Another fridge problem, no 12volt. (1 Viewer)

Oct 2, 2014
585
1,726
West Yorkshire
Funster No
33,675
MH
Rapido 866f
Exp
Since 2014
Rapido 866f 2014 on Fiat.
Dometic 8555 fridge (AES) possibly about 4 years old, replaced by a previous owner.

Hi, After a visit to Ferry Meadows, Peterborough where we had the fridge on electric hook up, we set off for home. Our check list, which for once we followed completely, has its final item as “check fridge” which involves Mrs RoR looking back at the fridge as I drive away to make sure the warning light has gone off and the fridge is presumably on 12v.
All good and off we go.
About 10 minutes later the fridge is beeping and flashing red. For the remaining 3 hours despite our efforts, switching on and off, restarting, etc , it remained so.
Once home, Checked fuses for fridge 20amp and 3 amp for D+ link, all good. These are at the CBE 12v fuse panel at the back, couldn’t find anymore fuses for fridge anywhere else.
Removed vent at back of fridge and with engine running and fridge on auto. Checked power supply (thick wires) to fridge they showed about 6 volts. Ah, alternator. Waited until dark and batts had settled to 12.8, turned engine on batts increase to 14.2. Alternator ok.
So then checked D+ link to back of fridge, also 14.2.
Does fridge work on 230volt?
Fridge and freezer both at 6C, then after 2 hours Fridge 4C good, Freezer -13C, so obviously working although freezer unimpressive.
Sat and read the Dometic guide books, they have a vague suggestion of “ heating element” and consult Dometic dealer.
Thought I might consult MotorhomeFun Knowledge team first. Is there anything else I should do, any alternative theory. TIA.
 
Oct 10, 2018
1,987
1,123
Bracklesham Bay, West Sussex
Funster No
56,646
MH
PVC
Exp
Since 2005
The fridge will work on 230v when on EHU using the 230v heating element , 12v with the engine running using the 12v heating element and gas if not on EHU and engine not running. With the engine running there should be starter battery voltage at the low current 12v D+ and high current 12v power input supply connections at the power control on the rear of the fridge
There may be another 20amp fuse near the starter battery for the high current 12v power supply

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Oct 10, 2018
1,987
1,123
Bracklesham Bay, West Sussex
Funster No
56,646
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PVC
Exp
Since 2005
To add , if your fridge has aP860 electronic control on the rear , I think you will find that , as well as the D+ signal with the engine running and an electronics 12v supply , there is also a high current heating element 12v input to the P860 with or without the engine running.( with the 12v element supply output being active when the engine is running)
 
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RidersofRohan
Oct 2, 2014
585
1,726
West Yorkshire
Funster No
33,675
MH
Rapido 866f
Exp
Since 2014
To add , if your fridge has aP860 electronic control on the rear , I think you will find that , as well as the D+ signal with the engine running and an electronics 12v supply , there is also a high current heating element 12v input to the P860 with or without the engine running.( with the 12v element supply output being active when the engine is running)
IMG_0705.jpeg

Under the multi cable it says P 860. If the grey wrapped cables are leading to the heating element that is so far out of reach it would mean pulling the fridge out. Might be beyond my confidence levels.

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Dec 12, 2010
5,406
21,548
Cumbria
Funster No
14,651
MH
C Class
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since 2011
I think the thick orange and white cables are the supply to the 12 volt element. Can you disconnect them at the right hand side of the choc block and get an Ohms reading from them, that will tell you if the heating element is fubard or not ?
 
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RidersofRohan
Oct 2, 2014
585
1,726
West Yorkshire
Funster No
33,675
MH
Rapido 866f
Exp
Since 2014
I think the thick orange and white cables are the supply to the 12 volt element. Can you disconnect them at the right hand side of the choc block and get an Ohms reading from them, that will tell you if the heating element is fubard or not ?
Just so I’m clear. Do you mean unscrewing and removing the right hand side wires ( the supply side) and then putting the prongs into where they were ( into the choc block)?
Presumably no need to run the engine.! Thanks I’ll try that later this afternoon.

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Oct 10, 2018
1,987
1,123
Bracklesham Bay, West Sussex
Funster No
56,646
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PVC
Exp
Since 2005
The red and white cables should be the high current 12v into the P860 with the grey covered cables being the feed out to the 12v element, there should be 12v at the choc box connections (constant supply, you do not have to disconnect to check but have fridge turned off and also it is not needed to have engine running , ), Also if the white cover will come off you could check if 12v is available at the grey covered element outward supply cables ,( with the engine running , fridge on ), to supply the heating element and , if it is ok , you could then disconnect the element supply cables from the P860 and check the element Ohms reading from the cable supply end
 
Last edited:
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RidersofRohan
Oct 2, 2014
585
1,726
West Yorkshire
Funster No
33,675
MH
Rapido 866f
Exp
Since 2014
IMG_0709.jpeg

