F1501984

I am the original owner of a 1984 150 supercab w/215k miles on it . While all of the other fluids and lubes have been done and changed regularly, the rear end has not enjoyed much attention. I have only changed the rear end oil once about 6 years ago when the miles were at about 205k. When I changed it that one time, the fluid was a little dark and smelled, but honestly it didn't look all that bad and the magnet just had a small amount of fines stuck to it. When I changed it, I used Valvoline synthetic 75w-90. Now I am wondering if I did the right thing or not by changing it. I've been reading where Ford trucks are supposed to come from the factory with gear oil that lasts the life of the truck but am assuming that is just for much newer models. Should I start changing the pumpkin oil more regularly and what weight oil should I be using in it ? It doesn't see any towing, a little lite hauling, no getting in water at all, mostly a daily driver locally, and a few occasional empty road trips of 500 miles or less. It has a 351HO, C6, and I cant remember what the rear end is, I think a 353 or 355, maybe LS but not sure, Ill have to check.

Steve83

An '84 diff is not "filled-for-life". Its interval is the same as this:


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You're describing gear ratios - NOT the rear axle type. Are there bolt heads around the back, or nuts around the front?


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It's time to invest in a Haynes:


(phone app link)

F1501984

Thanks Steve. The back of the differential is sealed or welded - no removable cover like most vehicles have.

I finally got underneath and took a brillo pad to the aluminum tag on the rear end and found the first characters to be WDM BK2.
When I look that up here
http://www.fordification.com/tech/rearends_ford03.htm
I find that is a 9 inch rear end, 3.50 ratio, 31 axle splines, and NL or not limited slip which sounds right as when both rear wheels are off the ground, and either wheel is rotated in the direction that would move the vehicle forward, the other wheel rotates the opposite direction.

I do not see a drain plug - looks like a fill plug only. Should I be using 80w-90, 75w-90, 75w-140 ? Synthetic or organic ?

Steve83

The bottom line of the tag doesn't require decoding:


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Old Ford axles didn't have drains, but I added an inexpensive Dorman HELP! transmission drain plug kit to this 9:


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All Ford axles of this era take 80W90. There is no advantage to using expensive oil, over NORMAL-quality oil. Obviously: never buy low-quality fluids or parts. I'd seriously consider this:
But it might be cheaper on eBay or elsewhere.

F1501984

My tag format is slightly different...

Steve83

Yeah, the date is on the top line (Apr 26, 1984), and the plant code (Sterling) is on the bottom.