We bought our RV three years ago, but I keep learning things about owning a motorhome. Recently I learned the fan is not supposed to run flat out all the time! So the "Linus-Tornado" is not an essential part of pulling into an unpaved campground!
The cost, and relatively short service life folks report for replacing the Sauer-Danfoss hardware is discouraging. The wax-valve has the advantage of providing non-electronic protection from overheating your coolant, but trying to tie in the responsiveness to A/C and CAC conditions wasn't too promising.
A little research aimed at asking just how critical are those aspects revealed some surprising sophistication in the programming of the S-D system. Not only does it respond to rising and falling temperatures, but it varies the fan speed in proportion to the demands, and "ramps-up" and "ramps-down" over a definable range of seconds, so when the fan changes speed, it does it kind of gracefully. In the example shown, the ramp up time is 25 seconds. That would sure minimize any shock loads on the fan motor! That description of the S-D Fan Controller actually makes an interesting read, especially the discussion of programming parameters in the last 6 pages or so. You can check it out here: https://assets.danfoss.com/documents...5en-US0303.pdf
Then I saw the post by Ron (IRV2 handle: RonandSue74, thread at: https://www.irv2.com/forums/f112/sau...on-501809.html
) in which he points out that a DC motor speed controller works on the same Pulse Width Modulation as the S-D controller. He spells out a terrific solution, resulting in a two-speed option with fan speed responding to coolant temp, for under $100. And that got me thinking, could I adapt his solution to also pick up fan speed if the CAC was running too hot, or when the dash A/C is turned on?
Well, unlike Ron, who reports he has run for two years with his solution working quite effectively, I can only say that I have a kind of "proof-of-concept" construction, which has not yet been stuck into a motorhome. But if the weather clears, we may cross that hurdle, soon.
For a cost of about $130, plus a few odds and ends I already had laying around, I think I have a box that will allow me to set a nice, calm, low fan speed for start-up and low to moderate temperature and throttle conditions -- like rolling through a campground -- but switching to a medium speed whenever coolant temp rises above 200F, and kicking the fan up to full speed any time the CAC goes over temp, or the dash A/C is turned on. At least that's the theory! So far, it works in my living room . . . .
I've attached (1) some info from Sauer Danfoss, (2) a decision tree for the logic I wanted to apply, (3) a sketch of the circuit I developed based on Ron's eye-opening comments, and (4) a list of what all went into it. Finally, there's (5) a link to a short video showing how I hope it will function. The LEDs and wires were all on hand, so they aren't listed. For what it's worth, the project box ended up pretty crowded. If I were doing it again, I'd buy size 18AWG wire throughout. I used scraps of wire I had on hand, some 16, some even 14. Much heavier than the loads require, and stiff and bulky. A guy could also opt for the next size larger box. I have not ventilated the box, but I have "run" it for an 8 hour test run in the living room. I hooked it to a 12v power source, and found no detectable heat buildup from any of the relays or controllers.
The actual power delivered to the fan controller is not exactly correlated to the digital displays I've used, and measuring voltage may be misleading. Remember, at full continuous electrical power, your fan runs its slowest. The PWM is rated to cycle at 16,000 times per second. So at say, a 20% duty cycle, it is passing 12v power in pulses starting every 1/16,000 of a second, but only lasting 20% of that time. I don't have an oscilloscope, but I'll bet that's not a perfect square wave. I found my several multi-meters seem to have different response rates to a varying voltage, and read this reduced power as reduced voltage. They just can't agree on what that voltage is. So trying to set power with a meter here can get tricky. But with the dials, I'm hoping to "tune it by ear!" As they say, YMMV.
I'll post additional experience details once it's in the bus. Can't wait for some dry weather!
And here's the link, for a video of the "dry-run":