Sargent EM50 Intelligent Interface Adaptor

I thought that I would just enquire of you good folk as to whether anyone has ever suffered from this problem.

It started innocently enough with my reversing camera ceasing to function whilst away recently. As it seemed to be totally dead, I thought that I would investigate the obvious when I got back home. There is no information in the manual anywhere as to how it is powered so I looked at the main van supply EC328 fuses without any enlightenment. I then looked at the Sargent EM50 "Intelligent Interface Adaptor" situated on my van behind the drivers seat. (This connects the Mercedes electrics to the habitation supplies and also operates the electric step, etc). I was very surprised to see that three of the fuses had completely melted (the plastic parts that is). Upon further checks I could see that it was the 20 amp fuse that feeds the fridge whilst travelling that had caused the problem (F7).

This had grossly overheated but I assume had not blown and the heat generated melted the two fuses either side - one of which I guess fed the reversing mirror! The metal parts of the fuse had welded themselves into the holder and the printed circuit board was badly blackened with the plastic door distorted.

Amazingly, I was away for four days after the mirror problem was noticed and at no time did I experience any fault with the fridge. I do remember smelling something electrically hot once but as we were travelling through an industrial area I put it down to it entering from outside.

I have now sent the EM50 off to Sargent for their fixed price "reconditioning" (I believe they just fit a new PCB) with a request for comment. Upon its return I will need to investigate what current is being taken by the fridge element as a gross overload here is the most likely cause of the problem. I have no desire for a repeat procedure.

What is of concern is that the fuse had overheated but not blown. I am aware of reports of cheap Chinese fuses that have been suspect of late, but all the fuses in the unit were the originals from new.
Somewhat worrying isn't it?

I would appreciate any inputs from anyone with a similar experience - a problem shared as they say......

Many thanks
Graham

Without seeing the damage it's not possible to say with certainty, but a common failure mode when a fuse is run at close to its capacity for long periods is that it can generate enough heat for the contacts at each end to lose some of their "springiness", which reduces the contact pressure, and then vibration can lead to arcing. Once that starts, a ~lot~ more heat is generated, and that's when the damage happens. However, I'm a bit surprised that this would occur with a 20 Amp fuse, unless the element in your fridge is quite large (from memory they usually only take 6-8 Amps).

Another possibility is that the PCB tracks might be under-sized - this may seem a bit unlikely, but I have heard this suggested of other Sargent components, so I wouldn't rule it out (though I should add I have not personally had a problem of that sort).

I'm also surprised that the EM50 would play any part in the functioning of the reverse view camera. Offhand I can't think of any reason why that would need to interface in any way to the habitation electronics, as in most vans it would need to interface only to the vehicle electronics, and would be supplied only from the vehicle battery.

The camera system is fed from the fridge fuse as well. The fridge takes circa 20A. A high resistance at the fuse caused the heat and the low voltage on the equipment side stopped the camera working. The contacts for the blade fuse on the EM50 may have relaxed to cause the high resistance or, as there is no body/guide around those contacts,one of the fuse blades may have not been in the contacts but touching the side of them. I had the latter after the fuse had been removed/replaced during an investigation into a fridge fault that turned out to be in the connector block at the back of the fridge.

Thanks for the responses, the great advantage of the forum is that the replies always supply food for thought. Thinking about it I agree that a poor fuse contact is a likely contender. Searching on the internet, I found a figure of 0.88 to 1.08 ohms for the resistance of the dc element in the fridge. This means about 12 to 14 amps, hence with only say 0.1 ohm extra contact resistance you would generate 14 to 20 watts - easily enough to melt the fuse mounts in the confined area of the box.
In terms of why it occurred, earlier this year I had been having a lot of problems with the electric step. Whilst travelling in a remote part of Northumberland in May in very wet weather I was beset with a wailing sound from what turned out to be an auxiliary buzzer in the EM50 unit. (Bet not many people realised there was such a thing, but there is!). As I was certain it was to do with the step (it had stopped working completely by then), I attempted to silence this buzzer by removing the various fuses in the EM50. Obviously because of the difficulty of access, I am guessing that I did not get this one back in properly and with a bit of subsequent vibration it eventually caused the effect. So I am beginning to imagine now that it was really my own fault!! This may be a bit of wishful thinking, hoping that the fridge element proves to be OK as I have ferries booked for France in a few weeks.
Time will tell when I receive the reconditioned EM50 back from Sargent when I can then check out the fridge and the reversing camera.
As an aside, I did get the step fully sorted in the end, but that is a whole new story.

Thanks again for the suggestions.

Graham

Thought I would just let you know the outcome in case it was of any use to other members.

Received back the EM50 from Sargent with a turn round of 3 days (great service, I always have found them to be very helpful).
Discussed with my mobile engineer and agreed that I would refit it and run up the fridge from the engine alternator supply and keep monitoring the fuse temperature. Any increase above just warm would then warrant a visit from him to measure actual currents taken.
No problem was found and the unit operated as it should with the fridge running OK.
Also, the reversing camera now works (thanks Peter B - you were right, it does come from the same supply).

So the moral is that no matter how difficult access is to the EM50, if you remove a fuse then make sure you replace it properly - with 12 to 14 amps flowing, contact resistance is important.

Now feeling a lot more confident of our forthcoming visit to France - after all these things are sent to try us - sacre bleu!!

Regards  Graham F

Thank you Graham: a timely reminder and I'm glad the outcome was good.

We checked ours yesterday, something we would not otherwise have thought to do but that is one less thing to be concerned about.

What price the days when the only thing you had to worry about was when your gas mantle broke every journey and you ran out of spares ?

I hope your French trip is trouble-free and the sun shines on you !