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Author Topic: 8.72 reading when multimeter is set to 20k ohms?  (Read 315 times)

marknoo

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8.72 reading when multimeter is set to 20k ohms?
« on: October 11, 2018, 07:38:35 PM »
I am attempting to determine is my thermistor in my refrigerator is good or bad. I tried the multimeter at 200 and at 2k and got no reading. When I upped the setting to 20k, I got an 8.72 reading.

THe service manual calls for a thermistor to be put in ice water for about 8 minutes, than you check the Ohms and if they are between 8000 and 10,000 the thermistor is good. If they are not, then the thermistor is bad.

This is sort of where I was told to start checking.

I made sure the chimney was clear, I did not smell any ammonia and I saw no green and yellow dust/residue. I also heard no "thumping or gurgling " so I guess it is not the cooling unit.

Also, the flue gets hot but somehow that heat is not doing something to make the cooling unit work. So, the flue is quite warm but the refrigerator does not cool at all.

(it just seems that if the meter had a 1k setting, 8.72 would be perfect but at 20k, I think it is too much. But I don't know, I had to watch a youtube video just to figure that out)

xrated

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Re: 8.72 reading when multimeter is set to 20k ohms?
« Reply #1 on: October 11, 2018, 07:58:36 PM »
If I'm understanding correctly what you did, your meter was showing 8720 ohms.  You said your set it on the 20K scale and 8.72K is 8720 ohms  And if that meter did have a 1K setting, and you are looking for between 8000 - 10,000 ohms, it would not do you any good.  In you case, if I'm understanding what you did, you chose the correct scale to try and read 8-10K ohms.
« Last Edit: October 11, 2018, 08:01:17 PM by xrated »
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Arch Hoagland

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Re: 8.72 reading when multimeter is set to 20k ohms?
« Reply #2 on: October 11, 2018, 08:12:42 PM »
You are good. 

That is just as xrated said...8,720 ohms


What model refrigerator? 
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marknoo

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Re: 8.72 reading when multimeter is set to 20k ohms?
« Reply #3 on: October 11, 2018, 09:01:28 PM »
I have a Dometic RM 2652. The service manual says that the thermistor, when put in ice water for about 8 minutes, should read between 8 - 10 thousand ohms and it does.

Therefore, it is not the Thermistor, correct?

What should I check next? The flue gets hot but that heat is not being pushed into the cooking system.
If I put my hand on the flue, it is hot. Hot enough you pull your hand away pretty quick so it does not get hurt. However, the tubes and some sort of reservoir next to the flue do not get hot.

Flue is hot (see pic)
https://imgur.com/a3S6QP4

tubing next to flue is NOT hot, it is just normal temperature (pic below)
https://imgur.com/iPeooiR

I keep reading and watching videos but I am not sure what to do next.

I read a great site on the cooling unit. There is no yellow green dust, no smell of ammonia, and no gurgling. According to this site it is likely not the cooling unit. Although it could be.
Here is the site:
http://www.rvrefrigerator.net/Troubleshooting.htm

Anyway, that is where I got the gurgling, ammonia and green/yellow dust information.

If the thermistor is within specs, than it is not bad. 
What should I check next? 
and would it change anything if i tried to get the propane working. right now it is only using electricity. I don't think the LP has ever been used for the fridgerator since I have owned it. (it is a 1996 Nomad trailer) so it is old

Willandgiselarv

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Re: 8.72 reading when multimeter is set to 20k ohms?
« Reply #4 on: October 11, 2018, 09:38:06 PM »
8.7k ohms..as others say it's good.
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Arch Hoagland

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Re: 8.72 reading when multimeter is set to 20k ohms?
« Reply #5 on: October 11, 2018, 10:08:46 PM »
Did it ever work correctly?

My Norcold takes 12 to 24 hours to cool and that is with it empty.

I always prechill everything before I put it in the refrigerator.

What RV do you have?  Perhaps someone with the same RV knows more about it.



