Norcold "3-way" temperature regulation... and a way to "fix" it?

[rickst29]

This all concerns the Norcold model N300.3rh which I have in my 2619 from year 2006; I don't know if TrailManor still uses this refrigerator in current models. In many Threads concerning this refrigerator, people (including me!) keep complaining about the way it tends to freeze lettuce in the coldest part of the night (just before dawn); and then warms up to 50F+ during the daytime (if you forgot to turn the danged thermostat dial back down).

This post includes a wiring diagram (please see the attachment, "norcold_n300.rh_3-way_wiring.jpg"). Then, in the following post, I discuss the electrical current path in each of the 3 operating modes (12VDC, 115VAC, and Propane).
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The diagram, copied from the Service Manual, labels the following parts with numbers, in circles next to the parts:

#1: 120VAC power cord ("hot", "neutral", and chassis ground).

#2: Wire Terminal Block. All connections are straight through. (1-8, 2-7, 3-6, and 4-5).
#3: AC power fuse. (3 amp fuse on the Norcold's 120VAC "hot" wire out from terminal block).
#12: DC power fuse. (20-amp fuse on the Norcold's +12VDC "hot" wire out from terminal block).
#11: +12VDC from the TM's converter/regulator. (A dedicated line, with another fuse within the TM's 12V distribution panel.)
#10: -12VDC "neutral". I'll SWAG that this is an actual white wire back to the TM's converter/regulator, but my TM is closed up right now... I'll edit/correct this statement if it turns out to be an adjacent chassis ground.

#4: The selector switch. (With settings for "Off", "DC/Battery", "AC plug-in", and "Propane flame".)
#5: The thermocouple -- which has NOTHING to do with the thermostat. (It is only used for safety and control of gas flow during propane operation.)
#6: the "dumb" thermostat.
#7, #8: propane ignition and flame status devices.

#16: The 12VDC heater (+12V "hot" coming in from terminal "W2" on the selector switch #4; -12V "neutral" straight back to the terminal block).

#17: The 120VAC heater. "Hot" comes on "BLK" from terminal "W1" on selector switch #4, into the "dumb" thermostat #6; when the thermostat feels that the heater circuit should be "on", it connects out on wire "BRN", which is ultimately connected to heater input wire "BLU".)

I'll provide some more diagrams in the next post, but you can already see the Norcold "temperature regulation" problems from my description:

• 12 Volts operation is NOT controlled by the thermostat. You turn the fridge to run on 12V, and it keeps going, and going, and going.... (But unlike the Energizer Bunny, this one hurts batteries.)
• The "dumb" thermostat, controlling AC and Propane, is just like the ones which are used in cheaper, non-digital kitchen refrigerators. But unlike the kitchen refrigerator thermostat, it doesn't sit INSIDE the food storage area. It's sitting on that front control panel, with one side (and the whole dial face) exposed to TM living space. The simple thermometer which is inside of it gets affected, a lot, by temperature changes within the TM living space. There is not a good correlation between "dumb thermostat" temperature and refrigerator interior temperature. That's why it works so badly.
• And finally (as I mentioned in connection with the parts list) the thermocouple has nothing to do with the temperature control thermostat. (It's used for propane control ONLY.)

In the next post, I'll try to explain the flow of electricity (for control and actual heating) in the 3 operating modes.

[rickst29]

The current flow diagram, from the Service Manual, is attached. The large box, #17, is the selector switch. The AC heater is shown as #4; the DC heater is shown as #14. The AC "hot" connection through the Thermostat is at #3. (I have no idea why it is shown with this symbol; it functions as a simple on/off switch in this case.) And the group of components at the lower left, #6-7-8-11, is the Propane control section. (#6 is the gas safety valve; #7 is the thermocouple interrupter; #8 is the front-panel flame meter; and #11 is the thermocouple.) I see that the flame meter "red" wire goes into the thermostat in order to complete a grounding connection, and although their picture shows no thermostat involvement in Propane mode, I think that the thermostat's dial setting does have an effect here.