This is the m/meter set to ohms with probes in the block with a very good connection with the thick wires leading heating element. No engine, fridge off. See post #8. OL open circuit.
The thinner Permanent Supply wires at their choc block were at 14.0v., similar to battery voltage.
Then I removed the white/ grey cover where all these wires go and pulled out the heating element wires and set meter to ohms. With a fiddly connection to the heating element wires this read 1.2ohms, it was too fiddly to get a photo. I was hoping for 40+ after watching an American doing the same on you tube. When I googled Ohms law my calculator came up 0.84.. so closer than I thought but still 50% out.
Then switched the fridge on, and the engine on.
Voltage at the block was still at 6 volts as in post #1, and tried to get a reading at the white panel connections and still around 6 volts.
So not really sure now if it is the heating element, good job if it isn’t as the positioning of the element would require the fridge removing and I ain’t doing that as it means touching the gas supply.
Can’t understand the 6 volts when the engine battery is at 14+*, would have thought either Nil or near 14 not 6.
*when engine running.

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Last edited:
Oct 10, 2018
1,987
1,123
Bracklesham Bay, West Sussex
Funster No
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MH
PVC
Exp
Since 2005
What is the voltage at the thicker permanent red + white supply cables into the P860 without the engine running (Thetford give a resistance reading of between 0.61 and 0.86 as ok for some of their 12v elements so your reading may be ok )
 
Last edited:
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RidersofRohan
Oct 2, 2014
585
1,726
West Yorkshire
Funster No
33,675
MH
Rapido 866f
Exp
Since 2014
What is the voltage at the thicker permanent red + white supply cables into the P860 without the engine running (Thetford give a resistance reading of between 0.61 and 0.86 as ok for some of their 12v elements so your reading may be ok )
Just been out now at 1948, failing light.
The thickest cables at the top choc block picture #12, aren’t permanent and without the engine running are 0.001 volt.
The permanent supply cables at the bottom choc block are at 13.48 as solar has just about finished.
Both sets of cables go to the white cover as in picture #6 from their respective blocks.
Thanks again for your help.
 
Oct 10, 2018
1,987
1,123
Bracklesham Bay, West Sussex
Funster No
56,646
MH
PVC
Exp
Since 2005
The larger cables that go in the choc box where your probes are shown should be a 12v permanent supply from your starter battery into the P860 via the fuse panel fridge (20 amp ?) fuse ,the other permanent supply (which seems ok ) at the bottom choc box for the control electronics should be from your leisure battery via the fuse panel. (3amp ? ) It could be worth checking that the 20amp ? fridge fuse on the fuse panel has power going to it as you do not appear to have the 12v permanent supply at the larger red and white cables at theP860/choc box .
The red and white larger cables going into the P860 should be 12+v at all times and the 2 x (brown ? ) cables inside the grey sleeve going from the P860 to the 12v element should be 12+v with the engine running and 0v with the engine not running

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Last edited:
Oct 10, 2018
1,987
1,123
Bracklesham Bay, West Sussex
Funster No
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MH
PVC
Exp
Since 2005
RidersofRohan ,If you check the suggestions out , and no fault is found , the wiring information are the Dometic fridge wiring details when a P860 module is fitted but it is possible that the CBE fuse/distribution unit you have supplies power to the fridge connections through a different route .
 
Apr 27, 2016
6,870
7,983
Manchester
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42,762
MH
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Since the 80s
The heater element has failed. The ohms reading shows O.L (= Overload), on the MΩ range. That means the resistance is too high to measure, even on the megaohms scale.

The resistance of a 12V heater element should be about 1.5 ohms. That is quite a low resistance, near the lower limit of what a multimeter will reliably measure. But you should be able to distinguish a good heater element (about 1.5Ω) from an open-circuit failed one (O.L) or a short-circuit failed one (about 0.2Ω, or about the same as what you get when touching the two probes together).
 
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RidersofRohan
Oct 2, 2014
585
1,726
West Yorkshire
Funster No
33,675
MH
Rapido 866f
Exp
Since 2014
The heater element has failed. The ohms reading shows O.L (= Overload), on the MΩ range. That means the resistance is too high to measure, even on the megaohms scale.

The resistance of a 12V heater element should be about 1.5 ohms. That is quite a low resistance, near the lower limit of what a multimeter will reliably measure. But you should be able to distinguish a good heater element (about 1.5Ω) from an open-circuit failed one (O.L) or a short-circuit failed one (about 0.2Ω, or about the same as what you get when touching the two probes together).
Thanks for your reply. When I opened up the white box on the P860 and pulled out the connector to the heating element, I inserted the probes into the ends of the cable connector I had pulled out and it read 1.2 ohms.
Does the P860 box determine from the D+ link if the engine is running and shut the power to the heating element if the engine is not running? If it does then the O.L reading I first took at the choc block which is before the P860 might be wrong as the engine wasn’t running and so would the pathway to the heating element be closed off by the P860?
I really don’t know where the decision making on power to the heating element is made as the fuses for the fridge 30 amp and D+ 3amp are in the CBE board.
Waiting for the rain to stop and will try again.