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regval

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Re: 8.72 reading when multimeter is set to 20k ohms?
« Reply #6 on: October 11, 2018, 11:48:22 PM »
I believe this a continuation from another thread:
 http://www.rvforum.net/SMF_forum/index.php/topic,117579.msg1065473.html#msg1065473

As Lou Schneider said in the earlier thread, the cooling unit appears to have failed.
The thermistor is just a sensing device to tell the control board to turn the cooling unit "heating" system on or off.  If you feel heat in the flue, the the heating system is on and should remain on (flue is hot) until the cooling unit does it's thing.  As described, it boils liquid to steam, the steam rises and condenses and flows back to the bottom of the pipes and is recycled.  If you don't feel any substantial heat on the pipes on the bottom left of the refrigerator, then the cooling unit has a restriction in the liquid lines above the boiler. Most restrictions are caused by solidified anti-corrosive agent.  This can cause a reduced flow of the liquid back to the boiler, causing overheating and a common cause of boiler overheating is running the refrigerator extremely off level once, twice or more, as it can be cumulative. Maybe this isn't the cause in your case, but the result is the same.
Anyway, that's my opinion.  Time for a replacement.

« Last Edit: October 11, 2018, 11:55:42 PM by regval »
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John From Detroit

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Re: 8.72 reading when multimeter is set to 20k ohms?
« Reply #7 on: October 12, 2018, 09:51:35 AM »
Without seeing your meter 8.72 on the 20K range would likely be 8720 ohms or right "in range"  but  stress I'd need to see the meter and likely use it to test a known resistor.
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Gary RV_Wizard

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Re: 8.72 reading when multimeter is set to 20k ohms?
« Reply #8 on: October 12, 2018, 10:21:07 AM »
Quote
I made sure the chimney was clear, I did not smell any ammonia and I saw no green and yellow dust/residue. I also heard no "thumping or gurgling " so I guess it is not the cooling unit.
Invalid conclusion. Any of those symptoms would clearly indicate a failed cooling unit, but the lack of them does NOT mean it is OK.

Since your boiler is very hot, the function of the thermister was never in doubt.  It's just a thermostat and it is doing its job if the heater (gas or electric) is on and heating the boiler tube.

The tubing should be extremely hot from the boiler up to the condensor at the top, but only warm beyond that.
The indications are that the cooling unit has leaked and lost its hydrogen gas and maybe some of the ammonia as well.
Gary
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marknoo

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Re: 8.72 reading when multimeter is set to 20k ohms?
« Reply #9 on: October 12, 2018, 10:32:12 AM »
So it is a cooling unit.

Then I am going to stop working on it.

My assumption is that both electric and propane use the same broken cooling unit and the only way to make this one work is to replace the cooling unit. The other option is to replace the fridge.

Is that right?

Lou Schneider

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Re: 8.72 reading when multimeter is set to 20k ohms?
« Reply #10 on: October 12, 2018, 04:11:14 PM »
Yes, unfortunately.

reniram

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Re: 8.72 reading when multimeter is set to 20k ohms?
« Reply #11 on: October 12, 2018, 04:30:08 PM »
You have pretty much eliminated all possible faults to the unit and more than likely replacement will be the only answer..I have one last ditch attempt to revive the old girl and that is to remove the whole refrigerator and thoroughly clean all the external stuff and tip it upside down a few times to mix the refrigerant chemicals .. They do settle over the years...We used to do this years ago to the old ammonia residential units and had a fair amount of success...Maybe you won't want to mess around doing this especially if it is a pita to get it out..but new ones are pretty pricey so it may be worth a try..it may even take off running again...my 2c

grashley

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Re: 8.72 reading when multimeter is set to 20k ohms?
« Reply #12 on: October 12, 2018, 06:44:35 PM »
For clarification on the meter, a 100 scale will read values from 0 up to 100.  A 1K (1,000) scale will read from 0 to 1,000.  The 20K reads up to 20,000, and is the lowest range greater than your reading.  It was the proper range.
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marknoo

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Re: 8.72 reading when multimeter is set to 20k ohms?
« Reply #13 on: October 12, 2018, 07:17:40 PM »
I thought about burping it, like reniram suggested, but if it is that big of a PITA than I won't want to do it twice.

I don't want to take it out, get it running, than have to take it out again.

I haven't ever taken one in and out before but i think the whole process will be a nightmare and I won't want to have to wonder when it is going to stop working again.

On the other hand if it comes in and out fairly easily, maybe I will try burping.  Either way, thanks for all the help on this everyone.