DC Operating Mode

Norcold specified the operating limits of DC mode at 13.5 Volts minimum, 15.4 Volts maximum. But there's nothing to interrupt or test the voltage along the DC current path; once it's running, it will happily suck your TM and TV batteries down to the point where your TV needs a "jump" in order to restart the vehicle. (As so many have learned, the hard way... ) Current and heater power are voltage dependent. At 14.0 volts, output is 150 watts and current is 10.7 Amps. At higher voltage, more current goes through the heater, and output is higher.

This mode corresponds to the current path #12 (+12VDC) --> #13 (DC fuse) --> selector switch --> DC heater #14 --> Grounding Connection #15. Why this picture uses totally different numbers, I have no idea. The real numbers, so that you can match up with the diagram in post #1, are as follows: #11 (+12VDC) --> #12 (fuse, same number) --> selector switch #4 --> DC heater #16 (different number!) --> Grounding connection # 10.

The big surprise here is the fact that both diagrams agree: DC operating mode never goes through the "dumb" thermostat for any management. There's simply no control connection, in or out, along this path. (hmmmm, I'm not impressed.... and don't let me get started on their use of a WHITE!!! wire for the HOT DC connection through the fuse and into the selector switch.)

AC Operating Mode

This is the top diagram, shown as #1, 120VAC "hot"; --> #2, AC Fuse; --> selector switch; --> #3, rotary "dumb" thermostat; --> #4, AC Heater; goes to #5, neutral wire into the AC plug. The padt numbers which changed are #2, the AC fuse (#3 in the previous diagram); #3, the "dumb" thermostat (formerly labeled as #6); the AC Heater (formerly labeled as #9); and the neutral connection back to the AC power distribution panel (not previously numbered separately.)

No surprises here, it's exactly as I had expected to see.

Propane Operating Mode

This is all a mystery to me, and I'm not going to touch anything involved in the regulation of this mode. I'm only going after the DC and AC circuits....

[rickst29]

Attachment shows the locations of the two relays, in a modified version of the wiring diagram from post #1. Each of the two Solid State Relays ("SSR" devices) supports load current of up to 25 Amps, but they are not the same. The AC relay is on the right, labeled "B". (It interrupts the "Hot" 120VAC lead coming from the "dumb thermostat"). Don't make this connection too close to the heater, within the heat resistant "BLU" wire; try to put it at considerable distance so that the relay doesn't get cooked by heat coming down the wire. (I don't know where the junction between "BRN" and "BLU" occurs; but if it is easy to access, then that would be a good place.)

These relays get warm when they're switched too often, but that doesn't happen in this application. No heatsink is needed here. The harder-to-find DC relay is labeled "A", and occurs along the the +12V "YELLOW" wire from the selector switch. As with the 120VAC "BLU" wire, cut in at a location with considerable distance from the from the heater.

Good accessibility assures that you can bypass the relay, if the control system blows up during a trip. If you put both power leads on the same "load" contact of the relay and tighten the screw down, then the circuit is always "on" -- no matter was has gone wrong inside the relay, or back in the PID. (If you want to be strictly "proper" about your emergency wiring, you'll use a small terminal block like #2 -- one lead per screw terminal, never two. )
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The last issue, which I have not covered until this morning's edit, is the relay control wiring. I am not sure whether they should be wired in series, or in parallel. (I'll guess that either way works, but I'll be testing it the bench before I wire it up in the TM.)

The question is whether to wire the SSR control leads in Series (through each other), or in parallel at the PID controller.

Probably, BOTH ways work, but Series is more simple. Both SSR devices images on the website show a "switch on" voltage of 3-32V, and the PID controllers dish out 10 volts. So, unless the internal resistance of each one is badly "out-of-whack" versus the other, they'll both be seeing enough voltage (across the control leads) to make the switch.

Sorry, editing this post has DESTROYED the description of parts (which was here when I finished last night). That parts list and discussion (the MEAT of this whole Thread !!!) is now below, in post number 12. Grrr, must have coffee first, before tearing big things apart without verifying that I've made a viable backup copy.

[ThePair]

Wow.

This is...awesome!

I don't know if I'll ever attempt this...but it's fantastic that you put in the time and effort to write all of this up for the rest of us.

Thanks

I am very interested in this. I find that I have to change the temperature setting twice a day. One setting for overnight and another for during the day.

I sure would like to not have to REMEMBER to do that.

