I am venturing out to change the oil in my differentials. How do I determine what ratio they are and if they are slip or not? I see different oil specifications. I decoded my VIN and it doesn't seem to indicate what the differentials are. There is a tag on the rear differential that says S-935 J-1, another that says 75W140 Synth Oil Only. I assume that is the only oil that should be used but what about slip or no slip? Would it be safe to use the same oil in the front?

 

Open the driver's door and there will be a tag that has an axle code on it. From the code it can be determined what's in there. As far as limited slip goes you can just use the code, or you raise both rear wheels off the ground and try to turn one. If the other rear wheel turns in the opposite direction then it's not limited slip. If you can't turn it, or if both wheels will only turn together when in neutral, then it's limited slip.

 

No, Ford specs a different gear oil for the front diff. Not knowing what year vehicle yours is, I'd suggest looking in the owners manual for your year specific spec.

 

There is usually a tag on one of the cover bolts that will have the gear ratio stamped into it, example; 3.73-1 is an open dif. and 3.73L-1 would be a traction locker dif.

 

If the axle code on your door tag begins with a number, it is non-limited slip. If it begins with an "H" it is Limited slip.

I don't know what year vwhicle you are refering to, but if you indicate the year and the axle code we can tell you the ratio for that year.

 

Sorry, totally forgot the year - it's a 1999.

I found quite a few numbers on the rear axle:
75w/40 synthetic only - I know this is the type of oil
S-935 J-1 - Not sure what this is
39759B10 - Not sure what this is either

When I last did this I apparently did not document enough - but I remember adding a small additive per the dealer and it never seemed to work right since. - hence me now asking the pros and experts. Thank you all for your time and effort.

 

Your truck came with the tag I described earlier, you must have forgotten to put it back on when you changed the gear oil before.
The additive is a friction modifier for Traction Lock differentials, you don't need it with an open rear. So... I assume you have a Traction Lock rear and you will need another when you change the fluid next.

 

Not be belabor the point because I believe you have answered my question (other than is there a preference on the additive?) What does the tag look like? I am very meticulous about putting things back (there is a tag on there that talks about the weight and synthetic). Will the door tag verify slip or not? Because after I changed the oil, there seemed to be more "binding" than I remember - not sure if the two are related though. Thanks again for the feedback. Its the best site ever!

 

I don't believe there is any difference in the friction modifier, just be sure to add it in before filling to the top with synthetic gear oil. It is not needed in the front as that is an open dif..
The binding that you feel should be when making pretty tight turns as the clutches are resisting the need to allow the wheels to differentiate their speed and that is normal. If you have both rear wheels off the ground, just rotate one wheel while someone holds the other and the truck is in neutral and the parking brake is off. The wheel on the opposite side will try to rotate in the same direction that you are turning your wheel and if the other person is stronger you will not be able to rotate the wheel. If you can rotate the whhl while the other is held you either have an open dif. or the clutches are worn out and in either case you do not need the friction modifier. Your dif. will operate just like a non locker if you choose not to have it rebuilt. If you don't need it as a locker just leave it alone.

 

Traction lock test from my manual;

Traction-Lok® Differential Check Road Test

Place one wheel on a dry surface and the other wheel on ice, mud or snow.
Gradually open the throttle to obtain maximum traction prior to break away. The ability to move the vehicle demonstrates correct performance of a Traction-Lok® rear axle assembly (4006).
When starting with one wheel on an excessively slippery surface, a slight application of the parking brake may be necessary to help energize the Traction-Lok® feature of the differential. Release the brake when traction is established. Use light throttle on starting to provide maximum traction.
If, with unequal traction, both wheels slip, the limited slip rear axle has done all it can possibly do.
In extreme cases of differences in traction, the wheel with the least traction may spin after the Traction-Lok® has transferred as much torque as possible to the non-slipping wheel.

 

I am afraid someone will be watching when I test this - smile. Just kidding, although it wouldn't be the first time I should be on worlds dumbest things to try..

I will try your previous suggestion when i get home from work tonight.

MotoMel - thank you a ton for all your threads and suggestions and advice. I have been part of a lot of silly questions along with many others and see your always offering the path to follow - so thank you for your commitment.

 

Thankyou. It is always a pleasure to help when I am able. I've had a lot of help along my path and sharing some of what I've learned is a way of paying back a little.

 

Here is a snippit of the door tag - can you determine what ratio the axles are from this?


It says H6 under axle.
DoorTag.jpg picture by jwhopson - Photobucket

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If you look at post 3 here it shows 3.73 limited slip
https://www.ford-trucks.com/forums/1...xle-codes.html

 

H6 is a 3.73 with Limited Slip.