Rear Differential

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Rear Differential

Postby 90 MarkVII BB » Wed Jun 15, 2016 11:50 am

Hey all!

I was under my Mark today and removed the rear diff fill plug. Upon unscrewing, I had an audible hiss. Pressure had built up in the diff.

Is there a vent tube on our rear differentials? I would think pressure is no good, as pressure could potentially push seals and cause leaks.

If so, where is the vent located - I didn't see one on the pumpkin. Is it a rubber hose, or one of the "S" shaped metal tube deals?

Any helps is appreciated.

~90 MarkVII BB
"Ivory" - 1990 Mark VII Bill Blass Edition
3G Alternator, Arnott Air springs front/rear, hardwired dashcam, Tigerpaw whitewalls, all 4 brake lines new NiCopp, all else factory.
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Re: Rear Differential

Postby oldray » Wed Jun 15, 2016 2:45 pm

Usually it’s a rubber hose on a nipple on top of the housing but I’m not sure on the mark, I have one on the floor in the shop so if no real expert comes along in the next week or so I will have a look for you, but you might have to remind me, kind of forget things sometimes. :)
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Re: Rear Differential

Postby 90 MarkVII BB » Thu Jun 16, 2016 11:05 am

oldray wrote:Usually it’s a rubber hose on a nipple on top of the housing but I’m not sure on the mark, I have one on the floor in the shop so if no real expert comes along in the next week or so I will have a look for you, but you might have to remind me, kind of forget things sometimes. :)


I got under there myself. If I'm looking at the right thing, its just a short metal "loose" cap on the pass side of rear axle. How I think it works is that when pressure gets too high, it lifts this cap so the excess pressure can escape. In that case there'd always be some level of pressure in the diff, just not enough to lift the cap to release.

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"Ivory" - 1990 Mark VII Bill Blass Edition
3G Alternator, Arnott Air springs front/rear, hardwired dashcam, Tigerpaw whitewalls, all 4 brake lines new NiCopp, all else factory.
In the works - Backup cam, Addco#415.
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Re: Rear Differential

Postby oldray » Thu Jun 16, 2016 1:18 pm

Yes, I looked this AM and that's it.
I would guess in certain circumstances (mine) that the rust could build up and plug the cap.
Because I can (and have) I'm going to run a short hose into a frame mounted fuel filter not that that it helps, it just looks cool, that no one will ever see.
My pinon seal is leaking and the rear yoke is lose, with the big grease / oil stain the same as in other under-body pictures of Mark VII's, I have found the part number for a non-flanged yoke because i think I might have to change my drive shaft but I am now wondering if it's not the flange weight but the double u-joint yoke that causes the seal / yoke problem?
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Re: Rear Differential

Postby CDW6212R » Thu Jun 16, 2016 1:23 pm

Those metal vent caps aren't really meant to "move", but they should allow air to move in or out at any time. The function of the cap is to not let water spray easily get into the diff. The trucks usually have a nipple there instead, and a rubber hose attaches, leading up to the body and hung down loosely. My new 98 Explorer had the hose broken off there at the nipple, so I cut off the old piece and pushed the hose back on.
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Re: Rear Differential

Postby 90 MarkVII BB » Thu Jun 16, 2016 4:25 pm

Mine behaves sort of like a safety cap on a pill bottle. It spins and moves up and down slightly but doesn't unscrew or pull off (maybe I didn't pull hard enough). I didn't want to test it in case it came off and didn't go back on.

Does it vent correctly?, I'd tend to say no if I had the hiss at the fill plug. Should there be no hiss at all?

Can I just pull it off and clean underneath?
"Ivory" - 1990 Mark VII Bill Blass Edition
3G Alternator, Arnott Air springs front/rear, hardwired dashcam, Tigerpaw whitewalls, all 4 brake lines new NiCopp, all else factory.
In the works - Backup cam, Addco#415.
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Re: Rear Differential

Postby CDW6212R » Thu Jun 16, 2016 10:13 pm

90 MarkVII BB wrote:Mine behaves sort of like a safety cap on a pill bottle. It spins and moves up and down slightly but doesn't unscrew or pull off (maybe I didn't pull hard enough). I didn't want to test it in case it came off and didn't go back on.

