changign front diff fluid

ranger952 · February 6, 2004

Hi all.

I have a 98 silverado, 11500, 4x4, Z71.

I switched my rear diff to syntheitc last weekend and want to do the front. What weight of stnthetic do I need?

Also, didnt I see somewhere that the vent tube or something has to be changed too? Is there a part number someone has for me?

Any tips on flushing or cleaning etc? Thanks!

(JayMan, I know you are out there somewhere dying to answer this. :-))

SilveradoWV · February 6, 2004

Synthetic 75w90 would probably work just fine!

ranger952 · February 7, 2004

How about the vent tube? Isnt there a difference between a black or white one? Have not checked yet to see which one I have....

JayMan · February 7, 2004

I'm not sure if this applies to 1998 trucks, but the TSB for running synthetic in the front diff in cold climates states you must change the vent grommet or it may leak.

The TSB covers 2002 and up trucks, not sure why they appear to have more cold temp issues. I think all trucks benefit from synthetics, especially in very cold temps.

I have a 2000, switched the front end to synthetic at 600 miles initial. Have changed every fall since, and not a leak or drip in sight.

If your 1998 front end it the same, look above and behind the fill plug and see if the vent grommet is white or black. Mine is black and is holding up well.

You will notice a big difference in cold temps, as the front end will engage much quicker with a synthetic 75W-90.

JayMan · February 7, 2004

Hi all.
I have a 98 silverado, 11500, 4x4, Z71.

I switched my rear diff to syntheitc last weekend and want to do the front. What weight of stnthetic do I need?

Also, didnt I see somewhere that the vent tube or something has to be changed too? Is there a part number someone has for me?

Any tips on flushing or cleaning etc? Thanks!

(JayMan, I know you are out there somewhere dying to answer this. :-))

Sorry Ranger952 it has been busy and hectic. Does your 1998 have 11,500 or 111,500 miles? I can't believe a 1998 would only have 11,500 miles on it.

On to your second question regarding the flushing the front diff:

See what kind of shape the drain plug is in when you pull it off.

If the magnet has a LOT of fuzz and black sludge, and the gear oil is real black and skanky, then a flush is in order. I had this problem with only 600 miles on my truck, which really surprised me. That was in August of 2000. I expected some break-in trash, but not THAT much!

My local NAPA dealer had a real steal on 80W-90 if I bought by the case. When I saw how nasty my front end was, I got a whole case, which cost like $14.

I left the skid plate off to make servicing it easier. I refilled with the NAPA stuff, drove to the highway and back, and drained & refilled.

FYI: I live about 5 miles at the end of a gravel sideroad, with a posted 30 MPH speed. So it was very easy for me to slowly drive in 4WD (No binding) to really warm up the front axle and get all the crap out. The old geezer neighbor was curious why I kept driving back-n-forth, and he finally went along for the ride just to have something to do. That's another story.

I did this every other day for two weeks until the NAPA gear oil was used up. After the third such drain & refill, the gear oil drained clean and stayed clean. Since I had no other use for the NAPA 80W-90 - it's way too cold here in winter to use - I kept the drain & refill thing going. It was well worth $14.

Otherwise I service the axle every fall using Mobil MobilLube SHC 75W-90, which is what I use in my commercial Eaton Dana HP-40 tandem axles. It has superior cold temp AND hot temp ratings. It has always drained clear, so maybe I'll extend the service interval to every second year. At $4.25 a quart in bulk, it's cheap enough to service ever year though.

I think the reason why the front end gear oil breaks down so fast AFTER the expected break-in trash is twofold: one, the capacity is small, around 3.5 pints, so nothing extra. Two, the cv joints and halfshafts are always turning, even in 2WD.

So even in 2WD, the front axle case bearings are always in motion. The case bearings are barely into the proper gear oil level - run it low and they run dry! - so that surface layer of gear oil has to do a lot of work by lubricating the roller case bearings.

Note: never ever put in any sort of detergent or solvent into an axle to "flush" it! I mean NEVER! The surfactants in a detergent, or the solvent, will react with gear oil and make it foam and break down. This will cause scoring of the R&P gears and early failure. Same as "flushing" using gasoline.

If you have access to industrial lubes, something like a light turbine oil, with R&O additives, can be used to quickly flush things out. I've never seen a turbine oil in less than a 5 g pail, usually a huge barrel, so it sure doesn't pay to buy it just to flush things out.

Let me know how it goes.

ranger952 · February 7, 2004

JayMan,

I meant "1500" for the size, not the mileage. It has 88,000 on it. I have flushed it a couple of times with regular fluid, so thats why I was wondering if I needed to really flush it good when switching to syn. I will anyway.

As usual, your info is helpful. Thanks again!!!

JayMan · February 7, 2004

Yeah if you've already tried to flush it a couple of times and it's still nasty, keep using a cheap 80W-90 to get the trash out.

If you experience cold temps, you'll find the synthetic 75W-90 allows the front end to engage much faster.

Heads up: some folks have reported leaks at the pinion seal and/or the case bearing seal. Make sure to keep an eye on that area after switching over.

So far, at 37,000 miles, my front and rear axle are oil-tight.

TonyD · February 9, 2004

I never changed front diff. I bought used and has 35,000 miles on my 2000 GMC 1500. I dont know what the previous owner did. Should I change with reg 75-90 acouple times before switching to synthetic?

SilveradoWV · February 9, 2004

Its up to you really! I changed mine at 30,000 for the first time and there was just a little gob of goo on the plug, fluid didnt look all that bad, just dark! Flushing it out a couple times wont hurt at all though!