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Old 02-19-2021, 10:33 AM   #1
Mlfiedler's Avatar
Country Coach Owners Club
Join Date: Aug 2017
Posts: 78
Yet Another Crack at the Fan Control

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:

Then I saw the post by Ron (IRV2 handle: RonandSue74, thread at: ) 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":
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Attached Files
File Type: pdf Page 31 from Danfoss Fan Control Manual.pdf (522.1 KB, 11 views)
File Type: pdf Parts List, Fan Control Project.pdf (843.1 KB, 14 views)
Mike & Lynn
2000 Country Coach Allure "La Pine", #30536 pushed by an 2014 Ruby Red Grand Cherokee. 38,650 miles run as of 10/17/2022. Retired 12/2017, and loving it.
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Old 02-19-2021, 12:26 PM   #2
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Location: Minnesota
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Mike - very interesting! I've been troubleshooting an SD FDC and have read Ron's informative thread as well. I'm leaning toward a Wax valve because of simplicity and reliability. But am open to other Non SD solutions!!
2000 CC Magna #5734
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Old 02-19-2021, 06:15 PM   #3
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Mike thatís great! I think youíre on to a great alternative to the SD. I noticed you said there was a ramp up to full speed. Both of the controllers I had would not ramp up. When the temperatures reached 199 deg, it would just go to full power until the temperatures came down to around 182 deg. When a/c was turned on or when CAC temperatures rose the fan went to a medium speed. If I didnít already have the wax valve, Iíd probably give your project a try. Fred Koval from the Country Coach owners forum made something similar about 10 years ago, but for liability concerns he would share the parts list.

I put together my wax valve conversion for under $400.I used basically the same parts as the Source kit. Thereís a good write up here, explaining how the wax valve works and parts needed;

2003 Country Coach Intrigue 36'
Cummins ISL 400
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Old 02-19-2021, 09:32 PM   #4
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Funny thing, Bill. You and I seem to be on the same street, headed opposite ways . . . if this box fails to work as planned, or just fails inside a reasonable time frame, I’m headed for the wax valve. And your $400 is the best quote I’ve seen on the subject! The reality of simple physical systems with very little to go wrong can be hard to beat. On the question of “ramp up”, etc. I think the complexity of S-D creating a system where each engine or coach builder could spec the performance they wanted turned into a problem. I think some coaches went to owners with either mismatched programming, or maybe with no options set. Cause I’ve read about others whose original, and even replacement fan controllers didn’t perform per that manual.
Right now, I’ve got to figure out how to get the S-D sensor out of the CAC duct! Put close to three hours into installing the system today. The sensor from the coolant line backed right out. The air temp sensor has rounded off the hex shoulder, and is now resisting a pipe wrench! Brass can be so soft! Looks like rain for the next three or four days.
Mike & Lynn
2000 Country Coach Allure "La Pine", #30536 pushed by an 2014 Ruby Red Grand Cherokee. 38,650 miles run as of 10/17/2022. Retired 12/2017, and loving it.
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Old 02-27-2021, 08:28 PM   #5
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As Maxwell Smart used to say, "Chief, I only missed it by that much!"

Well, learned a bunch more about proportional hydraulic valves in the last few days. Got my little magic box installed in the coach, and all the wires and sensors in place. Turned on the key, and found BOTH digital displays were lit up, reading about the numbers I had intended. But it should have been impossible for both to receive operating power at once. Still have not found how that was happening. I disconnected everything, took it in the house and connected my 12V starter battery. Only one side lit up at a time, no matter how I tried to stress the system. But the process of opening the box to look for a short, perhaps between two wires not intended to connect, convinced me to rewire the whole circuit with lighter wire, and leave slack to simplify reopening the box next time!

So, rewired fully with #18AWG. Upon completion, tested it, and the right hand PWM controller ranged only between 100% and 96%, then dropping to 000%. Could not coax it to the 30% range even though the knob turned full circle.

Trying to figure out how the electrical load of the S-D solenoid could induce the first error, or lead in any way to the second, I started researching how such a solenoid responds to a PWM current.

Ooooops. Hydraulic proportioning valve functions are optimized by PWM frequencies between 25hz and 200hz. The PWM controllers I was using operate at 16 Kilo hertz. Not quite the same thing.

So, long story short, it's on to some other solution!
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Old 02-28-2021, 04:24 AM   #6
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I’ve been wondering if an array of 12V fans would be able to replace the hydraulic fan completely. Is this just crazy thinking???

2003 Country Coach 32í
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Old 02-28-2021, 08:59 AM   #7
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Yes the solenoid valve is very sensitive the PWM frequency input from the Fan Drive Controller. Not all coached have the same type of solenoid valve. Sauer Danfoss tech support could not tell me what the frequency should be set at for our coach when I replaced the FDC. When I worked with the technician to program our new FDC we found that varying the output frequency from the controller made a large difference in the fan speed. By "trial and error" 60hz turned out to be the best setting for our FDC. The output amperage was also very important with the higher amperage leading to a slower fan speed. The stock controllers are programmed at 0.6 output amps which may not be high enough to control all types of valves (it was not for my coach). The FDC is rated to, I believe, 1.2 output amps but we were able to push the output to 1.5 amps in my coach which has worked perfectly in controlling the fan speed as it should. Mark
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