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The 68RFE and 66RFE automatic transmissions for Ram trucks

In 1998, Chrysler launched the 45RFE transmission, which used three planetary gear sets instead of the two usually used in a 4-speed automatic. This was, via the 545RFE, the rough basis for the 68RFE, launched in 2007, and the 66RFE, a lighter-duty version of the 68RFE launched in 2012 (we’re told that the main differences are in the torque converter).

545RFE

The 68RFE, a six-speed, heavier-duty automatic, was based on the 545RFE, using a similar clutch and hydraulic control system, but changing to different (and heavier duty) planetary gears with different tooth counts. Numerous parts, including the shafts and clutches and the torque converter, were redesigned to handle the high torque of the Cummins 6.7 liter diesel. Pump capacity was increased to deal with heat more effectively.

Gear 545RFE
65RFE
AS68RC* 66RFE
68RFE
1 3.00 3.74 3.23
2 1.67 1.96 1.84
3** 1.50 1.34 1.41
4 1.00 1.00 1.00
5 .075 0.77 0.816
6 0.67 0.63 0.625
Reverse 3.00 or 2.21 3.54 4.445

* Aisin.    
** On 545/65: “2 prime” normally unused.

The 68RFE has roughly the same capacity as the Aisin automatic also available in Ram heavy duty pickups, but the Aisin allows for a power takeoff (PTO) and the 68RFE does not. The Aisin AS68RC also has a steeper first gear for better off the line power, but thanks to wider ratios, retains the same sixth gear ratio. (The Getrag six speed manual available on these trucks had a 5.94:1 first gear and 0.74 sixth).

A “Tow/Haul” mode provided faster, crisper shifts to cut wear on the transmission, and reduced gear searching by holding lower gears longer.  (Details: under partial throttle, shifting into gears above 3rd is delayed; and the system is likely to kick down under part throttle into 4th and 5th.) It still allows for the use of overdrive, though owners recommend that overdrive be used with caution.

The gear ratios were designed to maintain speed on long or high grades, without sacrificing efficiency at highway speeds. Unlike the 545RFE, it uses all gears in normal acceleration, not reserving any for kickdown.

Electronic Range Select lets drivers limit the highest gear and make manual shifts. In the Ram Heavy Duty, a +/- switch is on the column shifter, and you press it up or down, holding your finger down on one or the other to go back to Drive. The gear shows up in the gauge cluster, in big print. This is mostly useful for towing and downhill or long uphill runs. Allpar’s test drive commended the transmission (in 2010) for rapid kickdowns and general responsiveness.

The 68RFE has dual lubricant filters, including an internal main sump pan filter and an external canister-type pump return filter (part 05179267); it uses Mopar ATF +4, a synthetic-based fluid used on all Chrysler-engineered automatics (the Aisin AS68RC requires a different fluid).

D68RFE automatic transmissionesigned to work with the Cummins engine, the 68RFE has a lower first gear than the 545 (or the Allison used by GM) for better launches and a wider spread, overall, of ratios. The torque converter clutch is engaged in every forward gear but first, based on factors such as temperature, gear, tow/haul mode, exhaust brake activation, etc. The transmission was designed to include electronic range selection. Because it uses a unique computer, it cannot be retrofitted back into earlier trucks. The assembly weighs 263 lb.

Owners have pointed to weak spots in the transmission, as made from 2007-2011; the torque converter was specifically highlighted as a problem. Partly for that reason, when, in 2012, Ram started selling a higher-output Cummins diesel truck with 350 horsepower and 800 lb-ft of torque, they also beefed up the torque converter and modified the shift algorithms, presumably switching to firmer, faster shifts that would result in less heat.

The 66RFE and 68RFE provide a wider overall gear range than the 545RFE/65RFE, which can help in towing. The transmissions can be identified via a label on the transmission case which has the part number and build sequence; these numbers are also stamped into the case, below the label. The 66 and 68 have one set of splines on the overdrive shaft; the 545/65 has two.

While the 66RFE resembles the 545RFE from the outside, its extension housing and output shaft are shared with the 68RFE. The 66RFE has its torque converter recessed inside the case, while it sticks out of the 68RFE; these differences are noticeable when the transmission is taken out.

All three automatics have a one-piece case, electronic control, two stage hydraulic pump (which shuts the secondary side at higher speeds, for efficiency), and similar valve bodies. They all use three planetary gearsets, three driving clutches, three holding clutches, and an overrunning clutch. The 66 uses some parts derived or taken from the 545 (mainly in the front: torque converter and pump; overdrive, underdrive, and reverse clutches; and valve body), and some derived or taken from the 68 (low/reverse clutches, planetary gearsets, and 2C clutch).

The reaction sun gear is welded to the 4C/reverse hub on the heavier-duty transmissions; in addition, the input and reverse planetary assembly includes the reaction annulus, welded to the reverse carrier. They do not use a #7 thrust bearing; the #11 bearing is in the input and reverse planetary assembly. Otherwise, the planetary gearset thrust bearings are the same on all RFE transmissions.

Service notes: 66RFE and 68RFE automatic transmissions

The transmissions (like all Mopar truck automatics, starting in model-year 2010) are controlled by a transmission-control computer kept with the powertrain computer, in a single module. During limp mode, the manual valve provides reverse, neutral, and fourth gear.

Two filters are used, a spin-on type on the cooler return, and a sump filter on the valve body; as with all RFE transmissions starting in model-year 2010, they all use the filter previously reserved for 4x4s.

The underdrive, overdrive, and reverse clutch assembly snap rings are not interchangeable between transmissions. The tapered side of the two tapered snap rings in the input clutch must face up.

Underdrive clutch clearance is not adjustable, but reverse clutch clearance is, via a selectable snap ring. Overdrive clutch clearance is adjustable on the 68RFE (using a selectable overdrive/reverse pressure plate), but not on the others. 4C clutch clearance is adjustable via a selectable snap ring.

The 2C clutch on the 66 and 68RFE has one more disc than the 545RFE, and the 66 and 68RFE piston is shorter. The 2C reaction plate has to go in first when rebuilding the 2C clutch, not the separator plate.

The 68RFE automatic transmission was made in Indiana Transmission Plant I, which also made the 545RFE automatic.

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