Truck Fridge Digital Thermostat

[kiltym]

[Sodo]

I had trouble for some years due to 'too small' wires supplying the TruckFridge.
It was a total brain fart, caused by wiring it temporary (for testing) then I forgot.
Then some weeks later, had to go camping (at the drop of a hat), so I closed up the cabinet like that.
It was a couple years thinking something was wrong with my Truckfridge, I even had it tested by a Truckfridge repair center, and they said "it's fine".

But the TF repair facility is who told me to attach the INKBIRD at the C&T so at least I got that....

Be sure your wires that supply the compressor can provide at least 5A at the fridge.

One way to be certain it's getting the power it needs is to measure the voltage at the fat red wire ("+" terminal) while the compressor is running.

0.25v less than battery voltage is 98% which is sufficient.
_________________


'90 Westy EJ25, 2Peloquins, 3knobs, pressure-oiled GT mainshaft, filtered, cooled gearbox
'87 Tintop w 47k 53k, '12 SmallCar EJ25, cooled filtered gearbox
....KTMs, GasGas, SPOT mtb

[kiltym]

So I did some more testing this morning.

On the stock mechanical thermostat there is a resistor in line at 463 Ohms. This resistor will increase the speed of the compressor to about 2750rpm. You can find this information in this document:

https://www.secop.com/fileadmin/user_upload/SEPS/d...00p522.pdf

If you simply attach the inkbird in place of the stock thermostat (by removing the existing C & T wires and replacing with new wires to the Inkbird), this resistor is no longer present, and the compressor will run at 2000rpm. This is what was happening to me. It will create VERY long run times.

I found another thread about wiring the two thermostats in series, and simply leaving the mechanical one set to '7' at all times. The Inkbird will still control the on/off of the compressor, but it will put the resistor back in service and the compressor will run at the higher RPM again intended by the factory.

My electrical wiring is all fine, this is an issue of the resistor being removed from service by swapping to the Inkbird.

Here is the thread that talks about this some more:

https://www.cruisersforum.com/forums/f115/adding-resistor-to-danfoss-system-257889.html

Specifically look at post #5 and the comments around that.

[Sodo]

[kiltym]

[Sodo]

[kiltym]

[fxr]

That 463 ohm resistor is almost certainly 470 ohm (+/- a few percent). A precise 463 ohm (+/- 1 ohm) would be very expensive, and probably not available outside of a laboratory.
_________________
Jim Crowther
1984 1.9l EJ22 Westy Wolfsburg Edition
Vespa GTS 300

[kiltym]

[Sodo]

My brother is a control systems guy and explains it like this.

There are other factors not considered, such as efficiencies.
Like the compressor working so hard it's adding heat into the van that further exacerbates the problem.
But we ignore that stuff.

Just looking at the max cooling attainable (the Truckfridge's "Delta")

=========
Lets just assume the max differential the TruckFridge can do
is cool a “40 F ° Delta“.
And you set the INKBIRD at 34°F….
The ambient temp in your van is currently 73°F
At 73°-34°=39°F..... its all good👍🏽 TF can do 40° delta.

Ambient temp rises 2°, to 75°F, but TF can only do 40°.
75°-40°=35°F, the max it can cool to.

The INKBIRD is set at 34°F so it's commanding the TruckFridge to run continuously trying to get it down to 34°F but it cannot. It can only maintain —>35F<— inside the fridge.
So the Truckfridge runs its compressor continuously whenever ambient is greater than 74°F.

Consequently....
Putting the original Mechanical thermostat “in series” is the best bet.
It works off the INKBIRDs protocol whenever the temperature difference is attainable (below 74F).
When the delta becomes un-attainable ( ambient above 74F ) the INKBIRD is just FULL-ON and the TruckFridge is now operating on its OE mechanical system, as if there was no INKBIRD in the system at all.
Which could be as simple as a "50% duty-cycle".

I'm gonna put the Mechanical thermostat “in series”.
_________________


'90 Westy EJ25, 2Peloquins, 3knobs, pressure-oiled GT mainshaft, filtered, cooled gearbox
'87 Tintop w 47k 53k, '12 SmallCar EJ25, cooled filtered gearbox
....KTMs, GasGas, SPOT mtb

[westyventures]

Data point: My C60 and C85 Vitrifrigos are almost always running at the lowest setting on the dial, rarely more than 1/3 unless it's really hot inside the van. I regularly see 30-35 middle of the fridge. I removed the little compressor speed resistor plugged into the control module and I 'think' the compressor is running on the lowest speed now. Perhaps the difference is these fridges are vented through the wall allowing the heat to escape?

