Dometic RMS8501 Sparks, light, gas stops, repeat!

[Growly bear]

Hello All
Can someone help me to fault find on my Dometic RMS8501 fridge?

Burner replaced last year (its the type where the burner tube, igniter and thermocouple are bought as a complete unit) and all has been well until last time out when I started to notice a fluttering(?) sound coming from the burner. It was only very occasional and thought that it might be the wind blowing the flame.

Anyway, tested it this morning (as yes you've guessed it, we're away next week) and it's now worse. The flame goes out momentarily, the igniter sparks and it relights. The thermocouple is glowing red, but the burner doesn't roar. I hear the gas valve pinging and I'm not sure if it pings before the flame goes out or not.

I don't really know where to start. I've removed the burner assembly and cleaned the jet using lighter fluid. Swept the chimney and all clear.

As the gas valve operates how to I check whether it's the value or what controls it's operation?

Any thoughts would be great!

Thanks

 

[GeoffnDee]

growly_bear ,If your fridge has the earlier oblong power control board fitted there is a service manual for the early Dometic 8000 series fridges , with gas operation information , on page 2 of Manuals for Motorhome Stuff that can be found in the Resources section (in menu).
If your Dometic RMS8501 Fridge has a later square (P860 , on the rear)power control board , it should have electrical connections (red/red/black) to the gas valve which would be worth checking.

 

[Growly bear]

growly_bear ,If your fridge has the earlier oblong power control board fitted there is a service manual for the early Dometic 8000 series fridges , with gas operation information , on page 2 of Manuals for Motorhome Stuff that can be found in the Resources section (in menu).
If your Dometic RMS8501 Fridge has a later square (P860 , on the rear)power control board , it should have electrical connections (red/red/black) to the gas valve which would be worth checking.

Thanks for this.
Do I need to pull the fridge out to get to the pcb?
This is the gas valve wiring

 

[Growly bear]

Is the gas valve on or off or does it modulate? The burner doesn't seem to roar like it used to. I know that this type of burner doesn't have a pilot light so would expect it to be full beans or off. Maybe that's why its popping due to not enough gas getting through.

I recleaned the jet using alcohol and the bristle from a soft pair brush but its still the same...

 

[Bessy765]

Worth pouring boiling water over the gas valve and giving it a sharp tap. Worked on ours?

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[GeoffnDee]

I think you may have the earlier version RMS8501 in which case the control board/PCB will be at the front behind the controls ,it is shown in the service manual, along with the expected voltages at the gas valve. (As well as tapping it ,as suggested ,it would be worth trying cleaning the ground connection point on that gas valve shown)

 

[Growly bear]

Do you feel that the gas valve isn't opening fully and restricting flow?

I cleaned all female spades. The terminals on the valve are bright and shiny.

I also tried tapping it...

Here's a video of what its doing. If you listen carefully you can also hear the gas valve opening and closing

BTW I removed the cage to see the flame. It is now refitted!

 

[GeoffnDee]

It could be the valve that is not operating correctly but there is also a burner control device fitted to your model that could be faulty or have poor connections , checking the voltage at the valve may help to confirm that the burner control device is working ok.

 

[Growly bear]

It could be the valve that is not operating correctly but there is also a burner control device fitted to your model that could be faulty or have poor connections , checking the voltage at the valve may help to confirm that the burner control device is working ok.

OK. sounds like a plan! I'll do it first thing in the morning

Rewatching the video - the earth terminal looks poor to the body of the valve. Think I'll clean that up too!

 

[Growly bear]

Update:
Just measured voltages at both terminals to earth and get 9.8v. However, when the flame starts popping the voltage jumps all over the place. I don't have an oscilloscope to measure what it does, but I've not seen 0v

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[GeoffnDee]

The voltage supply should be 0.7v-0.9v with 48-50 Ohm per valve inductive resistance (switched on gas mode) to the two valves in the unit , and 1.5v at the burner control device P810 ,which controls the gas valve/flame control/flame failure/ignition , according to the service manual for the earlier Dometic 8000 series fridges.

 

[Growly bear]

The voltage supply should be 0.7v-0.9v with 48-50 Ohm per valve inductive resistance (switched on gas mode) to the two valves in the unit , and 1.5v at the burner control device P810 ,which controls the gas valve/flame control/flame failure/ignition , according to the service manual for the earlier Dometic 8000 series fridges.

Thanks. Before I double check whether I misread the decimal point, can I confirm that the resistance is measured between each valve and earth and not valve to valve? Also looking at the wiring diagram, the wires seem to change their colour from one end to the other betwen X110 and GFA. Where should I be measuring the 1.5v at the burner control?

Sorry for the questions!

 

[GeoffnDee]

If you have downloaded the manual , section 2.4 (page 15) shows the gas valve measuring points for voltage and resistance , also , section 2.3 (page 14) shows the measuring points on the burner control device (I think the change of colour will be a mistake, but the measuring points are shown as pin numbers so it is not a problem)

 

[Growly bear]

My burner control looks different. It doesn't have the single row connector block. It has a screwed on block, and to the right is a screw connection block, all of which is impossible for me to get my hands down the back of.

So my next step would be to take the fridge out (which is all sealed with silicon etc at the back). I've looked on line for my part and nothing comes up. The ones which do are over £120. The burner tube is also on its way out and will need replacing (only fitted last year at £90).

I've fitted 280ah lithium last year and 420w solar (just finished), so I think for now I'm going to look for a compressor fridge and fix this fridge when it's on the bench or sell it for parts.

Thanks for your help so far. It is much appreciated, and I'll resurrect this thread when I can get at it!

 

[GeoffnDee]

That is a later version power control module which does not have a separate burner control device (it is built into the P860) but it does control the gas valve , normally using red/red/black cables. I do not know if the gas valve, and its operation voltage , is the same as for the earlier power control modules .
This later module , and its wiring , can be seen on page 2 of Manuals for Motorhome Stuff in the Dometic 8xxxx Series installation manual (It shows a RMD fridge/freezer but I believe it could be the same as the later module you have)

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[Growly bear]

Thanks for the heads up. I must find time to look through those files as you clearly know them very well!
Yes, that looks like my PCM. I'll be back when I have time to play. Again, thanks very much!