It can easily reach 90+ degrees inside my TM while we are gone for the day, but the overnight low will drop low enough that the furnace kicks in at the lowest setting, which is about 55.

[rickst29]

Originally Posted by PopBeavers

I am very interested in this. I find that I have to change the temperature setting twice a day. One setting for overnight and another for during the day....

humidity keeps "your" temperatures (on the California side) much more stable than we get on my side. In addition to the 90+ days, we can go all the way down to 30F in the high desert. Yes, frost in July. Single-digit humidity readings wreak havoc! But I know you to be a "roughing it" frequently. I don't know your preference for propane versus generator in such situations. (Remember, this doesn't change the Propane situation at all.) You could, though, switch from propane to generator-charged battery through the night, and that would prevent discovery of exploded beer cans in the AM.... when you REALLY WANT that first beer.
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I know you to be more competent at TM Mods than I am, and you'll be out and about during April. If you end up buying it first, my contact is "Marco". Please tell him that Rick Stockton sent you, and he'll know exactly what you're talking about.
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Pair-from-ChiTown: thanks for the Kudos!

[ferdoh]

I thought that the propane ran on low flame all of the time and the stat would bring in high flame as needed, but I can't say for certain without testing. Looking at your wiring diagram, I can't see how it would though.

[rickst29]

By looking at an electrical wiring diagram, I didn't see that the propane supply tubing goes right through the Thermostat.

So the sequence of controls is this:

#1: Propane control circuits must activated (electrically) at the selector switch).

#2: Propane supply line goes into the thermostat, where it is valve- controlled. I don't know if it is a "fully open OR fully shut" type of valve, or whether there really is some finicky control done there. (I think that it's only On/Off, because the attached diagnostic procedure doesn't test for "if open, how much".)

#3: Propane line out of the Thermostat, where it becomes thermocouple controlled.

I've attached the propane "won't stay lit" diagnostics page, leaving out the various thermocouple voltage settings and their regulation of the valve (stuff at the bottom).

[Bill]

Rickst -

Wow! You've done a lot of work and a lot of thinking about this. You've gotten me off my duff to restart a project I've neglected for some time. I had an old dorm refrig that still worked fine, but the outer case rusted badly and I was told to get rid of it. So before it hit the trash, I opened up the case and rescued the thermostat-capillary-bulb assembly. My intent was to drill a small hole from the back of the refrig into the cooled space, insert the bulb, and add a relay as you described.

For whatever reason, the project got shelved. But as I pulled this thing apart, I noted that there is at least two feet of capillary tube in this assembly, and only the last few inches were in the cooled space. The rest was folded up and lying on a bed of insulation behind the cooled space, and the thing still maintained temperature well. So why is it OK to have most of the tube exposed to outside ambient? My guess is that the capillary tube has a very small bore, so the total volume of the capillary is small compared to the volume of the bulb. Therefore the temperature of the capillary contributes little to the overall effect. This is true of a household refrig as well, where there may be 6 feet of capillary snaked up the back of the cabinet.

On the other hand, the bulb was hanging free in the cooled space as you have suggested, supported only by a plastic clip near the base. So the bulb itself was exposed only to the air in the cabinet. This shows the same thinking you've mentioned - it is important to sample the air temperature, not the surface temperature in the cooled space. But like you, I don't have an explanation for the "backward effect" you mentioned.

As an electrical guy, I would normally gravitate toward an electrical solution to the problem, as you did. For whatever reason, I didn't this time - though I may do so to take advantage of your work.

One final thought. Both you and I have concluded that the problem is that the OEM thermostat is deficient in some way, and that a new and better one will cure it. But there is a more insidious possibility. Perhaps the cooling mechanism itself is barely adequate, so the cabinet temperature follows the ambient. I need to think about this - I'm not sure it is entirely consistent, either, with what we see.

I gather from your post that this project is still in the planning stage - you haven't actually done it yet. If that is right, do you have a schedule for doing it? If so, I will put mine on hold again until you have some results.

Bill

If we were on shore power and disconnected everthing when we were leaving but forgot to turn the frig knob to off will it switch automatically to the battery. If so I'm sure we must have completely drained our battery. YUCK!!
T