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Oct 10, 2018
1,987
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Bracklesham Bay, West Sussex
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Since 2005
As far as I know the P860 controls the element power supply switching using the D+ signal , but it possible that the CBE unit only allows current to the P860 element heavy duty input connections with the engine running. If there is no power at the thick red/white cable connections at the P860 without the engine running but there is with the engine running this would indicate that the CBE unit could be controlling the power supply to the P860 12v element input connections.
 
Apr 27, 2016
6,870
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Sorry, I assumed those two wires went directly to the heater element, not the fridge controls. In AES mode, the fridge electronics will control whether the 12V element is powered or not, but in manual mode it won't control that. So a supply switched by a relay triggered by the D+ signal is required for manual mode.

So from your tests, the 12V power goes to the fridge electronics, but doesn't get to the actual heater element, which has the correct resistance. That's not looking good for the fridge control board. In manual 12V mode it should route the power straight through to the element.
 
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RidersofRohan
Oct 2, 2014
585
1,726
West Yorkshire
Funster No
33,675
MH
Rapido 866f
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Since 2014
Ok it stopped raining and I did the following tests.
1. Take out fridge 30amp fuse at CBE. Tested fuse resistance = 0.002 ohms.
2. Check voltage at fuse, +probe into fuse socket and -probe on negative post in Cbe unit. - without engine 0.00v
- with engine 14.4v (so the Cbe unit is the gatekeeper for D+ controlled power supplies).
3. Back of fridge. Take out thick supply cables from choc block, covering live ends. Re check ohms with engine running from supply side of choc block (recreating test done at picture at #12) = 33.02 ohms. (Yesterday test done without engine was O.L).
4. Check voltage at thick cable with engine running = 14.4v , without engine 0.0v (I was surprised at the 14.4 as yesterday it was 6.0)
5. Remove white cover at P860 unit and extract Heating element cable, re test ohms at removed connector = 1.0 ohms, re test again = 0.8ohms.
Fridge now working on 12v, ran engine and fridge happy showing battery symbol on Aes. Turned engine off and fridge correctly warned of no 12v, switch engine back on and fridge happy again showing as battery on panel.
I am just bewildered by this.
Perhaps disturbing the wires and fuses, re fitting them has tightened up a loose connection allowing the correct voltage through to fridge.
Or perhaps it’s an intermittent fault somewhere, perhaps in the P860?
Also I’ve a feeling my multi meter is not very accurate on ohms.
Thanks to all who have patiently helped me through trying to understand what does what, although I’m not really convinced this is the end of it!

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Oct 10, 2018
1,987
1,123
Bracklesham Bay, West Sussex
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56,646
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PVC
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Since 2005
Good to hear that it is now working , if you have not already done so , it would be worth cutting the 4 x red/white cable ends off and remaking the connections in the choc box as these are a possible point where the voltage loss may have been due to corrosion in the cable strands .
 
Apr 27, 2016
6,870
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Manchester
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42,762
MH
A class Hymer
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Since the 80s
Good to hear it's working, one remote possibility is that the battery in the multimeter might be on the way out, that would give bad readings as it fails. If not that then I agree with GeoffnDee that bad or corroded connections are a likely cause.
 
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RidersofRohan
Oct 2, 2014
585
1,726
West Yorkshire
Funster No
33,675
MH
Rapido 866f
Exp
Since 2014
Good to hear it's working, one remote possibility is that the battery in the multimeter might be on the way out, that would give bad readings as it fails. If not that then I agree with GeoffnDee that bad or corroded connections are a likely cause.

Good to hear that it is now working , if you have not already done so , it would be worth cutting the 4 x red/white cable ends off and remaking the connections in the choc box as these are a possible point where the voltage loss may have been due to corrosion in the cable strands .
Hi both thanks again for your help.
Fridge still working correctly when I tested it this afternoon. I decided to do the cable ends on the choc block. Two cables had a crimp I had never seen before (not particularly surprising) but it was Green and the copper wires also, so good idea to replace. Simple job turned harder, I didn’t have any crimp that fit so I put the new cut and striped cables into the choc block ( the other two cables at the other side of the block had never been crimped either), I had to re site the choc block slightly so the shortened cables weren’t under pressure. The fridge still worked ok, but the reading on the choc block was 12.7v, whereas the vehicle battery reading was 13.7. Yesterday the two readings were the same, so I’ve lost 1.0 volt. Do I need to replicate the crimping arrangement of the two cables? Presumably I could use a smaller version of the one next to them in the picture.
Might even splash out on a new choc block!!
Thanks again.

IMG_0094.jpeg

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Oct 10, 2018
1,987
1,123
Bracklesham Bay, West Sussex
Funster No
56,646
MH
PVC
Exp
Since 2005
I dont think it would be a problem to not use crimped ends, if they are not available , as they were only one side anyway. (the crimp ends could have been Dometic fittings) but it could be worth fitting a new choc box to avoid possible future problems, or give it a very good clean inside.
I do not know if you are aware , but if you have any future problems , there is a installation manual for your model of fridge using the P860 electronic module with information within it that is very useful , it is on page 2 in the Manuals for Motorhome stuff section of Resources (Found at the bottom of this page)
 

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