Before you order a Saturn...
Before you order a Saturn Overdrive, measure the intermediate shaft at the center of the transfer case. A measurement of 11/8 inch equates to an input gear with 26 teeth, and a 11/4-inch shaft signifies a 29-tooth gear. However, some interchangeability exists, so the best way is to remove the cover and count the gear teeth and internal splines.
The rear cover is held on...
The rear cover is held on with five bolts, all above the oil level of the case. If you have a power-takeoff unit installed instead of this plate, you need to call Advance for a special adapter to use the PTO. Either way, clean the gasket and crud off the mounting surface. Its highly recommended to drain the oil from the transfer case and flush the insides. Removing the bottom cover and cleaning the pan are good ideas too.
With the transmission in gear...
With the transmission in gear (or in Park on an auto box), remove the nut from the mainshaft. Older nuts are castellated and are retained by a cotter pin, while new nuts are of the self-locking variety. Pull the nut and washer off the shaft, without dropping them down into the transfer case.
Put the transfer case in the...
Put the transfer case in the neutral position to ease removal of the transfer case input gear. The internal splines are straight-cut and the gear teeth are helical, causing the gear to rotate slightly as its pulled out. Count the teeth and the internal splines to verify that theyre the same number as the Saturn Overdrive youre installing.
Heres everything you...
Heres everything you need to replace an Overdrive; keep all these parts safe. If you need to sell yours for quick cash, youll need these parts to be mobile again. If your Overdrive ever fails on a trail (although highly unlikely), youll need these parts after you take off the Overdrive.
The Saturn Overdrive comes...
The Saturn Overdrive comes in two basic parts: the planetary assembly shown and the aluminum case that covers the planetary. Slide the planetary onto the transmission output shaft in the transfer case. A special nut inside the unit is tightened with a standard 1/2-inch extension to 100-120 lb-ft of torque. The back of the planetary hub (arrow) should protrude from the face of the case by 0.677 inch.
These two items retain the...
These two items retain the planetary by locking the special nut in place. The tab on the left indexes into the nut in the center, and the small tab (arrow) indexes into a recess. It only fits one way, and the nut may have to be tightened or loosened to get the tab to line up. The snap ring on the right holds the washer in place by snapping into an internal groove.
The tricky part is installing...
The tricky part is installing the special retaining washer and snap ring. The washer indexes into a recess inside the planetary, against the special nut. Using your finger as shown is one way to get the nut into the depths of the planetary, then fiddle with it until it drops into place.
Needle-nose pliers are used...
Needle-nose pliers are used to install the snap ring, and it will probably take a few tries to get it into place. The washer and snap ring must be correctly installed, or the special nut can back off, ruining the Overdrive unit.
The aluminum housing contains...
The aluminum housing contains the shifting mechanism and the synchronizers are packed in grease to hold it together. A sheet steel oil pickup tube (arrow) fits into the housing and must not fall out during assembly to the case. Use some sealer on the gasket and place it on the housing. Smear some sealer on the bolt threads as well.
Lift the housing into position...
Lift the housing into position and carefully slide the oil tube in between the planetary and the case. Line up the bolts into the case and turn the bolts a few threads in. Slide the housing into place, flush with the case, and tighten the bolts to 30 lb-ft. If the housing wont fit flush, remove it, verify that the brass synchronizer ring is indexed to the three steel keys, and try again.
The shift lever for our application...
The shift lever for our application fits the SM 420 transmission and bolts to the rear of the transmission on the cover. Most applications require that the sheetmetal be trimmed away or new holes be made in the floor, but a boot and ring are supplied for a clean installation. On some custom applications, the Overdrive lever as well as the transfer case levers must be heated and bent as needed.
A threaded rod and yoke allows...
A threaded rod and yoke allows adjustment to the shift rod, which couples to the shift lever and the Overdrive itself. A simple clevis pin and clip allows free movement of the shift rod. Adjust the rod so that the shift rail is limited to between 11/8 and 13/16 inch of movement in direct, or the synchro rings could bind up.
Advance supplies a floor boot...
Advance supplies a floor boot with the shifter kit, but this Jeep required bigger boots to cover the hacked-up floor. Place the rear axle on jackstands and fill the transfer case with oil. Run the Overdrive in the direct and Overdrive positions for about 10 minutes, then recheck the oil level. Now youre set to enjoy the Saturn Overdrive in your Jeep.
Screaming engines and whining gears may sound great for a while, but excessive engine revolutions, which are the genesis of those noises, can mean the death of any drivetrain component. Low-ratio axle gears are great for slow-speed crawling, but they make the engine rev too high at highway speed. In a modern Jeep, an Overdrive gear is usually the top gear in a manual or auto box, allowing a 20 percent or so reduction in engine rpm on the road. Thats fine for the newer models, but what can you do for an older rig?
For Jeeps fitted with the Spicer 18 transfer case, the Saturn Overdrive is the answer. Fitted to the PTO (power take-off) opening on the back of the case, the Saturn unit replaces the input gear. The manual-shifting, planetary design of the Saturn allows each gear of the transmission to be overdriven for six separate gears in a three-speed and eight gears on a four-speed. Combined with the low range of the transfer case and the ability to use it in two- or four-wheel drive, up to 32 different gearing combos can be had, and thats not counting Reverse.
By overdriving each gear, much better matches of speed-to-engine rpm can be made, rather than just a higher top gear. For instance, Second gear may be too low when youre climbing a grade and may cause the engine to wrap too high, while the engine just bogs down and wont pull the hill in Third. By simply shifting into Overdrive while in Second gear, a 0.75 percent gear reduction is accomplished, which more closely matches the engine rpm to the needs of the Jeep.
Old-timers will recognize the Saturn as the old Warn All-Range Overdrive that was introduced in 1961. Once Jeep started using the Spicer 20 transfer case, demand for the Warn unit decreased until production was halted and parts became scarce. Finally, Advance Adapters purchased the tooling for the unit and resumed production in the early 90s, much to the delight of the Jeeping community. Nearly all the parts are interchangeable with the new Saturn, and Advance even offers a manual on how to rebuild either unit.
In these days of $1.50-plus-per-gallon gas prices, the Saturn Overdrive makes even more sense. Easy to install, the Overdrive features a 25 percent gear reduction, which allows engine rpm to decrease while the road speed stays the same. Longer engine life is a result, and the fuel savings can be a big benefit. The Saturn fits military and civilian CJs, Wagons, pickups, and other rigs that use the 18 transfer caseas long as the input gear has 26 or 29 teeth. The early 27-tooth gear found on WWII models cant be used, but complete transfer cases can be interchanged with a minor amount of machining.
We installed the Saturn unit in a regular CJ-3A transfer case in about an hour, including a stop to take photos. Simple and durable, the unit is rated at 300 lb-ft of torque, and weve seen them live under the harshest of conditions.