front & rear diff oil, change or not [Archive]

Dave C

02-03-2004, 07:01

Greatings,
I've got 70k miles on my 99 sub 2500 4x4, 6.5. I changed the rear diff oil shortly after I bought it new in 1998 (per owner's manual instructions for locking differentials). The manual doesn't mention ever changing it again, just check and top off. Well last week I decided it should be changed, which I did. Now I just read where the rear diff oil should just be topped off not changed because it may loosen build up and shorten life of diff (too late now, we'll see). The oil looked like it needed changing (clouded color) also there is a dounut magnet (just like the one in the tranny) that was glued to a depression in the bottom if the diff. I easily pried it loose, cleaned it up, cleaned the gray goo from around where it had been in the diff (fine metal shavings from gears) and put it back in the depression (no glue).

I've never changed the front diff oil but have heard it's a good idea and that synthetic oil is best.

My questions:
1. Did I screw up by changing the front diff? I used 80w90 GL-5 per owner's manual.
2. Will that magnet be okay not being glued? It's not a real strong magnet but it does stick to the bottom.
3. Should I change the front diff oil?
4. If changing is a good thing, how often should it be done?

I've been a forum member for several months. The info provided by the members is priceless. I greatly appreciate any info. you guys could provide.

------------------------
1999 GMC burb, 4x4 2500 6.5 stock, so far

Dave C

02-03-2004, 07:05

Correction in question number 1. should read "Did I screw up by changing the rear diff?"

Dvldog 8793

02-03-2004, 07:35

Howdy
I have a rear dif cover that has a drain plug and a dip-stick. I change my oil about every 30-40,000 miles. In the Marines we changed the diff oil every year or any time there was water.(gray cloudy color) I think I would have glued that magnet back in. Someone was probably paid allot of money to figure out that it should be glued in ;) Also I use Synthetic oil in the diffs and it made an improvement in mileage during the winter months. I've seen standard gear oil almost solidified in the cold.
I change the front the same time that I do the rear just because I'm doing it.
Hope this helps!
L8r
Conley

joed

02-03-2004, 07:53

On my 98 with locking rear, I try to change the rear every 15-20K. I believe the manual indicates to change it every 15K if towing (at least that's the way I've interpreted it). Other members here have actually indicated a slight decrease in mpg when using synthetic in the locker. I can't say as I've always used regular 80-90w.

As for the front diff., I've been changing that about every 50K, along with the transfer case. More often probably wouldn't hurt. Since the front doesn't have a locker, syn. is probably a good choice.

Hope that helps. Joe.

Dave C

02-03-2004, 08:02

Thanks for the info. Changing it made sense to me to, and most of the TDP folks do seem to prefer synthetic oil, guess I should go with the flow . tongue.gif

As for the glued magnet, there was just a dab of glue under it. Someone I talked to thought that it may have been used to keep it in place during shipping (before being installed in the truck at the factory :confused: ) Anyway, re-gluing it would give me more peace of mind (not sure I can find any that would work in that oily place). I'll probably switch to synthetic soon and can try gluing it then (assuming it doesn't go flying into the diff gears :eek: ).

Dave C

02-03-2004, 08:17

Thanks Joed. I was typing my response to Dvldog while your reply was posted.

I may have overlooked the diff oil change recommendations in the manual since, at the time, I was'nt using the burb for towing. I have done some towing (horse trailer), but not recently. I'm glad I didn't wait any longer to change the oil. It doesn't get too cold here in Maryland so I'm probably okay with either type oil. I think I will use syn in the front though. We'll see about doing the rear again.

Regards

captainvector

02-04-2004, 09:37

Been thinking about doing the diff in my truck for a while now - like to know what brands/kinds of synthetic oil you guys are using.

Dvldog 8793

02-04-2004, 10:41

AMSOIL with friction modifier for the locker :D
L8r
Conley

Dave C

02-04-2004, 11:28

dvldog,
I have a set of service manuals for my 99. It says not to use a friction modifier in the rear diff, but it didn't say why. :confused: I don't really know much about the stuff. I assume you haven't had any problems since your using it. What does it do for the diff?

Dave

Dvldog 8793

02-04-2004, 12:06

Howdy
My truck has a Full-float Locker rear diff. When using a regular oil or the AC synthetic you have to add a friction modifier so the diff works and locks like it should. I was mistaken about the Amsoil, it comes with the modifier in already.
You should be able to check the tag on the diff and see if it is a locker or not.
hope this helps...
L8r
Conley

Dave C

02-04-2004, 14:14

I took another look at where I read "no friction modifier". Actually it was in the owner's manual under recommended lubs, and said to only use GM's rear oil part number 1052271 and not to add the modifier to it. Probably because it already has it in it (it's labled GL-4 but said not to use any other GL-4 oil, GM trying to sell it's high priced products I'm sure). The service manual doesn't mention the modifier but does say to use GL-5. Looks like I'll be draining it again :rolleyes: Think I'll go ahead and get Amsoil, do it right. ;) Still not sure about that magnet, not shown in the manuels. Maybe I'll "ask Dr. Lee" smile.gif .

