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Custom Oil Cooler Cures Hot G56 Transmissions

’Cooler Concept

Bruce W. Smith
Nov 28, 2018
Photographers: Bruce W. Smith
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.
Photo 2/21   |   Custom Oil 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.
Photo 3/21   |   Our Ram G56 transmission-cooler kit consists of (left to right) Trusted Design Services billet power-takeoff covers and oil-filter housing, Flex-a-Lite’s universal transmission-cooler assembly, Tilton Engineering Buna-style continuous-duty oil pump, and Edge Products’ Expandable Accessory System.
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.
Photo 4/21   |   Ruben Villalobos of Mobile Diesel Service removes the steel PTO cover plate from the driver side of the ’14 Ram 2500’s G56 six-speed manual transmission and installs a Trusted Design Services billet-aluminum cover in its place. The TDS cover is tapped and drilled to accept the fittings and temperature probe necessary for a circulating oil cooler.
Photo 5/21   |   Edge Products’ Expandable Accessory System lets a user simultaneously monitor a wide array of sensors in different locations on CTS and CTS2 monitors. We elected to use the EAS temperature-probe kit and wiring harnesses to monitor the G56’s fluid temperature, instead of mounting a separate gauge somewhere in the cab.
Photo 6/21   |   The PTO cover takes 3/8-inch O-ring boss fittings. Ruben then uses 45-degree fittings to run 3/8-inch hoses up and over the framerail to the filter and pump.
Photo 7/21   |   Ruben uses a piece of scrap diamond plate for the Flex-a-Lite cooler assembly’s mounting bracket that attaches to the skidplate’s crossmember. He also adds a little brace between the cooler and the framerail for extra support.
Photo 8/21   |   Wiring for the fan consists of a simple ground wire and a power feed to the same lead Ruben uses to power the Tilton pump.
Photo 9/21   |   A couple of self-tapping screws make attaching the TDS oil-filter housing to the Ram’s framerail an easy task. The housing bolts on from the back, so a thin hold-off bracket is used to attach the assembly to the frame.
Photo 10/21   |   A NAPA 1394 oil filter will stop debris that might be in the G56 from making its way to the Tilton pump.
Photo 11/21   |   Ruben mounted the pump up high, grounded a black lead to the frame, and ran the power wire into a switch on the dash. The low pressure of this setup supports using barbed fittings and regular 3/8-inch power-steering hose for this installation.
Photo 12/21   |   The Ram’s tall framerail works nicely as a mounting plate for the G56 oil-cooling system. Fluid circulates so that it is filtered before entering the pump and flowing into the cooler. Braided line can be used, as well as ordinary 3/8-inch power steering hose.
Photo 13/21   |   The Edge EAS wiring is all plug-and-play, going from the yellow wire of the probe to one end of the harness that then connects to the special connector/harness that allows users to daisy-chain multiple inputs from other sensors. That harness is connected to another that plugs into the back of CTS and CTS2 monitors.
Photo 14/21   |   Custom Oil Cooler Probe To Eas Connector
Photo 15/21   |   Custom Oil Cooler Eas To Egdects
Photo 16/21   |   A blank TDS PTO cover is bolted on the passenger side to maximize coolant capacity for the G56 (and the billet aluminum makes it look cool).
Photo 17/21   |   Edge Products’ Expandable Accessory System facilitates monitoring a wide range of data with a CTS2 monitor. The device can be configured to present data per a driver’s discretion. In this setup, the transmission temperature is in the first column, third down.
Photo 18/21   |   Trusted Design Services uses solid .850-inch-thick 6061-T6 aluminum bar stock for its G56 power-takeoff covers. Each side is end-milled to ensure perfect fit and function. Installing each cover increases fluid capacity by 5 ounces due to the backside recess.
Photo 19/21   |   This little 17-row, stacked-plate Flex-a-Lite cooler (PN 600117) measures just 11x6x3.25 inches and includes a 6.5-inch, thermostatic-controlled fan. We think it’s perfect for small-volume oil cooling.
Photo 20/21   |   Tilton Engineering’s 40-527 transfer pump flows .9 gpm, at a maximum of 50 psi. It’s a Buna-style, continuous-duty unit, so it can be left on for hours at a time without harm. This pump is used to cool differentials in race cars, so it’ll work fine circulating the fluid in a Ram’s G56 manual transmission.

Sidebar: Best G56 Lube

There 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.
Photo 21/21   |   Big-rig repair is Mobile Diesel Service’s bread and butter; the shop rebuilds and services a ton of Eaton transmissions on a regular basis. The lube of choice is SAE 50 Mobile Delvac 1 Synthetic transmission fluid. It’s also the same lubricant used to refill our ’14 Ram 2500’s G56, which is out of warranty. It takes a little more than 9 quarts once the filter, pump, cooler, and lines are primed.


Edge Products
Ogden, UT 84404
Fife, WA 98424
Tilton Engineering
Buellton, CA 93427
Mobile Diesel Service
Trusted Design Services