I have a 1990 F-150, 351W, Automatic Trans, 2WD short box... and would like to get a Posi rear end for it. Will I need to do any extras like computer mods, ABS Mods, or anything else I should be aware of?

I remember the time when you never had to worry about anything like this, just remove the old one, and install the new one... But with all these systems tied into each other, I am always worried about the problems I may run into.

Any tips would be great.
Thx
Randall

 

No need to worry about any computer changes or anything.
You can probably find everything you need on EBAY pretty cheap.
I got a low mileage posi carrier and gears for $125. The posi carriers go for around $50.
Just check the gear pattern and backlash and you're ready to go.

 

I agree. RABS (& even early 4WABS) treats both rear wheels as one anyway, so it would only help to keep them spinning together. Just be sure to install the tone ring on the replacement carrier so you still get an ABS signal.

 

Thx for the help! Can you give me a list of all parts I will need to do the conversion? Or would it be easier to just pick up a whole rear end?

 

[updated:LAST EDITED ON 16-Dec-02 AT 02:14 PM (EST)]Get a whole rear axle from a truck with AXLE code (on the door jamb sticker) beginning with "H" for factory limited slip. If you want 3.55 ratio, get H9; 3.08 is H8. There are other codes, but I don't remember them all.

If the junkyard has already pulled the axle, look at the tag on one of the cover bolts. The beginning of the second line is the ratio. "3 55" would be 3.55 ratio with an open diff; "3L55" would be 3.55 with limited slip.

 

 

Not to be a jerk, but "posi" is a Chevy term. Limited Slip (LS) is the proper Ford term. But I think it's much easier to replace the carrier than the entire axle. It's a PITA sometimes to find the right one getting good brakes on it. Some people have better junk yards than others. I myself would stick w/ the axle I have because I know where it has been and what kind of condition it's in. The LS swap is a piece of cake. I got an LS carrier on ebay for 50 bucks, put a rebuild kit on it for 75 bucks and it's like brand new and ready to be installed. Basically all you do is drain fluid/pull off cover. Remove bearing cap bolts and bearing caps (make sure they go back the same way). With a bar and block of wood pry out the carrier and have a buddy help you catch it and watch where the shims go so you know which one come from which side. Remove the ring gear and the excitor ring (ABS sensor's ring). Bolt it on the new carrier w. same side shims next to the bearings and put it back together. Thats basically all you need to do, although there are details I left out. Just said it so you have a basic idea.

Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

Check out my Gallery for a look-see.


Justin - One Happy FTE Member

 

Besides, chances are if you found an entire axle with the right gears, the LS would need new clutches anyway. Mind as well swap carriers on the axle you already have and put new clutches in the LS before you put it in. The rebuild kit from a ford dealership is what you want, new clutches, steel disks, friction modifier, shims, and a lock bolt for 75-80 bucks. Makes the carrier like new. Make sure the carrier you buy is 31 spline and has the side and spyder gears (complete).


Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

Check out my Gallery for a look-see.


Justin - One Happy FTE Member

 

I'd rather buy a whole LS axle with bad clutches. If you rebuild the carrier with new parts, the shims will be correct. If you use Mustang's method, the shims won't NECESSARILY be the right ones for the setup since you will have a different set of gears on the carier which will be mounted in a different axle tube than those shims were set for.

Also, the carrier doesn't have splines - they're in the side gears, so it doesn't really matter what you get if you go that route. You can always swap in your old side gears that match your old axleshafts.

 

IIRC the 31 spline carrier is slightly from the 28 spline in several dimensions.

 

I've never seen anything to indicate that - the only case changes I've ever heard about (including on my Ford CDs) is for ratios, but they don't all agree even on that. Check here:
http://www.drivetrain.com/ford8.8.html

 

Your talking about the side shims right? It's not hard to put new shims in it, I think it would be a PITA to find the right axle, and then have to rebuild the LS too. I think 9 out of 10 times the shims would be correct in my method of doing it. If they arn't, change the shim, it's not hard. But thats just my opinion. I wouldn't put a used axle in my truck w/o rebuilding it with new bearings and seals. Who knows why that axle is in the junkyard...let alone the brakes' condition. You don't know how many miles are on it and you don't know it's real condition or how long it's been sitting there. Seals could be dried up. I think it's a headache to do it that way.

Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

Check out my Gallery for a look-see.


Justin - One Happy FTE Member

 

I know the cross shaft is different (been burned on that one)and I'm pretty sure the bores in the ends of the carrier through which the axles fit are larger for the 31 spline axles. I believe the external dimensions of the carriers are identical, it's been a while since I had a pair side by side to compare.

 

StrangeRanger
"...it's been a while since I had a pair..."


.

Mustang
Actually, you have to adjust BOTH sets of shims to get it right, any time you change a gear, carrier, or housing that they're installed in.
http://www.differentials.com/techi.html
With a complete used axle & a worn out LS (and even seals), you're only replacing the internals of the diff, so no changes that affect the gear mesh. Your method mixes a ring gear & carrier from one housing with a different pinion gear in another housing that was shimmed for that axle's original carrier/ring gear.

I've put 2 used axles in my truck and didn't have any problems. (I ruined one from abuse. ) It's easy to find out how many miles are on it (by looking at the donor truck) and to check the overall condition before you buy. I always trade in my LLT brakes and put a fresh set on the replacement axle, so that's not really a problem either.

 

Steve, Yep, I know about the pinions and their shims, but he doesn't want to change gears, so the pinion doesn't have to be touched. Just remove the carrier swap the ring gear onto the new carrier.


Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

Check out my Gallery for a look-see.


Justin - One Happy FTE Member