Thursday, June 14, 2012

Keeping It Cool - Installing a Transmission Oil Cooler Fan - Part 1

Supplemental Transmission Cooler
   At some point in its life, my coach had a supplemental transmission oil cooler installed. This is a good thing. If you "cook" your transmission oil, bad things will happen. The fluid is cheap, your transmission isn't. On a recent trip driving through some mountainous terrain, I noticed my transmission temperatures were higher than I'd liked. I decided that adding a fan and maybe a thermostat to automatically turn it on and off  when needed would be a good idea.

Let's talk a bit about automatic transmission temperatures in general. The harder the transmission is working the hotter the fluid gets. Common Sense, right? Pull a heavily loaded RV up a long hill, sometimes with a trailer or toad behind, and the transmission temperature will climb. In fact, it can climb VERY fast. Especially when it's very hot and humid. Many RVs have some kind of transmission cooler already installed. Sometimes it's as "simple" as a few extra pipes inside the engine coolant radiator and sometimes it's a separate radiator entirely. My Aero Cruiser had the former and a previous owner added the latter. For some reason they decided NOT to add an electric fan on this supplemental cooler. I have NO idea why. Most manufacturers of transmission fluid say, if the fluid gets above 300 degrees Fahrenheit for any length of time permanent damage could be done to the transmission itself. Some manufacturers state you can get 100,000 miles from the fluid IF it remains at 175 degrees or below. This chart shows how the life is shortened with temperature increases:


Wow, 40 Miles! I'd hate to have to change the transmission fluid that often! Personally, I can accept every 25,000 miles. So, shooting for an average temperature of 200 degrees seems reasonable. I have seen 250 degrees on a long hill, mostly due to airflow issues. I am hoping the fan will fix this. I COULD remove the plumbing for the auxiliary cooling loop in the radiator and likely reduce the starting temperature a little since the transmission fluid wouldn't get heated by the engine coolant flow. The down-side is getting the transmission fluid up to operating temperature in colder weather. We'll see what the readings are this season and make changes accordingly.
The Fan.
Thermostat Switch and Relay
After researching online I selected a 9" 12 Volt 80 Watt Fan that would move enough air to make a dent in the temperature. When the fan arrived, it was missing several critical installation pieces. After a quick email to the seller, I received a package that came with all the mounting hardware including springs to absorb vibration caused by the road and the engine. I also ordered a remote thermostat that would be adjusted to turn the fan on at 175 degrees (or so) and off  again. This installs on the radiator itself and has a relay that feeds the power to the fan. The remote switch will allow me to manually turn the fan on if needed.

Looked to be easy to install. Of course the location of the transmission oil cooler and the fact that the 9" fan was actually 10" across (the BLADES are 9") meant I was going to have to either let it hang below the cooler or finagle an angled mounting. When I got to thinking about the angles install I believed the slight angle would actually pull cooler air from under the RV rather than warmer air from the engine compartment. Seems logical, no? This being decided, I gathered all my tools and the bits and pieces of the fan install kit and went down to the RV.

With everything laid out and ready to install. It began to rain. Again, a lot. Since I was going to have to lie down on the ground under the RV to install this fan, I decided (for once) to postpone the install until next week. We'll cover the actual mounting of the fan using the supplied pieces and wiring all of the electrical to make it all work. I feel bad this couldn't get done this week, as I REALLY want to get on the road...but getting soaked seemed a dismal prospect.

Tune in next week for the final assembly!

See Part 2 -

Be Seeing You, Down The Road,

Rich "The Wanderman"


  1. Just gotta get some grommets on yer house's front so you can attach a tarp from it over yer motorhome to create a temporary covered work area :).

    1. Matt,
      Yeah.....that would be great!

      Rich "The Wanderman"

    2. So, how much did that boosted tranny cooler kit cost? Is it designed to work with any early 89-93 360-1 v8 with the 46RH (A518) auto transmission or is it only for a specific year and make?

    3. Matt,
      The tranny cooler was installed when I bought the RV. I've seen good ones go for around 200.00 in the same size. I sourced the fan based on the diameter and the thermostat switch was adjustable to set on/off temps. I actually have a 727 transmission. Essentially bulletproof AND easy to fix/get parts for.

      Rich "The Wanderman"

  2. Hi,

    Where is the transmission cooler mounted? It looks like it's mounted in a side compartment OR is this in a Class A?

    Just trying to figure out where to mount a transmission cooler in our 1990 Class C (Ford E-350). There is no room in our engine compartment. I may have to do something up around the frame rails.



    1. P Dorris,
      Sorry, I believed the photos to be descriptive. This install is on my Class A 1991 Aero Cruiser. The Transmission oil cooler hangs somewhat below the main engine radiator. Though this fan (or one like it) could be installed behind (or in front of if you make it a "pusher") any cooler. Provided there is space.

      Think of the Class A similar to a Giant Van (where the engine is partially in the passenger compartment and has a small hood to access the front.

      If you look at some of the other entries, you'll see lots of pictures of my engine compartment and views from underneath and many angles of the exterior.

      Hope this helps,

      Rich "The Wanderman"

  3. You mentioned "maybe" removing the lines that run through the radiator. If your goal is to keep the transmission as cool as possible, I'd do that. The transmission will warm up just fine on its own in cold weather.

    I'd also replace those rubber hoses while I was down there. My personal choice would be steel tubing but new hoses would be easier and better than nothing. Age, heat and pressure are mortal enemies of rubber things.

    1. Paul,
      After I test to see what the temps are now, I will decide whether to remove the aux. cooling loop from the main radiator. I would then need a larger transmission oil cooler, rated as a primary cooler based upon laden weight.

      The hose you see is the overflow pipe from the coolant reservoir. Not essential, but saves a mess.

      Rich "the Wanderman"

  4. Hey Rich, your chart ended up being very helpful to me on my current cross country adventure, with my new Tru-Cool Max transmission cooler I've maxed out at 180 degrees as a my top of the hill fluid temp (Cools off as soon as I start back downhill) pulling the Rocky mountains.

    Definitely need to get a pusher fan, though, the all aluminum truck radiator I installed keeps the engine at a near constant temp regardless of how hard it's working, so the fan doesn't always spin up that would normally pull extra air through the two of them.

    1. Matt,
      Way Cool! (Pun Intended!) I've still got to do some air redirecting to get those temps setup correctly. I have a low pressure stagnant air "pool" behind the grill. It's created when air is pulled down through the open "floor" of the engine bay. My Aero Cruiser has the electrical box inside so it doesn't block and escaping air in the engine bay. I have a floor to test.. we'll see how it goes.

      Rich "The Wanderman"


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