Transfer Case and Differential sensitivity to fluid level

[vinu_neuro]

Over the weekend I changed out the driveline fluids on our Q5 (trans, front and rear diff, transfer case). The fluid capacity of all three except the trans is 1L, so I had purchased 1L of each.

The transfer case and rear diff did not overflow out of the fill hole despite each taking almost the whole 1L bottles. Around 20-30mL was left in the bottles as the pump couldn't get all of it from the bottom. I am wondering if the pump retained some fluid as well since there was no overflow (though it just pumped out air at the end).

How sensitive are center and rear diffs to having completely full fluid level? I will top them both off so I can sleep at night (including the $100/1L transfer case fluid), but is it ok to drive the car minimally in the mean time?

 

[The Critic]

What is the fluid level check procedure? Some differentials ask you to fill to a certain distance below the fill hole.

 

[Kira]

I prefer diffs and TCs to be a sip over than a sip under but I think it doesn't make a dime's worth of difference.
Reduced heat transfer and possible frothing comes to mind re underfilling.
Frothing and seal broaching are associated with overfills.

Ha-ha Dept.: I serviced my sister & bil's '89 Ford Probe (first year).
Its manual trans drained x cups of fluid when it should've drained x pints.
Book capacity filled the entire gearbox completely to the very top.....where the speedo drive gear (electronic gauges) constituted the fill port's cap. In fact, the volume of the gear itself was allowed for. Seriously, you leave something like 4mm of space so the gear sits in fluid and displaces the level to the very top.

The resulting solid feel of the transmission lever impressed them to no end. To this day they think I know everything about cars.
Actually, that's a pain sometimes.

 

[vinu_neuro]

What is the fluid level check procedure? Some differentials ask you to fill to a certain distance below the fill hole.

Audi procedure is to fill to the lower edge of the fill hole by letting the excess fluid flow out.

 

[racer12306]

How far below the lower edge is it? If we're talking a few millimeters, I can't see it being detrimental. A lot more than that when we're talking about such a small volume anyways needs to get the right fluid ASAP.

 

[Ursatdx]

Might buy one more Qt. and be sure they are filled to spec.

 

[MRC01]

When the differential is spinning, that oil is splashing around so much, it simply can't be too sensitive to small differences in level. So get it about right and don't sweat it. The ones I've worked on have a drain hole at the bottom and a fill hole higher up. Drain it, then fill until oil starts to seep out the fill hole. I've seen them take more oil than specified to fill up. That may be due to draining while warm, it can drain more completely than they expected when they wrote the manual. Or it could be residual oil coating the hoses you use to fill it that didn't make it into the case. This (drain while warm, and drain as completely as possible) is a good thing.

 

[Deleted member 89374]

Over the weekend I changed out the driveline fluids on our Q5 (trans, front and rear diff, transfer case). The fluid capacity of all three except the trans is 1L, so I had purchased 1L of each.

The transfer case and rear diff did not overflow out of the fill hole despite each taking almost the whole 1L bottles. Around 20-30mL was left in the bottles as the pump couldn't get all of it from the bottom. I am wondering if the pump retained some fluid as well since there was no overflow (though it just pumped out air at the end).

How sensitive are center and rear diffs to having completely full fluid level? I will top them both off so I can sleep at night (including the $100/1L transfer case fluid), but is it ok to drive the car minimally in the mean time?

Your vehicle probably wasn't level. To be more accurate, your transfer case wasn't. On my 2022 Durango GT Plus I lift the rear of the vehicle 8 inches off the ground and let the air out of the front left tire to properly fill the transfer case.

No, they aren't that sensitive to fluid. Think logically: the only time you can starve them is if you drive the vehicle like crazy. I prefer to slightly overfill my diffs and transfer case.

Do you have any limited slip diffs?

What are the recommended fluids per owners manual? I'm asking because there are usually better and cheaper alternatives, as most of these products use common additive packages.