Having a Fridge Electrics mare

[dunnah01]

The 240v element on our Dometic RMD8505 has failed and I've had to replace it. Unfortunately it now trips the EHU when turned to 240v. On closer examination the rear PCB looks to be toast on the AC line but works fine on gas and 12v so I'm looking to 'hot wire' it to 12v as a temp measure (we're full timers so can't be without fridge freezer for long) until I can find a new pcb. The problem is that the D+ signal is showing as reversed polarity which makes me reluctant to bridge it to the 12v supply to trick the fridge into thinking the engine is running. Does anybody have any idea why D+ may be reversed??
We're on permanent EHU so the battery drain on 12v isn't a major concern for now.

 

[Lenny HB]

If the PCB failure you refer to it the black box on the back of the fridge as the are common fault.
Why can't you just switch the fridge to 12v?

 

[dunnah01]

If the PCB failure you refer to is the black box on the back of the fridge as the are common fault.
Why can't you just switch the fridge to 12v?

It only runs on 12v if the engine is running but we're parked up full time on free solar EHU so want to use leccy rather than the very precious lpg

 

[autorouter]

There are two kinds of engine running signal. The standard D+ is +12V when the engine is running and about zero when not running. The other type, called a D-, is what is called an 'active low'. Imagine a switch that connects to the earth (ie negative) when the engine is running, but is disconnected (floating) when not running.

If it's the D- type, then connecting it to 12V negative (ie chassis) will activate it, and disconnecting it will de-activate it. If you make sure the 'engine running' signal on the fridge input is not connected, then try connecting it to earth (chassis), then it might start working on 12V. If it doesn't then it may be the other type, ie D+.

I can see why you are reluctant to bridge it to +12V. You definitely need to make sure the D+/- from anywhere else is disconnected before you try wiring it as a D-. Otherwise you will blow the D+ fuse if you are lucky, or damage the D+ alternator signal if you are not.

 

[RedFrame]

Can't quite picture what you mean regarding reversed polarity.

On an 8505 there is a connector for D+, if 12v is supplied to this connector then it will run on 12v.

On an 8505 you can also use a 12v input on S+ to run on 12v.

The fridge is certainly not expecting D+ to be grounded, there should be no 12v signal on D+ at all if the van is not running, have you tried disconnecting the signal wire from the pcb and testing? Perhaps its the fried pcb that is creating the back feed?

Edit: Last para added

Cheers
Red.

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[Lenny HB]

On an 8505 there is a connector for D+, if 12v is supplied to this connector then it will run on 12v.

The way his van is wired it would run off the starter battery.

On an 8505 you can also use a 12v input on S+ to run on 12v.

Only if you have a perminant 12v supply wired to the fridge his van doesn't.

 

[Lenny HB]

It only runs on 12v if the engine is running but we're parked up full time on free solar EHU so want to use leccy rather than the very precious lpg

There are connections on the back of the 8505 for a perminant 12v supply you could wire these direct to your hab battery via a 20 amp fuse then it will run on 12v when you select it manually on the fridge control panel. Use at least 6mm sq cable.

 

[RedFrame]

The way his van is wired it would run off the starter battery.

Only if you have a perminant 12v supply wired to the fridge his van doesn't.

My point Lenny is that the fridge is expecting +12v on those connections.

I only mentioned the S+ as he could provide 12v on that to "trick" the fridge

Cheers
Red

 

[mitzimad]

if you wire the 240 direct to the element it must work but no thermostatic control

 

[dunnah01]

Mystery solved and I'm embarrased to admit it - SWMBO didn't think it mattered which way the cables were plugged into the multimeter when she was tidying it up yesterday. Now swapped around and showing correct polarity. I'll try hot-wiring again in the morning.
Thanks to all for taxing your grey matter and providing some more food for thought

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[Lenny HB]

But it only works if it has been suitably wired.
Mine is and I make use of the S+ which is connected to the AES output on my solar regulator

 

[Lenny HB]

if you wire the 240 direct to the element it must work but no thermostatic control

Those elements are not known for their longtivity, I doubt it would last long permantly on.

