Testing Norcold N611v Optical Display Board

[Oroman1]

My 1 year old travel trailer has a Norcold N611v refrigerator that runs too cold (25 - 30 degrees).  Tested thermistor and it is within specs.  Moving thermistor to different fin positions has no effect...frige still runs too cold.  There are no other manual temp controls with this model.  The service manual indicates that the optical display board is responsible for temperature monitoring and control.  Thermistor temps sent to the power board are monitored by the optical display board, and the optical display board directs the power board to turn on or off cooling cycle to keep frige within factory pre-set temp range.  So, it appears I need a new optical display board.  I'm told by a reputable RV technician that the problem can not be with the power board.  My question to the community is:  Before I buy a new optical board, does anyone know if the existing optical display board can be somehow bench tested with a multimeter to prove it is defective?

 

[Gary RV_Wizard]

I agree with your tech - it seems unlikely to me that its the optical board.  We are talking about the board logic, not failed components, and the new one will work the same.  A defective thermister is more likely, and that is testable.

I also wonder where you are measuring the temperature. RV fridges are very uneven in temperature, with various spots colder or warmer than average. Check different spots, and different ways of distributing the foodstuff inside the fridge.  Also, put the thermometer in a glass of water rather than just in open air. The water will stabilize the temperature reading. It may well be "working as designed".

 

[Oroman1]

I have tested the thermistor by removing from frige and using a glass of water at various temps.  At 32 degrees, I get consistently about 31,000 ohms, about what it should be.  At 40 degrees and up, I get readings about 2,500 ohms higher than the published mid range value.  At a 2,500 ohm difference, that does not seem like a lot.  Also, there is no thermistor fault code triggered.  It would seem that if the thermistor is working correctly at temps approaching and at 32 degrees, as is the case, the cooling should at least stop there.  But no, the cooling does not shut off until about 25 degrees.  I do plan on buying an infrared non contact temp gun to temp test all the time and internal frige.  Again, why would not the cooling stop at 32 degrees if the thermistor is reading correct at that temp?

 

[Ernie n Tara]

Most likely would seem to be a high resistance connection to (or between) the control board(s).

Ernie

 

[Oroman1]

What does that mean?  Bad wiring? Any way to troubleshoot?

 

[Ernie n Tara]

Bad connection.
Another approach to this would be to parallel the thermistor with a high value variable resistor, perhaps one million Ohms and gradually reduce the set value until the proper operation is attained. Once there, measure the value of the resistor and replace it with a fixed value resister.

Ernie

 

[regval]

Bad connection.
Another approach to this would be to parallel the thermistor with a high value variable resistor, perhaps one million Ohms and gradually reduce the set value until the proper operation is attained. Once there, measure the value of the resistor and replace it with a fixed value resister.

Ernie

The OP created a new post/thread, abandoning the previous discussion. Link is below
http://www.rvforum.net/SMF_forum/index.php/topic,120253.msg1099154.html#msg1099154