How to Install a Transmission Oil Cooler Fan – Part 1

September 8, 2013 chucker


Supplemental Transmission Cooler

At some point in it’s 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.
Be Seeing You, Down The Road,
Rich “The Wanderman