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Replacing your diff fluid is something that Ford says you can do without for the entire life of the vehicle, but I doubt that, especially if you use your truck for four-wheeling or re-gear for bigger tires, etc. In addition to changing out the differential fluid, adding a larger capacity differential cover, such as the one from PMLI will allow the differential to hold more fluid, and help keep temperatures down which means longer life for your gears. Not only that but it can add a lot to to the looks of the vehicle, replacing the ugly black or rust-colored differential cover that comes from the factory.
This is one of those jobs where if you don't know what you are doing, be sure to have help, or take it to a shop. If you mess up or have to give up partway through, your vehicle will be undriveable and might need a tow to the shop, so be sure of your mechanical abilities beforehand. That's not to say it's difficult, as its really not, you just want to be sure you are comfortable before proceeding.
Ford specifies that 70W-140 Synthetic MUST be used in the differential. This is important because when you are looking at different products, you'll find synthetic is a lot more expensive than standard gear oil, ranging anywhere from $7.50-$20 a quart, over the few dollars a quart for convential gear fluid. If you have a limited slip, you will want to ensure you use the friction modifier to ensure the limited slip continues to work.
1. The first step is to make sure you have a pan for the oil that will spill out underneath the differential. Gear oil is very thick and hard to clean up, so be careful! You can then proceed to loosen and remove the 12 bolts holding the differential cover in place.
2. After all the bolts are removed, you may have to do some gently prying to get the old differential cover off. A putty knife or gasket scraper will probably work best, but if those don't work you can use a flat-head screwdriver, just be sure to not gough the machined surface! The gear fluid will seep out or splash out depending on how you are able to pry off the diff cover. Be prepared to smell like gear fluid for a while if you get any on you!
3. Using a gasket scraper, clean all the old gasket material from the machined surface of the differential. This will ensure when you put the cover back on, or replace the cover, that you can make a new tight seal with the machined surface.
4. If you plan on re-using your differential cover, now is a good time to really clean that sucker out. You can use brake cleaner or simple green, but be sure to not leave any residue on the inside of the cover. Be sure not to use your mom's/wife's fancy towels to clean the cover out, or you will be spending more quality time with your truck than you expected.
5. You will then want to apply blue RTV silicone gasket maker product around the machined edge of the differential. Ideally, you should have a bead about 1/8" thick, and 1/8-1/4" wide. The gasket maker material should go between the differential and the bolt holes, and shouldn't cover the bolt holes, if you can help it. If your installation goes like mine, you will find that even with a clean surface, the gasket maker will not want to lie down properly and you may have to manually spread it with your fingers. This might be a good time to use rubber gloves as the blue gasket maker material took appxoimately 3-4 days to come off my hands even with repeated washings. You may also want to apply some gasket maker to the differential cover, but only if you are planning on putting a new gasket in between! (some prefer to, most instructions say it isn't needed and may actually cause fluid to leak prematurely).
6. Now you can put the differential cover back on. The PML Cover I replaced my factory differential cover with came with new bolts, others you may have to reuse factory bolts. You will want to hold it in place until you have 2 or 3 bolts on finger-tight to keep the cover from slipping. Similiar to how you tighten lug nuts, its best to work on bolts across from each other rather than in a circle, to precent the cover from going on lop-sided. Start by tightening all bolts to just finger tight, and then torquing to approximately 20 ft. pounds. THIS IS NOT A LOT OF PRESSURE! DO NOT OVERTIGHTEN! The damage that can be caused by overtightening could cost you hundreds or thousands to fix, so be sure to not apply more than 15-20 ft. pounds to these bolts. You may notice some gasket maker material squeezing out which is normal, and can be wiped away.
7. You can now remove the fill plug to begin filling up the differential with gear oil. To do so, on the front part of the differential, (opposite side from what you've been working on up until now) remove the fill plug with a 3/8" socket wrench/socket extension. You will notice some metal shavings and other grime built up on this fill plug, thats because this plug is magnetic and attracts an contaminants in your differential. Clean off an sludge you see on the plug before proceeding. The new cover from PML offers a "better" fill plug on the top of the cover, but the disadvantage is you don't know when your diff is at the proper capacity. You don't want to fill the diff all the way to the top or it will blow out your seals! Once you've changed the fluid once and have a good idea how much it will hold, the alternate fill location can be used.
8. There are a variety of ways to get oil back into that hole. One method is with a funnel/hose to allow gear fluid to pour in, or you can use a pump to pump the fluid directly from the bottle directly into the differential. The drawback to the pump, as I discovered, is that gear fluid is extremely thick and will test the limits of your hand pump! A pneumatic pump may be better.
9. Once the fluid is back in and has reached capacity (you will know because it will begin leaking fluid out fromt he hole you are putting it in) you can reinsert the fill plug. If you are reusing your factory differential cover, expect to use about 3.75 quarts of gear oil to reach capacity. If you are using an aftermarket cover such as the one from PML, your capacity will be greater. For our application, it sucked up about 4.25-4.5 quarts before reaching its capacity.
10. Wipe up any oil you spilled, and let the gasket maker cure about an hour before actually driving the vehicle.
Sources:
PML Incorporated
www.yourcovers.com
201 West Beach Avenue
Inglewood, CA 90302
(310) 671-4345
yourcovers@pmli.com