Strange 3 way fridge 12 volt electrical problem

[VXman]

Hi,

Came across this forum while searching the internet for answers to my issue. I haven't built my motorhome but thought maybe there are some experience people on here who may be able to help.

I have a motorhome with a 3 way Thetford fridge. (N4000 series) It has auto selection and only uses 12V when the engine is running. It uses a Schaudt ebl119 for electrical charging/distribution.

On a recent trip it started misbehaving. I noticed one day that the leisure batteries voltages started tumbling when I was driving (they would normally be charging when the engine is running) The fridge started flashing a red error light. If I switched the fridge off the leisure batteries would recover. What is odd is that the Leisure battery would not normally be used when the engine is running - it should be powered by the engine battery/alternator.

There is a 20 amp fridge fuse (according to the vehicle manual) by the leisure battery which melted. (I must admit I do not understand why there is a fridge fuse on the leisure battery when it should in practice never use the leisure battery as a power source) There is also a 20amp fuse by the engine battery which is also labelled as a fridge fuse. This one was ok. There is a 20 amp fridge fuse on the EBL which is also ok but doesn't seem to do anything anyway as I have removed it and nothing is effected.

The problem was intermittent initially. Sometimes it seem to happen when we stopped for a short break and then when the engine was restarted the problem started. However, later on it might be ok for 30 minutes or so of driving and then would start failing. Towards the end it seemed to be working but then I smelt burning from the 20A fridge fuse by the leisure battery and it had begun to melt again.

So a very confusing problem that I am struggling to understand. This had been working fine for over a year since the fridge was fitted. Fridge is fine on Gas and 240 volt.

Is it a fridge issue or a electrical issue and if the latter what could it be?

Would appreciate any ideas.

 

[st3v3]

It sounds like the split charge relay and the fridge relay are fed from a common point, but this isn't actually connected to the battery. So you have a link from the leisure battery to the fridge 12V.

 

[Squiffy]

Hi VXman,
The 20 amp fuses are in my view protecting the 12volt supplies from the liesure and vehicle (Cab) battery as you should be able to choose which battery to use when stationary with the engine off for running the 12volt Habitation system, though of course choosing the 12volt Cab battery is a last resort for situations where the liesure battery does not for whatever reason have enough power to run the Habitation 12volt system and should only be chosen for a short period of time, the obvious reason is to prevent the Cab battery also going flat and not being able to start the engine.

The fact that the liesure battery 20amp fuse melting is to my thinking either a poor fitting fuse holder causing arcing or a very high amp draw in the liesure 12volt system but not heavy enough to blow the fuse but a constant say 19.5amp draw which heats up the fuse element and hence melts the plastic case. This could be caused by several things, the fridge 12volt supply for running the electronics (Fridge computer not heating element) being one or an intermittent short circuit another. One thing for sure is the fridge 12volt heating supply will only be attached to the Cab battery via a relay controlled by the alternator d+. If the liesure battery is also connected to the fridge heater then someone has added that availability since the van has been produced. My guess would be that it is a problem with the fridge electronic control board or wiring to it but don't discount the poor fuse connection as in my experience the fuse holders are a source of a good many problems.

Good luck with you quest.
Phil.

P.s. In theory there should only be the 20amp fuse at the positive battery terminals, which is protecting the charge cct's from the battery's to the charge relay and Hab 12volt system. Any 12volt take off should come from fuses on your service control unit.

 

[VXman]

Hi VXman,

Thanks for your thoughtful response. I don't imagine that there has been any alteration to the motorhome electric as I bought it brand new from the dealer - unregistered so I am the first owner.

The way it is set up it is not possible to manually select leisure battery. If you try this the fridge just flashed an error. The manual states it can only be used when the engine is running. The EBL does have a 20 amp fuse for the Fridge but weirdly this doesn't seem to affect anything whether it is in or removed. I wonder whether there is a facility to run it off the leisure battery but the fridge doesn't allow it???

Regarding the power to the fridge 'computer'/display this is not affected by the 20 amp fuse on the leisure. Again, I am not sure what it does as it doesn't affect the running of the fridge whether it is in or out - or melted!

I do not believe there is a problem with this aspect of the fridge as I can use the fridge normally on Gas or 240v.

It does seem as the problem is related to the 12v heater element as the problem only comes when the fridge is running off 12V with engine running. As soon as I select Gas manually (with the engine still running) the problem disappears, fridge runs fine and the leisure battery voltage returns to normal.

The motorhome manual doesn't show any 20 amp fuse by the leisure battery (or a 1 amp that is there.) I think I will email Burstner and ask what those fuses are for and see what they say.

The only mention of a fridge related fuse in the manual (other than the one on the EBL) is a 20 amp by the starter battery which I believe is ok ( haven't looked recently as it is less accessible) Maybe this provides power to the fridge for the fridges electronics but that would need to be active at all times. I will have to experiment by taking this out at some point.

 

[Squiffy]

Thanks for your thoughtful response. I don't imagine that there has been any alteration to the motorhome electric as I bought it brand new from the dealer - unregistered so I am the first owner.

Hmmm, it would seem to me that your looking for a double coincidental fault, if the 20amp liesure battery fuse doesn't have any affect to the fridge electronics then there would seem to be a separate fault, especially as you say there is no connection to the fridge heating element. As to the melting fuse I'd defo check the fuse connection particularly where the fuse blades enter the fuse carrier, or even change the fuse carrier completely for a new one. Phil.

 

[VXman]

Re fuse holder, that is the plan. The fuse is completely melted inside and cannot be removed so I've little choice.