Thetford fridge does not work off battery

[markgedinburgh]

I have a Thetford N3000 series fridge in my Pegaso 590. It works absolutely fine on electric hook up and when connected to my gas cylinder. However when I want to drive and turn off the gas and turn on the engine the fridge will not connect to the battery. I have the fridge set to automatic setting. When I start up the engine the fridge tries to connect to gas, shows this setting for about 15 seconds and then the display starts flashing. If I try to connect to battery it keeps flashing. I have had a look at the fuse box and the fuse seems OK. Swapping the 30A fuse does not help.

Has anyone an idea how to solve this problem?

 

[David and Sally]

Are you holding the battery button on the fridge long enough?

Is there a trigger signal wire from the alternator/EBC that tells the fridge to connect to the main 12v supply? Could that have come loose?

 

[markgedinburgh]

Yes, have held it down for up to a minute without any joy.

 

[David and Sally]

The real experts are having their afternoon nap but their nurses will be waking them up soon and they will solve it in no time

 

[GeoffnDee]

There is normally 2 x 12v fuses for a fridge 1 x about 20 amp or more for the 12v heater element and 1 x around 2 amp for the fridge control system what make and model of PSU do you have ? ,

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[markgedinburgh]

Nordelettronica NE196-T

 

[XxAnthxX]

Obviously you have 12v (2-3amp) there for the control system so,

I‘d have a look at what’s happening with the D+ (engine running) signal line to the fridge, it sounds as though the fridge is not recognising the engine starting and not allowing the main 12v (20amp) power to the element.

 

[David and Sally]

Is it worth wiring two 12v connections - with suitable fuses direct to a battery to test the fridge is actually working?

 

[markgedinburgh]

The fridge is definitely working. Food chills and the freezer makes ice

 

[David and Sally]

The fridge is definitely working. Food chills and the freezer makes ice

I meant on 12v. Should have made that clear.

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[GeoffnDee]

Is the fuse F3 (7.5amp) and F2 (20amp) ok. also are the multiplugs on the PSU pressed in ok (JP13 , JP6 , JP4)

 

[autorouter]

The power to the 12V heater element that powers the fridge on 12V comes from that 20A fuse GeoffnDee described. It should be on when the engine is running, and off when it stops. It goes from that NE196-T box to the back of the fridge. This supply is entirely separate from the low-current supply that powers the fridge control board, which selects the power source and controls the gas ignition etc.

Usually you can access the back of the fridge by removing the lower external vent. You can probably access some terminal blocks there to chack that power is reaching the fridge from the NE196-T box. If it is, maybe the 12V element has failed. Replacement is usually reasonably easy, I'm sure there's Youtube videos on it.

 

[markgedinburgh]

Many thanks to all for your comments. The fuse and multiplugs are fine and given my limited electrical expertise and the possible problems I think I'll need to get some external help to fix it.

 

[markgedinburgh]

Just an update on the fridge. I took it to be looked at by an autoelectrician. He took the vent off behind the fridge and had a look at the electrics there and the 20 amp fuse was toasted. Part of the circuit board had suffered damage. You can see it in the 2 pictures. He replaced the fuse and the fridge now runs off the battery when the engine is running. He was quite concerned because when he tested the circuit the fridge was running at between 16.8 and 17 amps and the fuse on the main board and the device both are just 20 amps which he thinks is too low. He has advised me to get in touch with Thetford or a Thetford dealer because he feels it is a problem that could happen again.

 

[Jesspekjon]

Hi, where abouts is the fuse? I have the same problem at the moment. I have the vent panel off but can't find the fuse? Thanks

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[scotjimland]

the circuit the fridge was running at between 16.8 and 17 amps

12v element is about 125w (depending on model ) which will draw about 10A ..

doesn't sound right, but check the fridge specs for wattage (W) of the 12v element ..

W dived by 12 = current, in Amps

 

[pagey]

Hi, where abouts is the fuse? I have the same problem at the moment. I have the vent panel off but can't find the fuse? Thanks

Take off the lower fridge vent and you should see a black box remove the cover and you will see the circuit board above undo the 3 screws prise the board free and you will see 2 fuses on the board one is a 2 amp and the melted one is the 20 amp it's a common problem and the board has been updated now

 

[autorouter]

I agree with that auto electrician, if it's taking up to 17A then it could happen again. I don't know why it's so high, maybe the wrong heater element with a higher power has been used. Maybe the control board was tested as OK for a smaller fridge with a 100W element, and they just used the same board with a bigger fridge with a 200W element. Either way, the standard way to allow a board to control a bigger current without burning it out is to outsource the switching to a relay. All the high current passes through the relay, not the board. All the board does is provide a small control current to the relay coil, to switch it on and off. Something like the diagram. An inline fuse, a bit higher than 20A, is a good idea.

 

[pagey]

I agree with that auto electrician, if it's taking up to 17A then it could happen again. I don't know why it's so high, maybe the wrong heater element with a higher power has been used. Maybe the control board was tested as OK for a smaller fridge with a 100W element, and they just used the same board with a bigger fridge with a 200W element. Either way, the standard way to allow a board to control a bigger current without burning it out is to outsource the switching to a relay. All the high current passes through the relay, not the board. All the board does is provide a small control current to the relay coil, to switch it on and off. Something like the diagram. An inline fuse, a bit higher than 20A, is a good idea.View attachment 803678

Iv read that the problem is a design error in that the tracks on the board are too fine , the updated one is built stronger , I can't verify this as I read it but my fridge has the same problem and I saw this on the net

 

[autorouter]

Iv read that the problem is a design error in that the tracks on the board are too fine , the updated one is built stronger , I can't verify this as I read it but my fridge has the same problem and I saw this on the net

If the new control board is free of charge, and also so is the fitting, then that's the best way forward. But it would be cheaper to add a fuse and relay as in the diagram if you have to pay for a board or the fitting of it. And in my opinion a better solution anyway.

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[IRHP]

See post 16. https://www.motorhomefun.co.uk/forum/threads/dometic-3-way-fridge-issue.275970/ Thetford give a 3 year guarantee, I wrote to them and despite being just out of warranty they sent me a claim form. I filled it in and got a new upgraded pcb FOC.