This article originally published in Issue 58 of the Turbo Diesel Register.

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6.7 HPCR - Coverage of the '07.5 to current Model Trucks

68RFE TRANSMISSION — ASK THE ENGINEER

In early July there was lots of conversation about the new-for-'07.5 68RFE automatic transmission. I asked TDR writer Andy Mikonis if he could arrange a tour of the Kokomo, Indiana, transmission factory. He was set to make the trip, only to have to reschedule at the last minute.

As the magazine deadline was closing in, Andy and I opted to try “Plan B.” Plan B would be to use the TDR web site and TDR member questions and “Ask the Engineer.” Arrangements were made, and questions were written. Presenting, “Ask the Engineer.”

Please give us the basic specifications of the 68RFE, and compare them to the 48RE (size, weight, ratios, internal lockup pressures, clutch surfaces)

 
48RE
68RFE
Weight
248 lbs.
263 lbs.
Gear Ratios 1
2.45
3.23
  2
1.45
1.84
  3
1.00
1.41
  4
.69
1.00
  5
.
.82
  6
.
.63
Reverse
2.21
4.44
Lock-up Pressure
120 psi
120 psi
Clutch
Single-Face
Dual-Face

 

TDR: What type of fluid does it use? What are the maintenance intervals? What sort of filtration does it have?

The 68 uses ATF +4 (MS-9602). Maintenance intervals are 120,000 or 60,000 miles, depending on duty-cycle (see owners manual). The sump oil is filtered with a high-flow, advanced primary filter (different part numbers: 4x4 0513470AC; and 05015267AC 4x2). The fluid is filtered again when returning from transmission oil cooler, providing a double-team cleaning effort. The part numbers needed for the external filter is Mopar 05179267.

Filters for the 68REF transmmission.

TDR: Is the 68RFE an original design, or a modification of a previous transmission?

Although the 68 uses a clutch arrangement and hydraulic control system that are similar to the 545RFE transmission, the planetary geartrain configuration and tooth counts were altered to provide the 6-speed gear ratios. The input shaft, UD shaft, OD shaft, output shaft, UD clutch, OD clutch, 2C clutch, overrunning clutch, and the entire planetary geartrain were all designed to handle the 650 ftlbs of torque from the 6.7-liter. Also, pump capacity was increased to improve fluid flow through cooler, greatly reducing overall transmission heat. The 68 was engineered from the ground up and “supersized” to match the power stats of the 6.7-liter Cummins.

TDR: Was it developed from the beginning for a diesel engine application?

Yes. The 68RFE was designed to be mated with the 6.7-liter Cummins engine. Thus far this is the transmission’s only application.

TDR: Some people feel that the 68RFE is Dodge’s answer to the Allison transmission used with GM diesels. What features and benefits do we have over the Allison?

The transmissions are very similar. The 68 has a lower first gear ratio to improve acceleration, and a slightly wider overall gear ratio spread.

TDR: Does the transmission lock-up out of first gear? Do lockup pressures vary with RPM and load?

The 68RFE does not engage the torque converter clutch (TCC) in first gear. The TCC is applied in 2nd through 6th gears, and use varies depending on trans temperature, mode of operation (Tow/ Haul, exhaust brake, gear selection), etc. Yes, TCC pressures vary with RPM and load.

TDR: Does it have a manual shift capability?

The 68RFE has Electronic Range Select (ERS), which allows the driver to manually select which gear to operate. This feature gives the customer more options for different drive cycles.

TDR: Are exhaust brake controls programmed into the PCM?

Yes.

TDR: Why did you choose to use the Aisin 6-speed for Chassis Cabs and 68RFE in 2500/3500 trucks? Is the Aisin equal in strength?

The Aisin transmission is used where a Power Take Off (PTO) unit is required. The Aisin transmission has PTO capability which is well suited for the Chassis Cab models. The 68RFE and Aisin transmissions have similar duty cycle capability and are designed for their specific applications.

TDR: What are the designed shift points in normal and tow/haul mode?

The shift points vary based on several inputs, including load and driving behavior.

TDR: What functions are changed in tow/haul mode? Can overdrive and cruise control be used?

Part-throttle upshifts into 4th, 5th, and 6th gears are delayed, and part-throttle kickdown sensitivity into 4th and 5th gears is increased. These changes provide improved response, and reduce grade hunting and shift busyness when towing. Clutches are also applied more aggressively during tow/haul, preventing unnecessary heat generation during shift maneuvers. Yes, you can use overdrive and yes, you can use cruise control in tow/haul mode.

TDR: Can this transmission be retrofitted into earlier Ram trucks? Are the electronic controls heavily integrated in the PCM or does it have its own primary controller? Are the spline shaft, bell housing the same?

The interface to engine is similar (bell housing bolt pattern), but the 68RFE uses a separate transmission controller. The 68RFE controller interfaces with the engine controller for exhaust brake operation, etc. Therefore, retrofitting the 68RFE into a 5.9-liter truck is not feasible due to incompatibility with the 5.9-liter engine controller. Also, the 68RFE is fully electronic compared to semi-electronic 48RE.

TDR: What kind of testing was done on the 68RFE before production? How were problems associated with hauling heavy loads addressed (fluid overheating, bigger cooler)?

The 68RFE passed not only our standard battery of transmission tests (including wide open throttle testing, powertrain endurance testing, rock cycle testing, and fleet vehicle testing), but also unique-to-vehicle testing program (such as a high-speed WOT dyno trailer tow test at full GCW) for this application. The input shaft, UD shaft, OD shaft, output shaft, UD clutch, OD clutch, 2C clutch, overrunning clutch, and the entire planetary geartrain was upsized/revised to handle higher torques. Pump capacity was increased to improve fluid flow through cooler and reduce overall transmission heat. Chrysler engineers “supersized” the transmission components to reliably handle the 6.7-liter Cummins engine and 650 ft-lbs of torque.

TDR: What type of torque converter clutch does it use? Will the torque converter hold without slipping when hauling heavy loads? What kind of load can it handle on a downhill with the exhaust brake applied and not slip for durability issues?

The 68RFE includes an upgraded torque converter that includes a larger dual-face torque converter clutch (TCC). This TCC yields greater engine torque capacity at highway speeds than its predecessor. The new 68 provides ample capability for aggressive engine braking at max GVW on steep grades via electronic algorithms which control the exhaust brake. We ran the 68 at high-speed WOT dyno trailer tow test at full GCW for acceleration testing and then pointed downhill for the hard-line engine brake test, again at full GCW. The engine braking system is electronically calibrated with the 68 to maximize braking force even under the harshest down-hill cycles.

TDR: Was using an electronically controlled manual, like Volkswagen’s DSG, explored?

Yes, but the DSG type system was not considered to be the optimal system, so was not selected for this vehicle.

TDR: What changes, if any, have been made to address breakage issues associated with snow plowing operations? Were the planetary gears and bands upgraded?

Chrysler engineers designed the 68 without any bands, which eliminates that particular issue. The input shaft, UD shaft, OD shaft, output shaft, UD clutch, OD clutch, 2C clutch, overrunning clutch, and the entire planetary geartrain is larger to handle much higher torque numbers. Transmission fluid pump capacity is increased to improve fluid flow through cooler and reduce overall transmission heat. Also, the 68 has a significantly higher reverse gear ratio compared to its predecessor, which reduces heat generation during plowing.

Cutaway illustration of 68RFE transmission.