Buggy program in Dometic Air conditioner

[fixpro256]

I have a Dometic Brisk II Duo-Therm purchased around 2018 in my Keystone rv. When it gets too cold and freeze mode is triggered, the compressor is supposed to shut off and the fan is supposed to go into high mode until the system thaws out. But, mine shuts the compressor and the fan off. Then it takes hours for it to thaw out because the fan is not running. Even if I put the fan in manual high or low, when freeze mode is triggered, the fan will shut off.

I have purchased a new thermistor and placed it closer to the area that freezes first (bottom left) and placed it further into the coil so it detects the freeze sooner. That helped reduce the freeze to thaw cycle, but did not get the fan to come on.

I then cut the leads on the old thermistor and connected a variable resistor in place of the thermistor. When I set the resistance to around 15k ohms, the compressor shuts off and the fan goes to high as it should. While it is in freeze mode, I can slide the resistor down to around 10k and the AC will kick out of freeze mode and go back to normal operation.

Even more interesting, with the thermistor plugged in, when the unit goes into freeze mode and the compressor and fan shut off, I can unplug the thermistor and plug in my resistor (set above 15k) and the fan will come on high (the compressor will stay off). I can then plug the thermistor back in, and the fan will shut off. Plug the resistor back in, and the fan will go to high again. I can keep swapping them back and forth until the thermistor's resistance drops below 10k (that kicks the computer out of freeze mode). Each time I use the resistor, the fan works as it should. Each time I use the thermistor, the fan shuts off.

I took apart the old thermistor. It is a simple thermistor only, no capacitor, no diode, no resistor, or anything else in series or parallel.

I have taken the circuit board out and checked it closely for cracks and burns. Cleaned with a small brush and put it back in. No change. All 120 volt AC and 12 volt DC connections have been replaced. The thermostat has been swapped with my other AC unit also.

I have never been able to get the freeze mode to work right with a thermistor, but it works every time when I trick it with a variable resistor.

It doesn't sound to me like there is a physical problem with the circuit board. I think it is a bug in the software. The part number on the box containing the circuit board is 3313191.000
Has anyone else ran into this problem?
Can anyone say they know their Brisk II Duo-Therm performs freeze mode correctly??? If so what is your circuit board part number?

Thanks!

[fixpro256]

Hi Dougrainer,
The short answer is, yes, I made sure of that.

The long answer, I usually set the temp to 77F in the morning. When the temp outside is under 80 or so, the system will reach 77F and cycle off and on from time to time as expected. When it gets up to about 85 or so outside, the system quits cycling and runs continuously. It starts to get frost on the lower part of the coil and then kicks into freeze mode even thou the room is above the thermostat setpoint.

With the fan on any of the three modes (auto/hi/low). When the freeze mode kicks in, the compressor and fan shut off at the same time. They will stay off for a few hours even if the room is 90F (because the fan is not circulating air).

I can force the system out of freeze mode by changing the fan mode to a different setting or by turning the system off (at the thermostat) and then back to cool.

When it gets up to about 90 or so outside, there is enough of a heat load to keep the coils from freezing, so it runs continuously but doesn't freeze.

Later in the day, when it starts to cool down outside, the system will freeze again. Even later in the evening, the system will go back to cycling as the temperature in the room fluctuates around the setpoint.

[dougrainer]

Well Engineer or PC guy. BOTH have the same problem in logic as it relates to RV's. The control board is NOT a "computer". There is NO programmable logic aftermarket. I read you first post about AC problems. You are trying to use your knowledge on RV components. Apples and oranges. Now, the Freeze thermistor does NOT--NOT allow the evap to freeze up. It STOPS the ice from forming BEFORE it causes a air flow restriction. Once the Thermister senses the freezing temps on the tubes of the evap it shuts off the Compressor. It should NOT stop the Fan IF the set temp has been reached and on AUTO. If the Fan is on Hi or Lo, then fan will run regardless of what the wall tstat does or the Freeze Thermister does. THIS is why the Upper control board would be at fault if the fan cuts off when on AUTO/COOL and the fan shuts off when the Compressor shuts off AND THE SET TEMP HAS NOT BEEN REACHED. Are you positive that the set temp has not been reached and the compressor and fan are shutting off because the AC has cooled and done its job? Doug

[fixpro256]

I'm not an engineer. I'm a computer guy. I don't know if it's a few people, or thousands. Since the circuit board appears to work fine with the DIY thermistor I made, or a variable resistor, I would think that it's a problem with the thermistor. BUT, I know very little about thermistors. Someone that is more of an engineer type would have to tell me why 2 thermisors that read the same resistance (about 20k at 35F) would make the computer respond differently (one makes the fan go off, the other makes the fan go "high").

Some service people are pretty sharp, some aren't. Most of the times that I've tried to use the service departments, the people I encounter aren't very good at troubleshooting. They charge me money, screwing things up and I have to go behind them to fix those problems also. I have, on rare occasions, found people that are pretty sharp in service. But I don't like the odds.

[dougrainer]

fixpro256 wrote:
Doughrainer, When you say "Per Dometic", what do you mean by that?

The original thermistor and the new Dometic thermistor that I purchased, both act the same.

I also purchased a pack of thermistors that are intended to solder into a circuit board. They are not the same, but similar. I soldered on some wires and surrounded in heatshrink. It works like it should. It turns the fan on high in freeze mode.

So the question is, is it the Dometic thermistors that are the problem or the circuit board? If there are only a few people with this problem, it isn't a big deal. But if thousands of us are having this problem, then Dometic should have their TS people just send a part as soon as a customer says they are having this problem. But so far, they are acting like this is an isolated and rare problem. They are telling me to call/pay a technician.

1. PER DOMETIC---That means I called Dometic and asked for the parameters. THEY said the upper control module.
2. I AM a Dometic Service tech(40 years and still working)
3. You MUST be an engineer. ONLY an Engineer would post what you posted and state there MUST be a inherent problem, instead of a minor glitch. HOW do you know if it is a few people, or possibly thousands?
4. I have rarely had a problem with the Dometic Freeze Thermisters. Maybe 2 or 3 in 20 years since this model thermistor was introduced. AND the failure was not sending the signal to shut the compressor down---failed to react to freezing evap temp.
5. If out of warranty, then you are OUT of warranty. If you are in warranty, get it to a Dometic service center. Doug

[fixpro256]

Doughrainer, When you say "Per Dometic", what do you mean by that?

The original thermistor and the new Dometic thermistor that I purchased, both act the same.

I also purchased a pack of thermistors that are intended to solder into a circuit board. They are not the same, but similar. I soldered on some wires and surrounded in heatshrink. It works like it should. It turns the fan on high in freeze mode.

So the question is, is it the Dometic thermistors that are the problem or the circuit board? If there are only a few people with this problem, it isn't a big deal. But if thousands of us are having this problem, then Dometic should have their TS people just send a part as soon as a customer says they are having this problem. But so far, they are acting like this is an isolated and rare problem. They are telling me to call/pay a technician.

[dougrainer]

Per Dometic, since you have swapped the tstat and freeze Thermister, the only thing left is the upper control board. The Wall tstat does not control this operation. You should be in warranty?(2 years). Doug

PS, long shot would be wrong freeze Thermister for your model control board