 

[RedFrame]

Mystery solved and I'm embarrased to admit it - SWMBO didn't think it mattered which way the cables were plugged into the multimeter when she was tidying it up yesterday. Now swapped around and showing correct polarity. I'll try hot-wiring again in the morning.
Thanks to all for taxing your grey matter and providing some more food for thought

Now that is funny...

Personally, I'd try running a permanent 12v feed to the S+ rather than D+ (that's just me), but either should suffice depending on damage to the PCB, as you probably know using either D+ or S+ will enable the fridge to continue to be thermostatically controlled.

Good luck, hope you get it sorted!

As for a replacement fridge I believe Drexxer has just obtained a replacement at a reasonable price, he may be able to give you specs...

Cheers
Red.

 

[Lenny HB]

Personally, I'd try running a permanent 12v feed to the S+ rather than D+ (that's just me), but either should suffice depending on damage to the PCB, as you probably know using either D+ or S+ will enable the fridge to continue to be thermostatically controlled.

That won't do anything if the fridge is not wired with a perminant supply.
If it is wired to a perminant supply all you have to do is press the 12v button

The S+ plus is just for a trigger signal for the solar it doesn't use any power apart from a few millamps.

 

[dunnah01]

That won't do anything if the fridge is not wired with a perminant supply.
If it is wired to a perminant supply all you have to do is press the 12v button

The S+ plus is just for a trigger signal for the solar it doesn't use any power apart from a few millamps.

Fridge is wired to a separate 170w 12v element by what look like 6mm wires off the leisure battery side so just needs the low amp trigger to allow running whilst engine is off .
240v is a separate 190w heating element but no way to get it working with fried pcb

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[dunnah01]

This is the PCB with a couple of fried 39k resistors on the AC side. The fat wires are the 12v element supply side

 

[dunnah01]

Well, tried providing 12v (& up to 14.4v) to both D+ and S+but nothing will trigger the fridge to work on manual 12v without the engine running .
The mystery is that it works fine on 12v with the engine running and the alternator D+ connected or a dedicated wire from the starter battery to the D+ connector. There must be another engine running trigger or VSR somewhere which I'll need to do a bit more digging to try to locate.

 

[Hoovie]

Well, tried providing 12v (& up to 14.4v) to both D+ and S+but nothing will trigger the fridge to work on manual 12v without the engine running .
The mystery is that it works fine on 12v with the engine running and the alternator D+ connected or a dedicated wire from the starter battery to the D+ connector. There must be another engine running trigger or VSR somewhere which I'll need to do a bit more digging to try to locate.

You may have already answered this (but I couldn't see it in the posts) ... do you have a 12V High-Power (as opposed to control) supply to the fridge without the engine running? The 12V supply is usually from the Engine Battery and there would be a Relay elsewhere that is activated by the D+ signal to connect the power cable. So not only would the fridge itself need to see the D+ signal, the 12V supply from Engine to Fridge would need to be activated.
In my Motorhome for example, the Fridge (Thetford in my case) would have the D+ control coming into the rear of the fridge alongside the 12V power cable, and the D+ would also activate a relay in a compartment behind the cab (and well away from the fridge).

 

[Richard n Ann]

This has a simple solution, Just switch the fridge to 12v and wire a thermostat module to the new mains heater element.

When you drive it's on 12v. When you plug in ehu the new stat powers the mains element.

Elitech Digital Temperature Controller Thermostat AC110V-220V External Temperature Sensor -50 ℃ ~ 99 ℃, STC-1000X Amazon product ASIN B087NF5LVX

 

[Lenny HB]

Well, tried providing 12v (& up to 14.4v) to both D+ and S+but nothing will trigger the fridge to work on manual 12v without the engine running .
The mystery is that it works fine on 12v with the engine running and the alternator D+ connected or a dedicated wire from the starter battery to the D+ connector. There must be another engine running trigger or VSR somewhere which I'll need to do a bit more digging to try to locate.

