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First off, I will say some dealers will say you don't need to change rear diff oil ever ( until you bearing get so beat up you need a new rear diff). Some will say it doesn't need it till 60,000 ( AFTER all the damage is done) The break in material is what does the damage. Getting it clean first and then let it go long term, or forever is the right way to do it.

Example a poster took my advice at I think 8,000 miles. He went in they completely poo pooed his wanting to change it. He said DO IT. When he came back to pick it up, the service manager said the mechanic came up to him and said he could not believe how black the oil was at 8,000 miles. The service manager said he was going to mention that to Hyundai to change early as it will save warranty repairs for Hyundai as he also knows rear diffs go out before or just after warranty. Money well spent getting rid of all the break in material that pounds away at your bearings.



So some of you may know I am anal on clean rear diff oil, as I found out there is an issue with these front wheel bias AWD systems from different manufactures. Before I bought my Kona AWD 1.6T I found out the rear diff on my wife's 2014 Rav4 was almost jet black at 22,000 miles. I made some post way back in 2018/2019 when my car was new my findings with my wife's car and I was not going to let my car get that bad as it is very hard on bearings. Plus, a new rear diff is $4,000 if you don't install it yourself. I read about some people's rear diffs going at 60,000, 100,000 to 145,000 from various manufactures. I knew mine was going to be thrashed on so clean oil is the best in keeping bearings happy.

I will put down what I have in my notes on my iPhone. It could be deviated from previous posts if I forgot to post it on my phone. I am not looking through old posts. So previous posts here I would say are more accurate then my phone as I have been known to forget to enter them. I do think this will be accurate though.

Also later in my posting I found out there are NO clutch plates in the rear diff as I was thinking the black dirty oil was wear and clutch material. In reality, I found out later through research help from a guy on Bob's, the Hyundai/Kia rear diffs have a fully encased clutch pack electromagnetic actuated clutch system. inside a steel tube. So all the dirt in the rear diff is wear metals from the ring and pinion and spider gears. The refill takes just a hair over a 1/2 a QT of oil so I was dumping for research purposes. Around $9 per dump of Redline 75W-90 GL-5 Hypoid gear oil.


1) First rear diff factory oil dump at 2,000 miles
surprisingly dirty and mid level black oil. Put in Redline 75W-90 GL-5 Hypoid gear oil .

2) Dumped at 6,800 miles Put in Redline 75W-90 GL-5 Hypoid gear oil. It was dirty again, but not as bad as the 2,000 mile dump. I think (??) here is where I changed my DCT With Redline 70w-75 GL-4 OEM speced DCT oil. This DCT dump had a light metal/silver off color of what I think is fine metal particals from gears mating, but not black oil. There is something in it though. I think it was here too I changed the transfer case and used the same Redline 75W-90 GL-5 Hypoid gear oil. It was so clear, I will change that at 35,000 to 40,000 miles

3) Dumped the DCT again at 8,000 (???) for Redline 75-85 GL-4 for an increase of viscosity for the front ring and pinion since I am towing. I talked directly to the Redline R&D chemist and he said it will give me a substantial increase in less wear then the OEM speced viscosity, he said it is worth the change if I was towing.

4) Dumped the rear diff fluid at 17,150 added Redline 75W-90 GL-5 Hypoid gear oil. and it was somewhat clear, still some black but far far cleaner then any before. Here is where we found the oil change happy spot.

5) Dumped the rear diff fluid at 27,900. Put in Redline 75W-90 GL-5 Hypoid gear oil. Just a bit of black, but basically clear. But I did not tow this year. Too busy with house reno.

6) I am leaving the transfer case alone till 35,000 to 40,000.

So there it is, Remember I am also flushing out metal particles as I go. So just doing at a later set time once will not get you a clean diff. I suggest to change rear diff at 8,000 to 10,000 and then again at 20,000 for the average AWD driver, then not again for 30,000 to 40,000 as your diff is now broken in and not shedding metals like it does when new.



Here is a link to some DCT stuff I posted way back that may be of interest. .

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Great information and observations. I always dumped the diff fluids in my cars after the first 5k miles. Always some metallic paste and dirty gear lube from break in. Then 30K miles after that, little to no metal paste found, but lube was usually pretty black.
 

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Also later in my posting I found out there are NO clutch plates in the rear diff as I was thinking the black dirty oil was wear and clutch material. In reality, I found out later through research help from a guy on Bob's, the Hyundai/Kia rear diffs have a fully encased clutch pack electromagnetic actuated clutch system. inside a steel tube. So all the dirt in the rear diff is wear metals from the ring and pinion and spider gears. ...
Interesting .... I stumbled upon this post after changing the gear oil on my Kona Electric out of caution at 19,000 km and discovering it to be as dark as black coffee. The gear oil specified is the same 70-75 GL-4 used in the DCT, just 1 litre and I can only assume it wasn't black to begin with. The ferrous loading was not terrible but due to the puzzling absence of a collection magnet it's all ground to dust rather than sequestered as slivers. The colour has not settled out when allowed to sit undisturbed for 2 days.

The gearbox is a simple single-ratio 2-stage reducer with all helical gears. Ball bearings on the primary and intermediate shaft and a conventional open diff final drive sitting in tapered rollers. I can't account for where the blackness comes from other than the possibility of a bearing outer race occasionally spinning in the aluminium casing. After twice the distance my ICE 4x4 leaves the oil golden clear in the (conventional) transfer and diffs. Even the manual trans oil stays fairly clean.

It looks like the drain and filler plugs in the EV are the same as the DCT's drain, M18x1.5 about 12mm long thread with a 24mm shallow hex head, steel, black finish. I need to locate a suitable magnetic drain plug. I replaced the oil with the Redline MT-LV.

Drinkware Liquid Bottle Tableware Fluid
 

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The first change is usually full of metallic paste from initial wear of gears, friction material, graphites.. A change of fluid should stay clear longer unless it’s friction, heat causing burnt fluid. Hopefully your next 19k wont look as bad…
 
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