Dometic Thermistor
- Thread starter jhild
- Start date
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If you have answers, please help by responding to the unanswered posts.
If you have answers, please help by responding to the unanswered posts.
Gary RV_Wizard
Site Team
Replace the wire? I wonder what could they think has happened to it? A bad connection on either end can be fixed more easily than replacing the entire wire, but if they think the wire is physically broken, then a new one is the best answer. Has the wire been tested for continuity from end to end?
I'd probably first do a test by bypassing the existing wire and running a temporary new one from the thermister inside the fridge, around through the inside door seal and [somehow] back to the main [lower] circuit board, to see if that helps. If it solves the too-cold problem, then proceed to a permanent repair.
The factory thermister wire runs from the P2 terminal on the lower board, up the back of the fridge and through the fridge wall to the interior, where it connects to the thermister. Usually you leave the old wire in place and run a new one along side, meaning you have to drill a new hole through the wall and feed the wire through. Avoiding any nearby wires, of course..
The service manual helps a little.
https://fourwheelcampers.com/NewDometicRefrigeratorManual.pdf
I'd probably first do a test by bypassing the existing wire and running a temporary new one from the thermister inside the fridge, around through the inside door seal and [somehow] back to the main [lower] circuit board, to see if that helps. If it solves the too-cold problem, then proceed to a permanent repair.
The factory thermister wire runs from the P2 terminal on the lower board, up the back of the fridge and through the fridge wall to the interior, where it connects to the thermister. Usually you leave the old wire in place and run a new one along side, meaning you have to drill a new hole through the wall and feed the wire through. Avoiding any nearby wires, of course..
The service manual helps a little.
https://fourwheelcampers.com/NewDometicRefrigeratorManual.pdf
8Muddypaws
Well-known member
Two things could cause this. The thermistor is basically a resistor that changes value based upon it's external temperature. Sometimes they go 'out of spec'.
The other thing is the added resistance of the wire. A little bit of corrosion goes a long way.
Maybe an adjustable thermistor is the answer. I've done some testing and prototyping for these guys.: https://www.snip-the-tip.com/cgi-bin/articca.cgi
The other thing is the added resistance of the wire. A little bit of corrosion goes a long way.
Maybe an adjustable thermistor is the answer. I've done some testing and prototyping for these guys.: https://www.snip-the-tip.com/cgi-bin/articca.cgi
Gary RV_Wizard
Site Team
Presumably the thermister has been checked and its resistance verified? What result? There is a procedure for that in the service manual. If it is physically accurate, then what about corrosion at the connections at either end? And you have tried moving the thermister location on the fins? Sorry, but we need to repeat the diagnostic process to give meaningful help.
How cold is "too cold"? Has the actual fin temperature been measured, or is just that some foods are freezing? And maybe other spots too warm? Sometimes it's just a matter of air circulation inside the fridge (actually, a lack of circulation in most cases). Moving foods around or adding an internal fan is sometimes the solution.
The system is really simple, so not all that much to go wrong.
How cold is "too cold"? Has the actual fin temperature been measured, or is just that some foods are freezing? And maybe other spots too warm? Sometimes it's just a matter of air circulation inside the fridge (actually, a lack of circulation in most cases). Moving foods around or adding an internal fan is sometimes the solution.
The system is really simple, so not all that much to go wrong.
Gary RV_Wizard
Site Team
Most people complain because their fridge can't maintain 34-38. On any setting.
8Muddypaws
Well-known member
OK, you said "The warmest setting". If the thermistor is mounted on the fins the warmest setting would be when the thermistor is right in the middle of the fin. The further it is from the center the colder the box will be. It's counter intuitive for many folks.
I did a bunch of things to mine so it would stay at 34 most of the time. ;D
I did a bunch of things to mine so it would stay at 34 most of the time. ;D
Gary RV_Wizard
Site Team
It depends on the physical design of the fins on the chill plate, but moving the thermister toward the edge (where the fin will be the warmest) will make the fridge run colder. Typically the coldest spot on the fins is at the center and either edge will be warmer. Moving the thermister "north" should be bringing it closer to the edge and thus making the fridge get colder. Not a lot colder, though. You typically won't see more than 2-3 degrees difference in the fridge interior temperature.
All this assumes the fins are continually getting warmed by air circulating (by convection) inside the fridge. If the fridge contents prevent much convection circulation, the fin temperature may be very close to uniform across its surface and moving the thermister will have almost zero effect.
I think your fins & thermister look like the attached photo. The normal "factory" position is at the vertical center of the fin and moving the thermister "north" means moving it outward toward the edge, just as 8muddypaws described
All this assumes the fins are continually getting warmed by air circulating (by convection) inside the fridge. If the fridge contents prevent much convection circulation, the fin temperature may be very close to uniform across its surface and moving the thermister will have almost zero effect.
