Custom Oil Cooler Cures Hot G56 Transmissions
Overheating the aluminum G56 five- and six-speed manual transmission is a concern for many ’05½-and-newer heavy-duty Ram owners who use their rigs for severe-duty tasks (hot-shot and excessive-weight towing, construction, ranch work, and such), especially during the heat of summer.
While normal operating temperature for the factory-approved ATF fluid gearbox is between 160 and 190 degrees Fahrenheit, it can top 250 degrees when ambient temperature nears triple digits while towing trailers that push a truck’s maximum GVWR. Trucks with larger exhausts compound the heat issue, because the exhaust is much closer to the transmission.
Fluid temperature is a big concern, because when ATF+4—the stock lubricant in G56s—exceeds a certain temperature, it begins losing its ability to adequately protect gears and bearings, just as it does inside an automatic transmission. Most heat charts for ATF show that once the fluid’s temperature reaches 180 degrees Fahrenheit, its rate of degradation doubles with every 20-degree increase.
Transmission experts say operating a G56 transmission when its fluid temperatures exceed 220 degrees for even short durations has a cumulative effect on its bearings and seals, shortening the transmission’s normal service-life expectancy. Even though a G56 is full of gears—not clutch packs and valvebodies like an automatic—even reducing the average fluid temperature from 200 degrees to 175 degrees will have a positive impact on improving the manual transmission’s long-term reliability.
While there are several bolt-on fin-type power takeoff covers available, they do very little to make a 20- to 25-degree heat drop, because there’s very little cooling airflow around the transmission case. That’s why the only effective method to cool a G56’s transmission-fluid temperature by any significant amount is to install a system that actually circulates the transmission fluid through a remote cooler.
Using a transmission cooler on the G56 requires a pump to circulate the fluid, as a manual-transmission’s fluid isn’t under pressure. Some owners install stock, radiator-mounted coolers used on automatic Rams, along with aftermarket pumps and filters to handle the circulation.
For our ’14 Ram 2500, we want a cooler system that is easier to resource and install. After doing some research in the truck-racing scene, we came up with the concept of pairing a Flex-a-Lite baby transmission cooler/fan combo with a Tilton Engineering positive-displacement pump that’s commonly used to circulate fluid through differentials and transmissions on race vehicles.
What we ended up with are three components that work really well together for this application. A Flex-a-Lite remote-mounted, 17-row, stacked-plate cooler (PN 600117) measures just 11x6x3.25 inches and includes a 6.5-inch, thermostatic-controlled fan. It’s also set up with 3/8-inch barbed fittings, which are fine for our low-pressure use.
The Tilton 40-527 transmission/differential oil cooler pump is a robust Buna (designed for oils and coolants) unit ideal for pumping oil through transmission and differential coolers, thanks to an internal bypass valve. The self-priming pump can also run continuously for longer than two hours without requiring time for cool down.
Finally, Trusted Design Services’ custom CNC-machined PTO covers (PN PTO-G56-CC-NPT) are used for making the connection between the G56 and pump/cooler combo an easy process. The covers are machined from billet 6061-T6 aluminum stock, with ports pre-drilled and tapped for the 3/8-inch “In” and “Out” fluid fittings, 1/8-inch NPT for a temperature probe, and a fill port that’s designed to accommodate 8 quarts of ATF (stock is 6.5 quarts)
Installing the transmission cooler is very straightforward. Ruben Villalobos, a technician at Mobile Diesel Service in Oakland, Oregon, mounted the Flex-a-Lite transmission cooler on top of the crossmember for the skidplate of our four-wheel-drive ’14 Ram 2500’s transfer case, bolted on the PTO covers (he also replaced the passenger-side stock cover with a billet plate for added fluid capacity and appearance), and mounted the Tilton pump and filter assembly to the outside of the framerail.
Ruben then ran rubber 3/8-inch power-steering hose from the “Out” fittings on the G56’s PTO cover to the filter, pump, cooler, and back to the “In” fitting on the billet cover. (Braided stainless coolant line can also be used for a more custom appearance.)
The more tedious aspect of this installation is routing the wiring harnesses that feeds information from the temperature probe in the PTO cover to an Edge Products CTS2 programmer/monitor mounted in the truck’s cab. We took advantage of the unit’s Expandable Accessory System by installing the temperature probe kit (PN 98608), along with a Universal Sensor Input (PN 98605) to tie directly into the monitor.
The CTS2 allows drivers to monitor the transmission-fluid temperature in real time. For example, when towing a trailer and fluid temperature tops 180 degrees, turning the Tilton pump on and activating the built-in thermocoupler on the cooler’s fan arms the cooler to automatically power up when the probe records 200 degrees. Airflow across the little radiator’s surface should drop the fluid temperature 20 to 25 degrees.
Is this custom G56 cooler upgrade for everyone? No. But for those who tow heavy and/or tow long distances in high ambient temperatures, it’s a sure way to keep the lubricants inside the manual transmission cool under pressure.
Sidebar: Best G56 LubeThere are lots of questions and an equal amount of theories as to which lubricant is best for the G56 manual transmission. What’s our take? Stick with ATF+4 until the truck’s warranty is expired. Mercedes-Benz, the company that manufactures G56 transmissions, recommends iterations of transmission fluid that are identical in makeup to Mobil Delvac 1 synthetic transmission fluid (SAE 50), according to laboratory tests. Some big rig shops use Delco synthetic transmission fluid (SAE 50), which is said to be specifically formulated to meet the latest extended-drain requirements for Eaton transmissions used in big rigs, with drain intervals of up to 500,000 miles.
Edge ProductsOgden, UT 84404
Flex-A-LiteFife, WA 98424
Tilton EngineeringBuellton, CA 93427
Mobile Diesel Service541-459-8939
Trusted Design Services210-364-3713