I'm changing the transmission fluid in my inlaws ML320. The guy at the dealer sold my father 8 liters of fluid. I told him to get 6qts, but the guy said 8 for the differential. I was planning on draining the pan, torque converter, and replacing the filter. I'm confused what they mean by differential. I was planning on not touching the differentials. From what I could tell, there are 3 differentials in that car. Front, center, rear. I noticed at least the front and center have a drain plug on the bottom, and fill on the top. Is it good practice to replace the diff fluid when the trans fluid/filter is changed? Which ones should I replace the fluid in? When filling the diffs on these cars do you just fill till it spills out the fill plug?

 

Why are you wanting to change the fluid?
Mileage? No need to change before 100k miles unless the box needs work on it for some other reason.
On a 4 X 4 there is a centre diff and it appears that this runs in trans fluid. Since the case is bigger it needs more fluid.
Setting the level is not as simple as you suggest.
Not familiar with the ML but I'm not sure whether the ML has a dipstick and sealed filler tube like on most other MB's. If this is the case you need to purchase / make a dipstick and purchase a replacement anti tamper fitting for the filling tube

 

I want to change the fluid because one of the trans cooler lines was leaking in 3 different spots where the rubber metal, and one at the transmission. The car has about 80k miles on her. I suspect the fluid has been leaking for some time and it's low. I think the car hums a little bit louder than it should. The ML has no dipstick. It has a cap with a locking tab that says "MB Tech Only" over the sealed fillter tube. I already purchased the dipstick from the internet. Somehow I got lucky and escaped for $31. People out there are trying to sell that thing for upwards of $80. I also scooped up a mitchell cd which has an a/t trans service procedure but it makes no mention of the differentials.