Thanks again
Dave

Beedee

02-05-2004, 01:23

There is no such thing as overservicing your diffs. Use a national brand synthetic and add the friction modifier if needed. There is a differerance between a locker and a limited slip, you need the modifier with a limited slip, but check the oil first to make sure that it isneeded. Your diffs do a lot of work, treat thwm with respect and they will be good to you.
Just for the record, I'm a heavy duty mechanic with 28 years under my belt and after seeing the differance that synthetic oils make in a diff, I wouldn't run anything else.

Dave C

02-05-2004, 05:23

Thanks BeeDee,

The info you guys have provided has kept me from screwing up (again). :D Ain't the first time TDP has come to the rescure and won't be the last I'm sure. I joined because I needed info for a starting problem. It paid for itself with the first repair. :D At this point I'm way ahead. smile.gif

Dave

Dimsdale

02-05-2004, 12:27

If it were me, I would definitely glue the magnet back into place. One has to imagine that the oil churning at highway speeds would be able overcome the magnetic attraction. Really wouldn't want to have the magnet getting squished between the ring and pinion!

Beedee

02-06-2004, 01:12

Just one thing to make sure of when you glue the magnet back down. Make sure that the glue will stand up to the hot oil and the synthetic oil and the friction modifier ect.......
sometimes the stuff that works good around the house doesen't work very well in that type of application. Might be worthwile checking with the local chevy dealer and see if they have something that they use.
Good luck

Dave C

02-12-2004, 04:16

Okay, rather than try to glue in the magnet and hope it stays put I removed it all together and replaced the diff cover with a Mag-Hytec cover. It was expensive but you get what you pay for and as far as I know it's the best. It has a dip stick and drain plug (both are magetized), cooling fins, and a much greater gear oil capacity. :D As for the gear oil and friction modifier, I checked with Gerg (Amsoil Lube Specialists). He recommended Series 2000 75W-90. It already has the FM in it. He said I could use the 80W-90 oil but I'd have to add the FM. ;) I'm using it in the front diff as well.

Thanks for the advice. smile.gif
Dave

Dvldog 8793

02-12-2004, 05:51

The Maghytech is a great product! :D Ihave both the trans and diff cover. When I changed the fluid/filter in my trans I damaged/lost a couple of the screws that come with it. I called them and they sent out a new mounting kit w/ O-rings for both of the covers that I had. Next day air FREE of charge! :D Customer service like that is worth ALLOT! I've had both covers on for about 100,000 miles and NEVER a drip! I also use the amsoil 2000. Seems like a good product.
L8r
Conley

GMCHORSHAM

02-13-2004, 00:42

Hi GMC,just thought I would pass this onto you,
even though I have read the quality feedback.
I changed front & rear diff oils on my truck just
before I set off on a 400mls trip pulling my Horse float with 2 horses all the gear including
camping goods.
The oil I used was Mobilube SHC 75W-90 synthetic
Gear and Diff oil.
The Truck seemed to run smoother than it had the previous year, to the same destination.
16.5 mls per gallon, which included a lot of stop start driving, and more weight.
I am very happy that I did change to synthetic.
Also use Mobil Delvac 1 5W-40 synthetic in the motor, again I am happy with the change.
Cannot help with the Diff magnet.
Have Fun, Safely.
Rod. ;)

Dave C

02-15-2004, 07:33

That's good to hear. I'll be making a 2,500 mile round trip to Florida next month. I'll see how my milage improves. smile.gif I also just flushed my tranny and used synthetic ATF. I used the trick of disconnecting the ATF cooling return line, idling the engine, and catching the old ATF in a bucket as I poured in new. Made a heck of mess but got the job done. :rolleyes: The tranny holds 13.5 quarts total but it only takes 5 quarts to fill the pan (after dropping pan and replacing filter). I wanted all synthetic, not a mix so it was worth it. ;) I also use Delvac 1300 15w40 (not synthetic). I've seen it mentioned here on the forum so I think it's a good product. :D

Thanks
Dave

GMCHORSHAM

02-16-2004, 01:48

Sorry Dave, no offence, titled my message to you as GMC, I have just realised that GMC is your "Avatar Image".I will pay more attention in future.
Futher; Synthetic ATM, what brand did you use??
This is next on my to do list.
Thanks.
Rod.

