Verne Simons
Senior Editor, Jp
Photographers:
John Cappa
Overdrives are cool, and while we all would like to have one, tossing a couple grand into a new transmission with an overdrive is not usually an option. What if you could bolt one on in just a few minutes and cruise at highway speeds without redlining your motor? You can if you own an old Jeep equipped with a Spicer 18-and for a lot less than an NV4500. This overdrive of which we speak was originally called the Warn All-range Overdrive, and now, Advance Adapters' Saturn Overdrive. This miracle of modern (well, not so modern) machinery enables the transfer case and transmission to have twice the amount of gears that the stock setup has, and way more than you will ever need. That's right-16 forward and four reverse gears if you are using a four-speed. By multiplying the transmission gears by 75 percent when engaged, the overdrive effectively reduces the ring-and-pinion ratio from 5.38:1 to 4.03:1 in two and four high. While Advance Adapters has offered completely new overdrives since the early '90s under the Saturn Overdrive namesake, many of the original Warn units can still be located in junkyards and Jeeps around the country. The Warn Overdrives were first sold in the '50s, so there should be a barrel full of them at many a scrap yard. If you happen upon one, don't pass it up because chances are you can get it cheap, especially if them junkyard boys ain't real sure what the heck that thang is. And if it doesn't work, don't worry-the Saturn parts can be used on the earlier Warn overdrives to get them back into running order. These overdrives are fairly easy to work on and make an old Jeep drive like a really old Cadillac instead of a really old tractor. We found a Saturn overdrive from '95 in need of a new shifter fork and synchro parts, so check out the captions to help you find and rebuild your overdrive. Part numbers, diagrams, and some disassembly instructions can be found in the Advance Adapters Overdrive instruction manual, so it's a good idea to get ahold of one before working on your overdrive.
Tips And Tricks
*Get those synchro rings lined up properly with the shift dogs, and use some grease to hold them in place.
*Make sure you reinstall the oil scoop in the Saturn/Warn before you reinstall it or else it will turn itself into overdrive blender.
*If the units pops out of gear, you may need a new shift detent spring and ball.
*Make sure you use the special sealing washers on the bolts and add some silicon sealant to the bolt closest to the top of the shift rod to keep it from peeing oil.
*The Saturn will have a date stamped into it on the back, outer rim around the back cover, while the Warn unit has a Warn tag on the back cover.
*Hold onto the cover plate, drive gear, and nut in case the overdrive goes south while out on the trail or road.
Spicy!
The Spicer 18 came with two common main drive gear-tooth counts, either 26 or 29, depending on the age of the transfer case and what it originally came in. This gear is what the overdrive replaces when installed, so you must make sure the overdrive's gear count is the same as the main drive gear it is replacing. Fortunately, both are still available through Advance, so if you've got the wrong one, you can easily swap it over. There is also a gear available to work with the Tera Low gearsets.