The heavy 12v connections will be switched via a relay when the engine is running.
I think my fridge as another pair of connections (brown & white) permantly connected to the leisure battery via the EBL.
Can't check until tomorrow as we are out for the day, I got dragged screaming to Benidorm.

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[dunnah01]

You may have already answered this (but I couldn't see it in the posts) ... do you have a 12V High-Power (as opposed to control) supply to the fridge without the engine running? The 12V supply is usually from the Engine Battery and there would be a Relay elsewhere that is activated by the D+ signal to connect the power cable. So not only would the fridge itself need to see the D+ signal, the 12V supply from Engine to Fridge would need to be activated.
In my Motorhome for example, the Fridge (Thetford in my case) would have the D+ control coming into the rear of the fridge alongside the 12V power cable, and the D+ would also activate a relay in a compartment behind the cab (and well away from the fridge).

The thick brown and red wires in post #16 are for high power 12v element and need to be activated by the relay on the left of the board which needs to be triggered by the D+ (or S+) - If I try to switch to 12v manually nothing seems to trigger the relay unless the engine is running. I think this may be a deliberate feature to only allow 12v either through AES or manual selection when the engine is running due to the relatively high power of the 12v element (170w) compared to many fridges.

The heavy 12v connections will be switched via a relay when the engine is running.
I think my fridge as another pair of connections (brown & white) permantly connected to the leisure battery via the EBL.
Can't check until tomorrow as we are out for the day, I got dragged screaming to Benidorm.

I can't find any any permanent heavy 12v connections to the leisure battery, only the relay switched ones unfortunately.

Benidorm! an acquired taste

I've managed to find a near match board (50-052003-H vs my 50-052003-L), the only difference seems to be a connector reed switch and 12v supply for the freezer compartment light. I think I'll admit defeat and give this PCB a try. It runs fine on gas so I'll just have to bite the bullet and hope we don't run out or can get a refill before we head off the France in a month.

Thanks again for all the suggestions and I'll keep you posted on how I get on with the new board

 

[Lenny HB]

Benidorm! an acquired taste

2 hours I've had enough can't see the attraction.

 

[Richard n Ann]

2 hours I've had enough can't see the attraction.

Get a few pints down you and enjoy wish we were there

 

[Lenny HB]

Get a few pints down you and enjoy wish we were there

It's crap not a place I would never visit again, more British Northerners than Spanish it's cold (19°) raining on and off and they are in shorts so uncouthed.

Edit:
Just realised you are one of them that's why you like it.

 

[Spriddler]

Get a few pints down you and enjoy wish we were there

Haven't you got a local Wetherspoons?

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[Hymerboy]

The 240v element on our Dometic RMD8505 has failed and I've had to replace it. Unfortunately it now trips the EHU when turned to 240v. On closer examination the rear PCB looks to be toast on the AC line but works fine on gas and 12v so I'm looking to 'hot wire' it to 12v as a temp measure (we're full timers so can't be without fridge freezer for long) until I can find a new pcb. The problem is that the D+ signal is showing as reversed polarity which makes me reluctant to bridge it to the 12v supply to trick the fridge into thinking the engine is running. Does anybody have any idea why D+ may be reversed??
We're on permanent EHU so the battery drain on 12v isn't a major concern for now.

This company will rebuild your pcb . enquiries@apuljackengineering.co.uk tel 01278588922. Hope this helps,Jack.

 

[dunnah01]

I've managed to find a near match board (50-052003-H vs my 50-052003-L), the only difference seems to be a connector reed switch and 12v supply for the freezer compartment light.

Just a bit of housekeeping. The difference between the two boards is that the H version (& E version) has a 2 pin connector X114 which supplies an electronic door lock which is activated on some single door fridges whilst driving. The L version replaces this with a 3 pin X115 connector which supplies 2 LED indicator lights on the front panel of 2 door digital display fridge/freezers to indicate that the fridge is either a) blue light normal, or b) flashing red light error. This is presumably because the digital display on 2 door versions is hidden behind the freezer door whereas on single door versions the digital display is always exposed so there is no need for the extra LEDs.