# Reverse polarity in Hymer 12volt system!



## Glandwr (Jun 12, 2006)

I thought that I should share this as it might mean others will check polarity before doing modifications to the system. I didn’t.

While doing some work rebedding a window I had to take the false bottom out of the over head locker above the passenger seat.

I found 2 cables both ending in junction boxes one 12volt habitation circuit and one from the EHU system.

Ah I thought with satnavs, blue tooth music, smart phones, tablets running Osmand etc. the 2 cigar sockets in the cab had spawned a network of splitters and extensions for chargers. Not only was it confusing and unsightly but I was getting concerned about the loading.

The cable in the cupboard looked pretty substantial 4mm I’d guess. So I thought I’d rig up 2 more sockets in the cab from it. Got suitable cable, fitted an inline fuse, tested it with a map light and slapped myself on the back proud of the fact that I had concealed all wiring. It looked a good job.

Then some of the chargers started to fail. First they would not work in the new sockets at all. Then they failed in the old ones. I was puzzled. Other things worked, I thought that it might have been the design of the new sockets.

Another replacement charger arrived today for my S2 thought I check it. Worked in the old socket great! Tried the new ones nothing! The maplight, extension and compressor worked in the new ones though!

I had simply relied on the cable colours to fit them. I thought that I would check polarity with a multimeter now. YES the polarity was reversed the tell tale minus sign told me!

Cost me a few quid and a lot of frustration. I hope that this will save it for someone.


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## pippin (Nov 15, 2007)

Mae ddrug genni Dick.

There have been various posts (including some by me) mentioning that Hymer 12VDC ccts use 
BROWN as the -ve and 
BLUE as the +ve.

Luckily I found out the easy way - sixth sense told me to check with my meter!

Good heads-up for other folk though.


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## Mrplodd (Mar 4, 2008)

What IDIOT at Hymer gave the OK for that stupid protocol ???


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## trevorf (May 16, 2005)

Its not just Hymers but other German built vans also. I remember a couple of years ago I did a hab service on a Dethleffs and thought some plonker had wired up the battery reverse polarity. When checked out with multimeter I found brown was -ve and blue +ve 8O 8O 
Well confusing. :lol: :lol: :lol: 


Trevor


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## bigtwin (May 24, 2008)

If I've understood Glandwr's post correctly (its the dc system he's referring to (edited - just noticed that the title is unambiguous!), this looks like a genuine case where both:

the term reverse polarity is appropriate

the existence of reverse polarity actually matters.

Glad you got it sorted though.

Ian


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## dalspa (Jul 1, 2008)

Found this out for first time with our first Hymer. The water pump didn't seem to pump very efficiently. The pump had been changed to an English one and the brown Hymer wire had been wired to the red wire from the pump. Checked out with meter to find, much to my surprise, that the blue was the +ve wire. Swapped over wires and pump worked properly afterwards! Always check out with my meter now. Oddly, I had just come into the house from checking the wires in the boxing above the hab. door (thinking of putting a 12v socket there for feed for Wifi router), when I saw this thread.
DavidL


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## pippin (Nov 15, 2007)

_What IDIOT at Hymer gave the OK for that stupid protocol ???_

German logik.

Brown is the colour of earth, which is down there, hence lower in potential.

Blue is the colour (of my true love's eyes, in the morning!!) of the sky which is up there, hence higher in potential.

Some years ago the convention was black wire for 240V AC live and white for neutral.


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## Pudsey_Bear (Sep 25, 2008)

Mrplodd said:


> What IDIOT at Hymer gave the OK for that stupid protocol ???


A very clever one, think how much work they will have got from people messing with their own vans electrics.


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## Techno100 (May 8, 2010)

I am not surprised
Here Brown was Positive and Red was Negative
http://www.motorhomefacts.com/ftopicp-1384802.html#1384802


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