[drj434343]

[Howesight]

In another thread (I will look to try to find it), I chimed in about the "Merlin" circuit board that I installed on my TF-65 at the same time I installed my "Inkbird" (that goofy name still makes me crazy!!) ITC-2000 digital thermostat.

The Merlin unit senses how long the run time is between on and off cycles. If the run cycle goes on past a pre-set limit, the Merlin applies a resistance to the SECOP compressor controller and ramps up one compressor speed range from the base speed of 2,000 rpm. Then, in the next compressor "on" cycle, if the run time is beyond a pre-set length of time, the Merlin again ramps up compressor speed to the next speed. ((2,750 rpm IIRC??).

This continues until the highest compressor speed is achieved, which I recall to be 3,750 rpm. Conversely, when the Merlin counts decreasing run times, it ramps down the compressor speed, again, in steps in the same fashion.

In the 2021 July "heat dome" event that cooked the Okanagon area of BC (114F!!), my TF-65 ran most of the time at the highest speed, dropping back a step or two only at night. My fridge contents stayed at the Inkbird temperature I set at, IIRC, 35F or thereabouts.

Indel has a device that senses when you have the engine running (thus alternator power) or have good solar power input and ramps up compressor speed to overcool the contents so less battery power is used later. It looks like a nice unit. Here is a link to a vendor showing a pic of a Westfalia install:

(Open the link and scroll down)
https://boattest.com/isotherm-intelligent-temperature-control

Frankly, I think that now that the marine market and RV (and Westfalia) markets are moving to LiFePo4 batteries, the usefulness of this expensvie Isotherm device is significantly reduced as the tendency is for most end users to go to larger capacity LiFePo4 batteries, reducing fears of running out of battery capacity.

By contrast, the Merlin device does seem to be great little device for those camping in areas with high ambient temperatures - - you get the electric power economy that Truckfridge and Secop intended to provide, plus easy access to the extra cooling power that is available in the Secop controller on the BD-35 compressor unit.

Mine has worked without complaint. It is still sold at a number of marine chandlers:

https://coastalclimatecontrol.com/images/PDF/Refer/Merlin-II-Instructions-v3.pdf

https://www.go2marine.com/Frigoboat-Merlin-II-Auto...ompressors

https://titanmarineair.com/product/frigoboat-merlin-ii-mini-smart-speed-controller/
_________________
'86 Syncro Westy SVX

[drj434343]

Thanks for the info. That Merlin II looks pretty cool, though expensive. It would be interesting to know what resistances get used to alter the compressor speed. I have the battery capacity to spare, so my interest is mostly in extracting the most cooling power from the fridge over efficient running.

I went ahead and connected my mechanical thermostat in series with the digital one and the fridge runs, but it occurred to me I don't have anyway to tell what speed the compressor is running.

People seem to be speaking to documented changes in compressor speed, is there anyway to tell?

[Howesight]

Here is a PDF from the inventors of the BD-35 compressor, namely, Danfoss. At page 4, they describe the different compressor speeds and the resistance values needed to engage those different compressor speeds.

It turns out that my recollection on the RPM steps was not quite accurate.

https://fmmsusa.com/wp-content/uploads/2016/09/BD35.BD50.Danfoss.pdf
_________________
'86 Syncro Westy SVX

[drj434343]

There must have been something wrong with my resistance measurements of the mechanical thermostat, as all the compressor documentation I've seen (including your link) suggests hundreds of ohms are needed to make substantial RPM changes.

My mechanical thermostat was working solo, so I am going to assume that in series with my digital controller I will be achieving higher compressor RPM. It will be interesting to see if I notice increased cooling; I was operating without resistance for about a year, and in hotter summer trips, the fridge would run sometimes 90% duty cycle, but it did always keep up.

It seems like a poor man's substitute for the Merlin would be a potentiometer wired in series with a digital controller, and equivalent compressor RPM's marked on the dial. Turn it up during hot days, turn it down at night...

[kiltym]

[drj434343]

Had a chance to camp this weekend and immediately noticed a cycling distance with the fridge after connecting the mechanical thermostat in series.

Amp draw looks like it went from 2.5 to 3.5 amps when running, but honestly I'm not sure if net usage was higher given how much less it ran.