hemivpr

My rear diff is leaking and needs replaced. I found an acceptable part but the salvage person is asking if my car is an HNM code car. Anybody know what an HNM code is and how I would know if my car is one? I have a 09' A8L D3 4.2. The salvage part is listed as A8/S8. I assume the diff is different for the S8 because of the higher output v10 but shouldn't a high output diff be able to handle a lower output 4.2? Just trying to keep the car I hate loving alive! Thanks!

Ste4lth

your car.

hemivpr

Thanks but thats just it, I don't know if my diff is HNM. I haven't pulled it to see directly. I wanted to order the part ahead to be able to pull and swap. I just thought there may be some other clue or it really didn't matter.

jlbell

If you run your vin on the Audi erWin website and pull the vehicle specific information you will be able to see your diff code. It costs $30 bucks, but at the same time you can download all service manuals and self-study programs.

MP4.2+6.0

Or, get under the car and look for the three digit code. Or, look for the build sticker and read it off of that. Search if info needed on where it is.

S8 is big red caution flag here to me. S8 gearing is different IIRC compared to 4.2 (I know "A8" W12 is), and that could flow through the diff set up. Would need to review in service manual--another possible source.

One way or another you need to verify code, unless you want to risk spin of the wheel with some indicators the house has better odds without you in the know.

jlbell

Also forgot you can provide your VIN to the a dealer service department and ask them for the code.

Giovanni Giovino 6.0+6.0TT

+1 !
You can not have an "S8" gear ratio in the rear end diff, and the 4.2 gear ratio in the front diff, you would tear the drivetrain apart...
Both front and rear diff's must match in the ratio's, regardless what you get, even if said to be the correct part, I would still check the ratio between the old and replacement unit to be 100% sure of matching ratio, 1 full spin of the wheel axel to spins of the pinion shaft.

volvopentaman

First question that arises: Why on earth do you have to replace a leaking diff?

The differentials are ALL very identical (except there are early ones with cast iron casing and later have aluminium alloy). And of course ratios differ. There is no distinguishing between engine output, they all handle roughly the same power. V8 420Nm torque output is very similar to V10 520Nm or W12 580Nm. These are divided on two axles, so each differential only handles a bit over half of that at maximum.

The differential only has 3 seals, one for each shaft, and some silocone gasket for the rear lid.

Take out the diff, clean it up, replace the seals and gaskets.

If your mechanic can't do this, he should not work on the car, or better not on ANY car. This is a diff just like on any RWD/4wd car.

Removal of the diff is like a 20min job if you have a lift, maybe an hour with a floor jack.

And as said, you MUST have the matching gear ratio, otherwise you will tear apart A LOT of stuff. First the driveshafts will probably blow, if they can take the hit a few seconds you will probably blow one of the differential spider gears.

There are only a TWO (HNM, HNK) gear ratios available for these in 2008-2009, but they MUST match up. This is why I recommend repairing instead of replacing. The spare part list says HNM differential is for 6cyl, 8cyl (also with HNK ratios), V10 and W12 (the last are listed with HNM and HNK ratios).

Ratios are 32/9 = 3,55 and 31/10 = 3,10

There is some kind of mixup in the spare part catalogue about ratios. In a few places it says HNM and HNK ratios are same, and in some it differs the ratios. But at least

BUT 2004-2006 models there are MANY gear ratios! HNM,HNK,JSR, HWB,HWA,GZG, EYC,HCL (V10/W12 seem to be available in the early models with all ratios, V6 models with only a few).

So be careful if you buy a second hand unit, AND check several times that they surely have same ratio, otherwise you WILL blow your drivetrain!

Cost of spare parts is max 100usd:

0AR409399B 60x10x45mm 2pc shaft seals for wheel drive shafts 14€/pc
0AR525275 65x10x28mm 1pc input shaft oil seal 15€/pc
Some silicone gasket sealant (if it's the cover leaking), costs like 15€/tube
1L of 75W90 or 75W140 GL5 differential oil (your choice of brand, anything works, no magic here) like 15-30€

So parts come at cost of like under 100€/USD. It should take no longer than 2-3h to replace the shaft seals. Make sure your mechanic inspects the shaft sealing surfaces that they are not corroded. If they are they should be carefully sanded down with like 1000grit paper.

hemivpr

I thank you all for you input. I managed to score a replacement diff with 40k (alleged) miles on it. I have not yet put it in and will do more comparison before I do. My reason for replacing the old is I have 175K on this original diff and it is leaking from one seal profusely (and has been for probably a month). I got the replacement because i assume the internal of the original are badly worn. So, pop on out, pop one in, maybe rebuild the old for the next 100k mile blowout.