Does it vent correctly?, I'd tend to say no if I had the hiss at the fill plug. Should there be no hiss at all?

Can I just pull it off and clean underneath?


That sounds correct, it will turn on itself but not come off. If it really has gotten plugged up, that would be very unusual and have to come from a wild circumstance(like car under water, way too much grease put in, car flipped at some point etc).

If you really think it is or was plugged, I'd take the axles out and thoroughly inspect it all, and clean out the housing tubes. If it did get clogged, that suggests the inside is not the best grease and there might be issues from moisture or contamination.

It is not hard to clean inside the housing, after removing the axles, and bearings. But that takes some time, especially the bearings and a Tracloc spring if it has it. I just went through that all with my 98 Explorer, for maintenance.

If it was actually dirty inside, with the bearings out you could reach almost all of it inside, enough to be able to spray that vent a lot with brake cleaner, and be able to wipe it out from inside. I used a long pry bar and paper towels, mine had a minimal dirty looking residue.
Don
1991 Red Special Edition, my 2nd SE, dead paint, DWS 255/45/17's on 95 Cobra wheels, and soon; DVD/Navi/backup camera, OBDII and the 347, before custom paint.
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Re: Rear Differential

Postby 90 MarkVII BB » Fri Jun 17, 2016 7:37 am

CDW6212R wrote:
90 MarkVII BB wrote:Mine behaves sort of like a safety cap on a pill bottle. It spins and moves up and down slightly but doesn't unscrew or pull off (maybe I didn't pull hard enough). I didn't want to test it in case it came off and didn't go back on.

Does it vent correctly?, I'd tend to say no if I had the hiss at the fill plug. Should there be no hiss at all?

Can I just pull it off and clean underneath?


That sounds correct, it will turn on itself but not come off. If it really has gotten plugged up, that would be very unusual and have to come from a wild circumstance(like car under water, way too much grease put in, car flipped at some point etc).

If you really think it is or was plugged, I'd take the axles out and thoroughly inspect it all, and clean out the housing tubes. If it did get clogged, that suggests the inside is not the best grease and there might be issues from moisture or contamination.

It is not hard to clean inside the housing, after removing the axles, and bearings. But that takes some time, especially the bearings and a Tracloc spring if it has it. I just went through that all with my 98 Explorer, for maintenance.

If it was actually dirty inside, with the bearings out you could reach almost all of it inside, enough to be able to spray that vent a lot with brake cleaner, and be able to wipe it out from inside. I used a long pry bar and paper towels, mine had a minimal dirty looking residue.


Thanks a bunch. Last time I changed the diff fluid (about 6 months ago, first time I've changed since I got the car, so who knows how long the interval was before that), everything seemed pretty clean. Old fluid was dark, almost grayish black. Refilled with valvoline 75w90 synthetic for limited slip, and motorcraft friction modifier. I have a Trac-loc. It took about 2 & 1/3 bottles of the fluid.

That too much?
"Ivory" - 1990 Mark VII Bill Blass Edition
3G Alternator, Arnott Air springs front/rear, hardwired dashcam, Tigerpaw whitewalls, all 4 brake lines new NiCopp, all else factory.
In the works - Backup cam, Addco#415.
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Re: Rear Differential

Postby CDW6212R » Fri Jun 17, 2016 2:17 pm

That sounds right. The spec is around 5 pints, which is close to 2.5 quarts.

My truck calls for 75-140 synthetic, so I used Amsoil Sever Gear and about six ounces of a Lubegard additive. The Lubegard isn't specifically for a LS, it didn't mention it, but so far it works fine in my LS. I'm going to upgrade later to the gear only diffs, a True Trac and a Torsen. Those don't take a modifier, which is a pain to go buy special for an LS diff.
Don
1991 Red Special Edition, my 2nd SE, dead paint, DWS 255/45/17's on 95 Cobra wheels, and soon; DVD/Navi/backup camera, OBDII and the 347, before custom paint.
98 Mountaineer with Continental DWS 255/55/18's, big sway bars, custom brakes. Soon to be A4WD with XP8 bumpers and wing.

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Re: Rear Differential

Postby CDW6212R » Fri Jun 17, 2016 2:20 pm

If there is any wetness around the rear brakes at the axle seals, it might be time to R&R the axles to look at them, do the seals, and maybe the bearings.

