Dometic Fridge/Freezer on 12 volts

[coach2000]

I have a problem with my Dometic Fridge/freezer if anyone can help please. Does anyone know of any other fuses than in the fusebox behind the drivers seat. It is a 3 way and when it changes over from gas to 12 volts when on auto a buzzer sounds behind the centre panel.

 

I have checked the connections behind the outside grill and there is no 12 volt supply on any of the terminals when the engine is running. This is the main supply before the relay. This seems strange because the controls on the front still work.

 

Clive.

[Robinhood]

I can't help with the location of the fuses, but I can possibly clarify your current 12V situation.

 

Dometic fridges with electronic controls have those controls supplied by a separate (permanent) 12V supply to that used by the 12V heating element.

 

The symptoms and warning you're experiencing are consistent with the control circuit being complete, but the heater circuit not so. (though that would imply a working 12V supply on the control circuit, which you're implying might not be the case? - the warning buzzer implies it is getting power).

 

The control circuit will be fused by a relatively low-value fuse; the heater circuit will be fused by a significantly larger one, 20 amps or more.

 

It would appear that the control circuit may still be functioning, whilst the heater circuit is not. The fuse for that heater circuit may be in the location you've identified (but will be separate to that for the controls) or may be somewhere else in the separate circuit (I would guess at somewhere near the leisure or vehicle battery).

 

Dometic advice has been to wire the heater circuit through an additional, external relay only operational when the engine is running (to protect the battery if the controls are switched to 12V manually) but I know for a fact that this is often omitted by converters (the installation relying, in auto mode, on the fridge's electronics). I mention it only in case CI have installed the additional relay, which would mean another item to find and check.

 

There is always a possibility that the electronics have failed (a not uncommon, and expensive issue), but elimination of circuit problems is a must before assuming this.

 

Can you identify and post the fridge model?

[Keithl]

From reading other past posts on the forum converters regularly fitted charge and fridge relays on your era of Fiat conversions (ie Pre X250) under the bonnet, located centrally immediately below the windscreen and under a plastic cover.

 

Try looking for 'add on' wires from the starter battery and follow them looking for relays and fuses.

 

Keith.

[Robinhood]

Keithl - 2020-05-16 9:09 AM

 

From reading other past posts on the forum converters regularly fitted charge and fridge relays on your era of Fiat conversions (ie Pre X250) under the bonnet, located centrally immediately below the windscreen and under a plastic cover.

 

 

 

...the most common location for Swift Group conversions, Keith, but not necessarily others. ;-)

[Derek Uzzell]

I’m guessing that Clive’s Dometic fridge/freezer is one of the RM 76x5 or RM 78x5 models.

 

I’m not sure if it would help with his problem, but there Operating Instructions for those appliances here

 

https://www.manualslib.com/manual/785941/Dometic-Rm-7601-L.html#manual

 

 

[Robinhood]

Derek,

 

my guess would be an RMT 7655 or equivalent (though the timeline for that 'van is, I think, possibly tight, as the fridges date from about 2004/5 I think).

 

I have the operating and installation/service manuals for that, but further possible guidance depends on knowing the actual model.

 

(the RMT76** AES series has the 12V control electrics and heater electrics separated on different/separate supplies and terminal blocks, and if this is the case, it would be interesting to see which blocks show no power on testing).

[coach2000]

Thank you all for the in depth replies. I think it i a RMT 7600 series. I will check the plate when I go to the storage again. The next thing I was going to do was lift up the fuse panel and check the pcb board below and connection plugs. It is an awkward job because of its position. Hopefully I will be able to tell if the 12 volts is going out on the circuit supplied by the 20 amp fuse.

Also if anyone has had experience of removing or servicing the burner jets on the built in oven above I would be grateful.

 

Clive.

[Robinhood]

Clive,

 

the terminal blocks on the back of an RMT7655 (T in that because it looks like you have the tech-tower version with the oven above) should probably be as per the attachment. (there were variations by year).

 

The 12V supply to the control electronics and the 12V heater for the fridge are wired entirely independently.

 

I was slightly confused by the three sets of terminal blocks in the attached picture, but from another wiring diagram I believe the permanent 12V connection is wired into the lower LH block, and existing fridge wiring passes this forward up to the upper block (C/D corresponding).

 

The heater supply is wired into the lower RH block, and may be a permanent 12V supply, or if wired to Dometic recommendation, should be an ignition switched supply via a relay somewhere before connection. (Dometic recommendation is not always followed - it isn't on my 'van).

 

It seems you are capable of checking power at the terminal blocks, so I would suggest:

 

i) Check the permanent 12V supply at the lower LH terminal block. As you are hearing an alarm, my expectation is that the electronics are supplied. If there is no power (then I would be surprised because of the alarm but,) check for a lower-rated fuse in your fuse array, which may be shared with other items.

 

ii) With the engine running, check the (nominally) ignition switched supply to the lower RH terminal block.

 

If this has a supply (and your previous posts imply not) then the next suspects are the 12V relay controlling the heater (the next item in the circuit after the connector block), or the electronic switching controlling that relay (hope it isn't).

