrosion with copper grease.

 

[Tony Dyson]

I'm thinking more for protection against corrosion with copper grease.

The protection against corrosion in an electrical connection has to work from both the inside as well as outside the electrical connection, especially in the grounding of a connection from a source such a SAM where you may have upwards of 30 fused supplies, each with the potential of sending a few mA of fault current down the wire.
If there are any signs of corrosion on or around a grounding connection that needs cleaning up then it would be better to use a conductive layer of protection on the cleaned conducting surfaces first then a dab on the outside to protect further from moisture ingress. Yes, any old grease would be better than nothing for the latter but I’m not entirely sure of the hydrophobic qualities of Copper Grease, whereas Petroleum Jelly is a known good electrical conductor, hydrophobic in nature and as that orange antiques dealer often liked claiming, ‘It’s as cheap as Chips’

 

[M80]

The protection against corrosion in an electrical connection has to work from both the inside as well as outside the electrical connection, especially in the grounding of a connection from a source such a SAM where you may have upwards of 30 fused supplies, each with the potential of sending a few mA of fault current down the wire.
If there are any signs of corrosion on or around a grounding connection that needs cleaning up then it would be better to use a conductive layer of protection on the cleaned conducting surfaces first then a dab on the outside to protect further from moisture ingress. Yes, any old grease would be better than nothing for the latter but I’m not entirely sure of the hydrophobic qualities of Copper Grease, whereas Petroleum Jelly is a known good electrical conductor, hydrophobic in nature and as that orange antiques dealer often liked claiming, ‘It’s as cheap as Chips’

I haven't attempted to trace for it but haven't come across the fuse enclosure grounding point. I would guess the cable would pass through the fire wall and be ground some where within the cab.
If I'm right it becomes unlikely there will be any deterioration issues there.
I suppose spraying over with cellulose would beat grease when such a terminal is cleaned.
My only issue with petroleum jelly is that it doesn't have good resistance to running off when warm, but I can see the conductivity advantage, although I've never compared it to copper grease.

The issues mentioned by the OP seem to relate to the canbus, and signal confusion. Ingress to the multi function light switch could cause that, if the windscreen was passing (been there with a C Class). But my experience with the 639 is to suspect the SAM as the 1st port of call.

The OP's SAM is likely to be the 639 545 03 01 [03], but there was a different type on earlier models.
They are different again post 2010, maybe only due to the introduction of the DRL's??

To guide drainage there are 4 heavy(ish) foam inserts around the scuttle. Often these are omitted on rebuild post repairs. Then there is a bonnet width rubber seal to hopefully protect under bonnet components.
The guttering at the sides is woeful and any side tilt of the van when parked would make it next to useless, but any overspill should travel down the inner wing.
There are grommets below the wiper spigots, cheap and worth replacing.
There are grommets within the scuttle from the wiper cable and blanks.

The fuse box enclosure can distort, leaving ingress possibilities at one corner. I've used a heat gun to reform one.
People replacing the lid often find it too awkward and fail to do it properly. The 4 slide clips need be pushed home. I have red paint on mine so at a glance, if I see red paint, I know they aren't.
Then the transit blocks on the side toward the engine need be complete and packed tight. There are 2 dummy blocks that could be omitted there.