Dvldog 8793

02-16-2004, 05:41

Howdy
Just saw the talk synthetic "ATM" :D Must be ATF!
Anyways IMHO, Mobile makes a great synthetic ATF. I also think Havoline makes a good product.
My trans temp dropped by 25-30 degrees when I switched. Seems like I worry about getting up to operating temp more than overheating! :D
L8r
Conley

GMCHORSHAM

02-17-2004, 00:13

Hay Dvldog8793, I was talking about a withdrawal
of old ATM fluid and replacing it with synthetic-
ATF. ;) ;)
Good pick up :D
A 25-30 degree drop has got to be a benefit, must get onto Synthetic ATF.
Thanks for the info.
Rod.

Dave C

02-17-2004, 09:44

I used Amsoil synthetic ATF only because all of their other products have such good reviews and are so widely used by other folks here on the forum. I don't have any personal experience with it. We'll see how it does. smile.gif

Dave

GMC Hauler

09-23-2004, 21:04

I just read all my RPO codes, and I noticed that i have the limited slip (G80), so i will have to add the friction modifier, as i didnt use an oil with it (mobil 1 synthetic) when i originally changed the fluid. When i changed the fluid, i had the rear end jacked up and tried the old trick of turning one wheel and seeing if the other wheel turns the opposite way. I believe it turned the opposite way. Does this mean that the unit needs to be serviced? What is the proper way to tell if the unit needs to be serviced (100K miles on it)? Where and what friction modifier do you recommend?
Also, i have several RPO codes that wernt on the list. This vehicle was a Canadian vehicle. I will list these with hopes that some of you can identify these. Thanks.
Codes:
AXP
C25
JAN
NY1
Q1W
R7K
R7M
SLM
VXS
VGS
YGK
ZGK
55L
6GL
7GL
6SA

If we cant identify all these codes here, can the dealer identify them?

rjwest

09-24-2004, 07:36

Some of the Gear oil manufactures list the compatible GM pn on the Bottle......

lineman

09-24-2004, 09:13

GmcHauler,Go to the Members AREA at the top of this page and click on it.When the page comes up scroll about 3/4 of the way down to the RPO (Regular Production Option )line and click on it.This list should give you what each letter and number combo stands for......Lineman

Desert 6.5

09-24-2004, 14:36

Lineman, you answered one of my questions too, Thanx

norm

09-24-2004, 16:57

GMC Hauler: the G80 is definitely the option code for your rear axle. I think the RPO listing for G80 is not entirely correct, as I have had two trucks with factory G80 axles (see postings in "driveline slack") both with the gov-lock rear locker. The build sheet and window sticker both list the G80 option as a "locking rear differential."

If you spin oposite directions when jacked, you probably have the gov-lock (a badly worn clutch-type posi unit can do this, too). The gov-lock remains "open" until a spinning wheel creates an RPM difference of a set amount, throws a governor and locks the spider gears up. To be sure, when you take the cover off to drain lube and inspect your gears, you will see the locking governor mechanism. The gov-lock is pretty unique.

GMC Hauler

09-24-2004, 22:34

Thanks for the replies. I had already printed out the RPO code list and highlighted the ones i had, although the ones i listed in this topic were not listed on that sheet. That is why i threw them on the forum, to see if anyone knew what these extra codes were.
I did take the cover off once to change the fluid and inspect, and i noticed that the differential was cone shaped and i couldnt see inside of it for any clutch packs. I just read the driveline slack post and will get back as to what i find. Thanks

[ 09-24-2004, 10:57 PM: Message edited by: GMC Hauler ]

Dihrdbowti

09-28-2004, 20:29

GMC Hauler In my 96 service manual I came up with the following: (I found some that were close if I couldn't come up with the exact match)

Codes:
AXP VIN Ident Position
C25 Wiper System: Rear Window
JAN Plant Code: Janesville, Wi
NY1 Shield Fuel Tank Steel
Q1W QIW Tire: LT245/75R16E
R7K no listing
R7M no listing
SLM no listing
VXS Vehicle Complete (always a nice thing I think)
VGS no listing
YGK Tire Rear LT245/75R16E
ZGK Tire Rear LT245/75R16E (always good to have the same size)
55L Secondary Color: Ext, Light Autumnwood Metalic
6GL Component Front LH Computer Selected Suspension
7GL Component Front RH Computer Selected Suspension
6SA 65A Stripe: Color Accent, Two tone Beige Metallic/Dark Autumnwood