 

If there is no supply on the heater terminal block then possibilities (in no particular order) are

- blown 20amp + main fridge fuse (not the same as the permanent supply fuse above)

- failed additional ignition control relay (if fitted as per Dometic recommendations)

- failed D+ signal to above relay (where fitted)

- broken circuit anywhere in the main heater wiring to the connector block

 

[coach2000]

Hi Robinhood. The connections are simpler than the ones you show. There is only one terminal block that is the lower right one A and B

If I remember right the large L and N cables and a N relay wire are connected to these, not sure if a wire from L went into the fridge and a return to the relay.

 

Clive.

 

[Robinhood]

Unless there was an early change to the design, the RM 76** series definitely had separate connections for the control and heater supplies.

 

The previous RM 65/7** series, however, only had a single 3-way connector block with +/- and a D+ signal (the supply to both the controls and the heater being common).

 

The RM67** series has rotary controls for energy and temperature selection. The earlier RM67** series has "push" controls overlayed by a plasitic panel.

 

If it is this previous series, then there definitely shouldn't be an additional relay in the supply circuit, and the fridge 12V should be connected directly to the leisure battery(ies), so you can discount that bit of trouble-shooting..

 

I'm still confused that you think there is no supply at this connection, however - the fact that you have working controls implies that, if it is a common connector block, power is getting through. I would be inclined to double-check this supply, certainly check the D+ signal at the connector block, and reseat the fridge fuse.

 

I do think, however, that these fridges "alarm" by lights, rather than a buzzer. There are various postings on the web detailing a buzz noticeable from the front but caused by the relay on the back (or more worryingly, by the electronics - there are definitely relays on the RM76** control board) in the case where there is either a dodgy/intermiitent 12V supply (or more likely the D+ supply) or the relay is failing. I would also check and reseat the relay (and listen with the engine running to see if the relay is causing the buzzing).

[coach2000]

Robinhood. It is the earlier one with rotary knobs. There is no D+ connection, only A and B. I have checked and re seated the 20amp fuse.

When you say the D+ relay. Is it mounted at the back of the fridge, or below the fuses. The relay at the back of my fridge is not mounted on a board. It just has the crimps pushed on.

[Robinhood]

If it has the rotary knobs, it is most likely to be the (later) RMT7655 TecTower. (they were introduced 2004/5). I have assumed that in the following.

 

In this case, the connector block you're referencing is the 12V heater supply, (the bottom right one in the diagram I've posted previously) and this does only have positive and negative wires to A and B..

 

But, for an AES fridge, there will definitely be a further, permanent supply (C and D in the same diagram) and a D+ supply from the alternator, which drives the electronics to switch to 12V when the engine is running.

 

The attached picture from the maintenance manual, whilst indistinct, shows the two lower terminal blocks from the previous diagram. Given that the three other wires C, D and D+ from the lower left block are shown on the wiring diagram simply passed to the corresponding connections on the upper terminal block, there is just a chance that this was a modification for ease of wiring, that yours is simply wired straight into that upper block, and the lower doesn't exist.

 

I would want to know that C/D and D+ were all connected and working, but only after other possible issues had been eliminated.

 

Given that you say that there is no power on the A/B connections, I think you need to re-check as I've posted above, viz:

 

With the engine running, check the (nominally ignition switched) input supply to the lower RH terminal block (A/B).

 

If this has a supply (and your previous posts imply not) then the next suspects are the 12V relay controlling the heater (the next

item in the circuit after the connector block, tagged on the back of the fridge, as you say), or the electronic switching controlling

that relay (I hope it isn't).

 

That 12V relay on the back switches in response to the D+ signal from the alternator, which is first fed to

the fridge electronics before being fed back to the heater relay.

 

If there is no supply on the heater terminal block (A/B) then possibilities (in no particular order) are

- blown 20+amp main fridge fuse (not the same as the permanent supply fuse above)

- failed additional ignition control relay (if fitted as per Dometic recommendations)

- failed D+ signal to that ignition control relay (where fitted)

- broken circuit anywhere in the main heater wiring to the connector block

 

BUT

 

The buzzing, however, makes me think that there is a possible problem with a relay.

 

The heater relay directly in circuit just after the connections A/B is, as you say, just tagged onto the back of the fridge. Try setting the fridge to auto and starting the engine and check whether this is where the buzzing is coming from. If it is, then the likelihood is a failed relay (a cheap generic fix) or a poor D+ signal (which I think is 12V on the "85" terminal of that relay)..

 

If it isn't then the buzz may well be coming from the fridge control PCB, which most certainly also has relays on board. Looking at the wiring diagram I think the PCB has a relay that sends the D+ (or the S+ if solar is connected) to the heater relay when auto or 12V are selected. If this relay is stuck it is likely to buzz (and may prove costly to remedy).

 

In case it helps, the service manual I'm using is downloadable from here:

 

https://www.4mcculloch.co.uk/images/mediator/559/599523023.pdf

 

It has connection instructions and (internal) wiring diagrams.

