Hacking the Secop / DanFoss Compressor

Ever since I installed my Isotherm Cruise 219 I have been able to travel anywhere in the USA, in any weather, and have food safely stored in my refrigerator and freezer, which is more than I can say about my factory Dometic propane fridge.

Still, its not perfect. If I had it to do over again, I would have selected a dual compressor fridge. The extra power and precise ability to tune both compartments is even better.

I’ve had numerous issues with my fridge, some self inflicted. The self inflicted one is, I purchased this guy:

This is the very cool Secop “ITC” controller, with many features and looks nice on the wall. As I soon discovered though, it had a major drawback.

Rather than the crude mechanical thermostat that is standard equipment, it controls the temperature with a probe and its rather precise. The problem is, it caused me to have a tremendous frost problem – as bad as the Dometic. Thats because it short-cycles the compressor continuously, running in 15 minute or so cycles, with an equal amount of “off” time. This results in a continuous ice coating of the back wall. Its never off long enough to have a melt session.

This led me down an endless rabbit hold with the manufacturer, who started sending me new doors and such in the useless quest to seal the refrigerator better.

But it turns out frost is not really caused by bad door seals as the Internet seems to believe. Home refrigerators have heaters and perform defrost cycles daily. (And every time you open the door a blast of humidity enters, so its somewhat hopeless to keep humidity out.)

RV refrigerators have no such ability. The only way to get it “frost free” is to turn it off long enough for the temperature on the back wall to rise close to 40 (which is still a safe zone) and then enjoy the massive ice melt. The crude mechanical thermostat did a great job of that – by virtue of it running the compressor longer – and then the resultant longer downtime. So the trick to get the fridge somewhat frost-free is extended run time followed by extended off time.

So you’d think I could just remove and put back the factory thermostat, right? Well, no. It turns out the ITC reprograms your Secop compressor!

Once reprogrammed, it will no longer work correctly with a mechanical thermostat. You could replace the Secop controller of course, but I started thinking that what I really need is a better thermostat, that more accurately would generate defrost cycles, and for starters I would have to reprogram my compressor so I could get rid of the ITC.

And that search led me to the discovery of this nifty software aptly named “Tool4Kool”:

This package is designed for refrigerator manufacturers to program the Secop compressors on the assembly line. There are a lot of useful settings such as battery cutoff voltages and such, and most importantly the thermostat type. I wasn’t sure if you needed to buy a key to use the software, but it seems I had further to go down the rabbit hole.

The next step is, I needed some way to actually connect a refrigerator to my laptop PC, and it turns out they have a brand new product just for that:

This is the all new Secop-105N9518. Its a gateway that uses a standard USB-A so you can connect directly to the Secop controller. Again, designed for refrigerator manufacturers. Neat! Except you can’t actually buy it in the USA (we don’t make any Marine/RV refrigerators I guess).

In fact, because its so new you can’t actually buy it anywhere in the world practically! Maybe its just not advertised – you have to be a refrigerator factory?

I did eventually find it here:

Hello! This is Jabsales in Maribor, Slovenia. These nice folks have it right on their website. The price was very reasonable (shipping not so much). They required a wire in advance but to their credit shipped the next day, so it was a very good buying experience (not counting the hurt of the overnight international Fedex).

So with software and interface in hand I hooked it up and, it worked!

Took4Kool did everything it promised and more. To my surprise I didn’t need any software key and moments later I was reprogramming the Secop compressor.

The first thing I did was, most manufacturers (including Isotherm) have a resistor on the mechanical thermostat connection. This resistor sets the maximum thermostat speed. I verified mine had the correct resistor for maximum (3500 rpm).

But I’m hoping to build a better thermostat, and I don’t want to ever run the compressor at less than max anyway, so I removed the resistor jumper, and reprogammed the compressor to just look for a straight on/off at the the thermostat contacts. And then I programmed the Secop to hardwire the compressor speed.

This will allow me to go even further down the rabbit hole, and make a thermostat that is more to my liking. My thermostat will have specific programmed defrost cycles, and be tweaked for my particular refrigerator. More on that to come soon…

November 2023

Still working on my new thermostat. In the meantime the Secop gateway stopped working, complaining that I needed the dreaded product key. Fortunately they list a bunch of the right on their product page, and the “general” one of ZH3TT6RBAD1GW8G68ZM1N8G62ZMCORVB3OEGBL12UZNAR worked.

I’m pasting it here in case it disappears from the internet some day, but as of now it works fine. It seems to me that the gateway is powered entirely from the laptop USB, so leaving it connected semi-permanently is just fine as it turns off anyway once the laptop is removed.

I’ll post more on my experimental thermostat soon, but in the meantime here is the early development version. This is just for testing, I’ll obviously have to solder something substantial to go bouncing down the road in my RV. It uses (2) Arduino Nano 33s and a 5 volt relay to turn the Secop compressor on and off. The processor on the left handles the Bluetooth connection to (2) Sensorpush Bluetooth sensors, and the one on the right handles the control panel and Internet (WiFi) connection for remote control and monitoring.

Nov 2023 Update: I built the thermostat; testing it now. Details at: Build a Better Refrigerator/Secop Thermostat

2 thoughts on “Hacking the Secop / DanFoss Compressor

  1. When I was working to improve my NovaKool I tried in van to get the earlier version of this tool. I no longer have a danfos / secop refrigerator compressor or I would by this and follow you down the same rabbit hole. I look forward to your next posting.

    Gamma1966 on sprinter forum.

  2. Thanks Harry,

    I don’t know why its so hard to find. Even the previous model is hard to come by. Maybe its not supposed to be sold to the public but this HVAC supplier in Slovenia had a bunch in stock, who knows.

    We’ll see if this leads anywhere. My goal is to focus on frost-free, not power consumption, which is not of major importance to me.

    At the very least if I can get my custom thermostat working it features an Internet interface to monitor and change settings so that would add some value.

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