I have a '98 F-150 4x4, and the clutch pack has been locking up for about six months now.The tag on my rear end states S 909 B-1 L 55975 7K29 , and the door jam says H-9 which is 3.55 I have found a used rear end for $300 from a 2000 F-150 that has disc brakes,and he figures it has 8.8 3.75, but he states the tag is the same but there is a 3 before the L55 part. Is he right about the gear ratio then, or are these rear ends a good swap. I would like the four wheel brakes but am not into swapping gears to match the front of my truck. Any help would be great,
Thanks

 

i don't know much about a alot, but things id look at :

your 4x4, so if you were to switch to 3.73's in the rear wouldn't you have to do it in the front too to not burn anything up?

Going from rear drums to disks id check to see if the master cylinders are the same part number. Possibly look to see if the proportioning valve for the disk/drum is the same as the disk/disk set-up.

Anyone correct me if im wrong, and im sure more will chime in eventually

 

3L55 is 3.55 limited slip. Your axle is 9.75", not 8.8. I would not put a 8.8 behind a 5.4L engine.

 

I forgot to mention my truck does not have ABS, so no issues there. The motor is the 5.4, with 4 speed auto. I don't want to buy this used rear end and have to change anything as far as gearing to match the front. So Glc, you figure the gearing is the same, but the strength of the used one is less desireable? I do not use the truck off road, but do pull a small boat or a 18 foot travel trailer at times. Help me understant the differances between the 8.8 and the 9.75. Bottom line, am I better off fixxing the one I have, or should I swap to something else all together? Am I doing any long term damage, or is there a breakdown in the future by just leaving things alone? Will a fluid swap or addative change a thing? Thanks again

 

If the clutches are locking, then more than likely they still have friction material on them. Is it possible that you got water in the chunk? Take the fill plug out and stick your finger in it. See if the oil is milky. If it is, change the oil and add Ford friction modifier. If the oil is not milky, leave the oil alone and add a bottle of the friction modifier. After you add the modifier, drive it for a little while and see if it improves. If not you can pick up a new clutch pack on eBay for under $100.00. If you make friends with the guys in the shop at the Ford dealer, you may get one of them to put the clutches in for you on the side. Usually cheap.

 

It's a lot easier replacing the clutch packs in yours than swapping out the whole axle.

 

Do you have LSD additive in the rear? The only times a LS rear was grabby for me, it was easily fixed by adding LS additive. The Ford stuff isn't that great. My dealer stocks the Ford Racing additive (Blue, super stinky stuff) because they feel it's much better than the standard Ford stuff.

 

I checked the oil in Diff. and it was not contaminated with water. There was a fair bit of metal on the plug, but nothing really bad. As a last cheap attemt, I am going to put in a bottle of friction modifier. I purchased the vehicle last year and have not touched the rear end at all, so not sure if it was added in there before or if it is all original. I had a closer look at my Diff. ID tag, and it matches the used one for sale perfectly,so........ that is the logical step I think as I want to add a set of overload springs anyways. Any mods to brake lines or swap labor is no big deal to me unless someone can come up with a reason as to why I should not go this way. Changing a diff. is a job I can do, but LS clutch pack is not something I think I should tackle.

 

Rear disks use a different brake proportioning valve then drums. Not sure if that's in the master cylinder or some other valve in the plumbing. Probably not a big deal, as people swap drums for disks all the time...

Ask the seller to count the number of bolts on the diff cover. 10 = 8.8. 12 = 9.75. (don't tell him this before he counts, and don't tell him what you need!).

If it's a 9.75, and has the 3.55, for $300, you are good to go and you just need to figure out the rear disk valving. If it's the 8.8, then I'd forget it.

If it's a 9.75, and has the 3.73, then you have a decision to make and will have to get new front gears or swap the rear to 3.55's....

As you probably already know, swapping gears requires all that micro measuring and some special tools and knowing what to look for to do it.... Not something the DIY guy who has never done one should really do on their own... Believe me, I'm a DIY guy myself, but won't touch something like that...

If it was me, I'd verify the rear end size and somehow, you have to verify the gear set that's in it... The tag should tell... Actually, there are two tags, so make sure you are comparing the same tags?? The "3Lxx" part is the gear ratio, so make sure!! A 3.73 LS will be "3L73" and the bolt count will verify the size of the rear end...

Good luck!

Mitch

 

Fix the one you have. Ff you go to disk you will need to adjust for the rear brakes grabbing harder. Your truck most likely has RABS (REAR ANTILOCK BRAKE SYSTEM). All the 98s had that as a base. The 98 lariat had disk brakes so it can be done. If you have a 5.4 you have a 9.75 inch ring gear differential. Any other engine has the 8.8 inch ring gear. Changing a gear set involves removing the dear differential and pinion gear setting up/adjusting the tolerances. Changing the clutch pack does not require any adjustment so it will be the cheapest/easiest. I would throw a bottle of friction modifier in the rear and see if that helps. The metal on the magnet is normal wear. If you have metal chunks, well if you did you would be writing about this loud clunk.....

Regards

Jean Marc Chartier

 

The size is also on the tag - look for 975 or 88 on it.

I'd consider changing the fluid in yours for now in addition to adding modifier. 75W140 full synthetic.