 

If the buzzing is coming from the control electronics, then elimination of other problems is still vital, in case they are having a knock-on effect (e.g. a poor D+ signal causing relay flutter), but failure of the PCB is not uncommon, and replacements are expensive. I diagnosed a 230V issue in a friend's RMT7655's PCB, and we managed to get it fixed (very) cheaply by identifying the failed component, a relay in fact, and replacing it. This may or may not be an option on yours (but I wouldn't want to raise your hopes).

[Brian Kirby]

Just a thought. The van is CI (Italian). Are the electrics by any chance CBE?

 

If so, the D+ is simulated from the DS 300 distribution box, and is supplied by a 3A fuse specifically dedicated to the AES fridge supply (Fuse 10 in the CBE manual). There is also a link between two terminals on the DS 300 that must be in place with an AES fridge. If either is defective, the AES function won't work. Any use?

[Robinhood]

Brian Kirby - 2020-05-17 5:16 PM

 

Any use?

 

I don't think so, Brian. The buzzing makes me think there's another issue.

 

Your post did, however, remind me of another thing to try; Instead of letting the fridge switch to 12V using "Auto", what happens with the engine running and the panel switched to 12V manually? (My suspicion is that it might exhibit the same buzzing, but it's worth a try).

 

(and the electrics might be Nordelletronica ;-) )

[Brian Kirby]

Ah well, Bob, it was but a shot in the dark. :-)

 

All the best.

[coach2000]

Hi Brian What are CBE electrics?

 

[coach2000]

Robinhood I will try on 12 volts manual and check the readings again and listen for buzzing sound. On auto there is no 12 volts on A and B. No 12 volts on the wires to the supply coil . So even if the rely operated there would be no 12 volts from A and B to complete the power circuit.

This is why I think there is a break in one of the 12v circuits coming from further forward. ie fuse control panel or seperate fuse.

There is not a lot more I can do until I can get back to the van as it is a fair distance from where I live.

 

Thanks.

Clive.

[Brian Kirby]

coach2000 - 2020-05-17 6:44 PM

Hi Brian What are CBE electrics?

It's an Italian firm (now American owned by DexKo, who also own Al-Ko and quite a few others!) who make caravan/motorhome 12V/230V electrical installations - generally supplied to manufacturers as kits complete with prefabricated wiring harnesses for ease of installation. Mains switch with RCD and MCBs, intelligent battery charger/power pack, main 12V control panel, and 12V distribution panel with fuses.

[coach2000]

Robinhood! I found the C and D connections hidden at the back of my spare key safe.

I managed to do some extra checks yesterday and found that the J4 plug for the fridge had come off the PCB under the fuse control panel. This stopped the buzzer but did put the 12 volts back onto C and D. These terminals were dead until replacing the plug under fuse-board.

However there is still no 12 volts on A and B with the engine running or when the fridge is switched to 12v or auto.

As far as I can tell there is 12v going out from the PCB plug but does not get to A and B. I am reluctant to take out the fuse PCB to get to the rats nest of wires below in case I cause more problems.

That's all I can do for this week. Once again thanks for your help with the diagrams.

 

Clive.

[Robinhood]

I'm still rather confused how you are testing these, since A/B and C/D are connection points that don't rely on the functioning of the fridge, or any loose plugs thereof.

 

 

A/B are the direct external high-current 12V supply for the 12V heater. Power on these is dependent only on the external wiring continuity, and possibly, dependent on whether there is an external relay or not, the fact that the engine is running.

 

C/D (and the associated D+) are lower current and supply the fridge electronics and switching logic. C/D should show a permanent external power supply.

 

If there really is no current on A/B, it shouldn't be an issue with the fridge itself (I'm not discounting other issues in the fridge electronics, it is simply that A/B are the external supply points, and thus should be powered from elsewhere).

 

A/B should be directly connected to the leisure battery(ies), (or unconventionally the vehicle battery) but., external to the fridge the positive to B should definitely have a 20amp (or so) fuse in line (and this is potentially the fuse you've found, but there may be another in-line somewhere) and again external it may have an additional ignition controlled relay as well (Dometic recommend this, but some converters omit it).

 

If there really is no power at A/B with the engine running you need to check full wiring continuity, the fuse(s), and, if fitted the additional relay function.

 

Your original post implied that the fridge was attempting to switch with the engine running. It does this in response to the D+ signal (which should be connected in the same block as C/D) so if switching is being attempted it appears a D+ signal is reaching the fridge electronics. This gets passed to the relay on the back of the fridge to switch in the 12v heater. For peace of mind I think I'd want to check continuity on the 87/30 terminals of the relay with the engine running to ensure it is actually switching.

[coach2000]

Just an update on the no 12v problem. I have had other jobs to do so finally got back to have another look. After going under the van and concluding that the supply cables were inside the inside the insulated floor, I decided to be brave and have another look under the fuse control panel to see if the cables were visible on exiting the floor.

I found them straight away as there was another plug that had come out and was missed the 1st time. The cables from this plug were identical size and colour to the appliance end. The plug itself was difficult to spot not being on the edge of the PCB and was not an obvious shape for power cables.

All's well that ends well so thanks once again for everyone's help.

 

Clive.