I think your fins & thermister look like the attached photo. The normal "factory" position is at the vertical center of the fin and moving the thermister "north" means moving it outward toward the edge, just as 8muddypaws described
Attachments
Gary RV_Wizard
Site Team
7.4k seems just a little low (the service manual says 8k-10k), so that might cause the controller board to over-react slightly in response to the reading.
regval
Well-known member
Just some additional information for what it's worth: I have a DM2852 refrigerator and was curious awhile back on what effect the thermistor resistance had on the control board operation.
In Auto mode with 120V AC power applied to the refrigerator, I attached a Fluke DVM to the terminals providing power to the boiler electrical heating element.
Disconnecting the thermistor from the control board, I attached a variable resistor to the control board thermistor connector and measured the following:
At 8.2K ohms, the control board switched ON the 120V to the heating element. (Starts the cooling cycle)
At 10.0K ohms, the control board switched OFF the 120V to the heading element. (Ends the cooling cycle) There is an audible click when the control board's relay is energized/de-energized.
According to the resistance range for a dometic thermistor, a value of 8.111K equates to a temperature of 37F and 10.223k ohms equates to about 29F.
Also understand that the cooling fins in the refrigerator compartment must cool below the desired compartment temperature in order to keep the compartment itself at the desired temperature.
Here's a link to the dometic thermistor value range
https://www.arprv.com/dometic-thermistor.php
In Auto mode with 120V AC power applied to the refrigerator, I attached a Fluke DVM to the terminals providing power to the boiler electrical heating element.
Disconnecting the thermistor from the control board, I attached a variable resistor to the control board thermistor connector and measured the following:
At 8.2K ohms, the control board switched ON the 120V to the heating element. (Starts the cooling cycle)
At 10.0K ohms, the control board switched OFF the 120V to the heading element. (Ends the cooling cycle) There is an audible click when the control board's relay is energized/de-energized.
According to the resistance range for a dometic thermistor, a value of 8.111K equates to a temperature of 37F and 10.223k ohms equates to about 29F.
Also understand that the cooling fins in the refrigerator compartment must cool below the desired compartment temperature in order to keep the compartment itself at the desired temperature.
Here's a link to the dometic thermistor value range
https://www.arprv.com/dometic-thermistor.php
I have re-tested the thermistor after I cooled the fridge. It is now at around 8.4 ohms, which is normal. I purchased a fridge thermometer and am now testing the placement of the thermistor at various points on the fin to see where the optimum temperature will be.
Gary RV_Wizard
Site Team
That's a sound approach, but you need to place a thermometer in a glass or dish of water and wait several hours to let it stabilize after a change. Also be consistent where you place the thermometer - the interior of an RV fridge cools very unevenly, so moving it around or shifting other fridge contents will often cause a difference in the reading.
Ok, i have tried just about everything to get the fridge to warm-up.
1. I tested the thermistor after it cooled down. Ohm reading was 8.4
2. I purchased a thermometer (not the bulb/mercury type) and put it in the fridge.
3. I tried placing the thermistor bracket in various places for the past several days. I kept notes and once I got the fridge to 40 degrees. After that I could not replicate this temperature. The temp hovers around 30-34 no matter where I place the thermistor.
My service guy suggests it might be the circuit board. I'm hoping that isn't it.
Possible solutions:replace the thermistor. Replace the circuit board or:
Have any of you trip the Dometic "Snip the Tip" repair?
Thanks everyone. Appreciate your help
1. I tested the thermistor after it cooled down. Ohm reading was 8.4
2. I purchased a thermometer (not the bulb/mercury type) and put it in the fridge.
3. I tried placing the thermistor bracket in various places for the past several days. I kept notes and once I got the fridge to 40 degrees. After that I could not replicate this temperature. The temp hovers around 30-34 no matter where I place the thermistor.
My service guy suggests it might be the circuit board. I'm hoping that isn't it.
Possible solutions:replace the thermistor. Replace the circuit board or:
Have any of you trip the Dometic "Snip the Tip" repair?
Thanks everyone. Appreciate your help
8Muddypaws
Well-known member
I worked with Snip-The-Tip By testing prototypes of their device for Norcold 1200 series refers. (I convinced them to make them in the first place).
The engineer who founded the company and probably still owns & runs it is very knowledgeable about all thing RV. If you go to his website you can probably ask him if He thinks his product might fix your problem.
The engineer who founded the company and probably still owns & runs it is very knowledgeable about all thing RV. If you go to his website you can probably ask him if He thinks his product might fix your problem.
8Muddypaws
Well-known member
https://www.snip-the-tip.com/cgi-bin/articca.cgi
