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Rear Differential

16508 Views 37 Replies 18 Participants Last post by  DaleB
Does any body know if the rear differential (for 4WD) is really needed very time you have an engine oil change?
I sent my MDX last weekend to the dealer for the first oil change (@4200 miles) and they suggested changing the rear differential oil. I didn't have them change it because the cost was $87.99 +tax.
I checked in the manual and it mentions that the engine oil and rear differential oil needs to be changed every 7500 miles.
First of all, does the oil need to be changed or just check its level?
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The VTM-4 is the type of rear differential used in the MDX. The terms for the mDX are therefore interchangeable.

There is a TSB for the MDX that emphasizes the initial interval for the rear diff (VTM) must be 7500 miles. They use a special fluid and you'll get proper break-in. I'm not surprised a dealership is going AGAINST a factory recommendation- despite Acura's assurance that "ask your dealer- they are factory trained and know what is best for your new vehicle" (right)

I've done three changes, and eack time the fluid I drained out was perfect. Still, follow the factory schedule, and don't buy the dealers' garbage.

Use ONLY VTM4 fluid. this is the only stuff you should put in the Honda Variable Torque Management Rear differential... ($19 a gallon)

On another front, watch the dealer on the transmission service. 30k (severe) or 45k (normal service)- heard of one guy that had a dealer suggest it every 15k...

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Easier than the engine oil...

Yep. Two bolts- actually you take a 3/8" socket drive extension and insert that directly into the bolt head (has a square hole)

The fill is super easy: use a $4 hand pump from the auto parts store, and just pump into the fill until it runs out. Remove the nozzle, When it stops, the level is *just* at the edge of the hole. Replace the bolt and tighten.

Most differentials are 'on all the time', that is they are simply changing the drive shaft rotation to 2 90 degree outputs. A simple gear mechanism. Perhaps with a viscous coupling for Limited slip.

The VTM4 is active, and is controlled by electromagnetic clutches, and gets a lot of on-off-on-off. This is why the fluid is critical, and the change intervals are frequent. My RWD Q45 has a service to check the rear diff at 90k. Prior to that, you only look inside if you see a leak. For conventional diffs, I'd recommend Synthetic and change it at 60k.

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That is, new washers AND properly tightening...

The part numbers for the two washers are (94109-20000 and 90471-PX4-000). If I recall, one is 18 and the other is 20mm. (Some dealers are only selling the first number for both locations...I don't know why.)

35 ft-lbs for each

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Where ya going with this?

Just because there is "nothing to indicate any particular break-in element to the fluid" are you claiming this to be proof there is, in fact, nothing special about the fluid?

(In fact Acura has indicated otherwise, but not in writing.)

Likewise, there MAY be competing issues with changing the fluid: Acura engineers may have wanted a 7500 mile interval on breakin, but also realized that fluid age was an issue, and therefore limited it to a 6 month limit. Again, being able to construct a scenario in which the 6 month limit occurs at 3000 miles does not mean it should be changed at 3000 miles, or even that it is equal to change it at 3000 versus 7500.

We really have no idea what is optimal, so I just follow the MANUFACTURERS recommendations (not the dealer).
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I really don't know the specific technical answer to that question.

Of course, we should all follow the maintenance schedule, just to protect the warranty.

Oil doesn't have a 'shelf life', so why would it need to be replaced if it sat in your crankcase for 6 months with zero miles versus in the box?!

I think the answer is elsewhere:

In some motor applications, they do not track mileage: my tractor, jet engines, trucks, etc all use hour meters that measure running time. Running time is THE measure for wear and oil performance. Look over at theCastrol/Valvoline/Mobil sites and many technical reports list hours not miles. In fact, 50 hours is a typical interval for industrial diesel oil change. Just so happens that 50 hours at 60 MPH is... 3000 miles.

But, if you only drive 8 miles a day to work, at 8 miles an hour, you will rack up 60 hours in three months- but only 480 miles.

Since the MDX (and any other passenger vehicle in the US) has no engine runtime meter, the time/distance metric is likely the best compromise.

It is also the reason the "severe Service schedule" includes short trips, idling, etc.

My 2 cents

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I'm away from the service manual, but my recollection is that it is less than 50 ftlbs- (36?-sorry, don't recall)

Are you sure you've got the correct plug? Both use a 3/8" drive extension to fit right into the plug.

If it is indeed drastically overtightened, the dealer should fix it for free.

Drives me nuts...

Got new tires and specifically told them, on the work order, to hand torque the wheel nuts.

Picked the car up with my SnapOn torque wrench and they were over 100 ftlbs. Pissed me off. Had them put it up on the lift, release the torque and I re-did it.

The engineers set specs for a reason, some more important that others, but it kills me how rarely a dealership looks at a torque spec.

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