The front and rear differentials in your 1994 Isuzu Trooper allow the wheels to turn at their proper speed whenever you make a turn. The differentials are under the vehicle, hidden from view, and easy to overlook during your maintenance schedule. However, just like the engine, they require attention to prevent rapid bearing wear and damage to the pinion and ring-gear teeth. By changing the gear oil at home, you can help maintain differential performance and increase its service life.
Park your Isuzu on a level ground and raise the front and rear with a floor jack. Support it with two jack stands under the front frame and two jack stands under the rear frame.
Remove dirt from around the filler plug located on one side of the differential and the drain plug underneath with a wire brush to prevent contamination of internal components. Try to loosen both plugs with a ratchet and socket. This way you will make sure the plugs are not rusted and frozen in place. If you are unable to loosen the plugs, soak them with penetrating oil for several hours before changing the oil. On Trooper models with mounting bolts on the side cover of the differential instead of a drain plug, try to loosen the mounting bolts as well. Otherwise, soak the bolts with penetrating oil for a few hours.
Place a 3-quart drain pan under the differential and remove the drain plug at the bottom of the differential. Remove the filler plug with a ratchet and socket or gradually loosen the side-cover mounting bolts with the ratchet and socket until you remove all the bolts. Carefully pry the cover off with a screwdriver and remove the old gasket. Let the oil drain completely.
Remove dirt, oil and grease form the case cover and housing with a brush, shop rag and solvent, if your Trooper comes equipped with it. Apply a coat of gasket sealant to the housing and cover mating surfaces. Place a new gasket on the cover and replace the cover on the housing. Install the mounting bolts finger tight and then tighten the bolts to 30 foot-pounds with a torque wrench.
Replace the drain plug with the ratchet and socket.
Fill the front differential case with 3.2 pints and the rear differential case with 3.9 pints of standard GL-5 hypoid type gear oil. Use a viscosity of SAE 75W-90 in the front axle for all temperatures. For normal temperatures at 10 degrees C, use SAE 140W. For temperatures below 10C, use SAE 80W/90. For temperatures between -17.7 and 32.2C (-18 to 32 C), use SAE 80W/90. Pour the oil through the filler hole with a hand pump. Check with your index finger that the oil is at the lower edge of the filler hole.
Replace the filler plug with the ratchet and socket.
Check for leaks around the drain plug or cover and lower your Trooper with the floor jack.
Isuzu recommends replacing the differential oil every 15,000 miles the first 30,000 miles during the vehicle break-in period and every 30,000 miles thereafter.
If your Trooper model is equipped with the optional limited slip differential rear axle, Isuzu recommends GL-5 Limited Slip Differential Gear Lubricant with the correct viscosity for the normal temperature in your area. If necessary, consult with your local dealer.