I had one axle showing a hint of surface cracking(at the bearing contact surface), no pitting yet, but maybe a year or two later it might be time to change that one.
Don
1991 Red Special Edition, my 2nd SE, dead paint, DWS 255/45/17's on 95 Cobra wheels, and soon; DVD/Navi/backup camera, OBDII and the 347, before custom paint.
98 Mountaineer with Continental DWS 255/55/18's, big sway bars, custom brakes. Soon to be A4WD with XP8 bumpers and wing.

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Re: Rear Differential

Postby oldray » Fri Jun 17, 2016 3:00 pm

Would it be better if I moved my question about the double u-joint to a new post so that I don’t hi-jack this one? :)
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Re: Rear Differential

Postby CDW6212R » Fri Jun 17, 2016 6:21 pm

oldray wrote:Would it be better if I moved my question about the double u-joint to a new post so that I don’t hi-jack this one? :)

Just a quick thought Ray, I think your pinion seal leaking is not being caused by the type of flange etc. It was likely just the rear having poor grease in it, lots of hard usage, or something wore inside and it hurt the pinion bearings or shaft. That most likely won't happen again if the parts are all made good, set up well, and high quality grease is used. Regards,
Don
1991 Red Special Edition, my 2nd SE, dead paint, DWS 255/45/17's on 95 Cobra wheels, and soon; DVD/Navi/backup camera, OBDII and the 347, before custom paint.
98 Mountaineer with Continental DWS 255/55/18's, big sway bars, custom brakes. Soon to be A4WD with XP8 bumpers and wing.

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Re: Rear Differential

Postby robertbweltzien » Sat Jul 09, 2016 12:40 pm

You guys are lucky. My rear sounds like a friggin' knife grinder. The pinion yoke came loose a few years ago so I removed it and tightened it with blue locktite. The cardan joint was fine. Tightening solved the vibration problem but the (pinion bearing?) noise started a few months ago. The oil has been changed three times and I never saw any metal chips, just some spalling of the side gears and it's always had a lot of backlash since I bought it 13 years ago with 83,000 miles. It also doesn't leak or have any looseness at the pinion yoke. Ring and pinion gears look OK with a normal contact pattern. The noise has lately become less noticeable and is only heard at 30 mph or less, sounding like riding on knobby tires and varying with vehicle speed. With 180,000 miles on the clock at this point I'll keep driving it until it blows or leaks. I thought the 8.8 was a good rear but now I have my doubts, as the 9" in my older Fords never gave me any problems. Let me know if any of you guys have a good rear for an '89 LSC. I have to use the car to give my sick wife rides to the hospital and the clinic. I'm in North central Georgia.

I'd like to put a 3.55 Traction-Lok unit in the old merchant marine.

Bob
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Re: Rear Differential

Postby CDW6212R » Sat Jul 09, 2016 1:28 pm

I'm sorry to hear that your 8.8 hasn't been good for you Bob. I think it probably had/has some issues with clearances of the gears or the diff, itself. It may be fixable by installing new gears and bearings, but the proper setup procedure is the key, and you wouldn't know until it's been done and then driven.

Replacing the whole rear may be the best answer, if that one didn't work out with a full rebuild. I do have a friend with at least one good Mark VII rear, but I don't think any of his spares have a LS diff, or 3.55's.

I have 3.73's in mine that I will be swapping back to the original 3.27's. That will be better for the 347 I will install next. I like the 3.73's but it does make it too easy to spin the tires at the start. The 3.27's are a great ratio, but the stock torque converter is a bit too low stall, to get the car going well from a start. The AOD has close ratios, and the wider ratios of the 4R70W trans would be a good upgrade(I think better than the gears). That would only help though if the AOD was being rebuilt and the parts were handy to acquire.

If you have some expert there to really check out your rear, you might do best to have that done, and get it rebuilt if it checks out well. Ford gears should still cost between about $75 used, to $150 new, bearings etc in the $50 range I think.
Don
1991 Red Special Edition, my 2nd SE, dead paint, DWS 255/45/17's on 95 Cobra wheels, and soon; DVD/Navi/backup camera, OBDII and the 347, before custom paint.
98 Mountaineer with Continental DWS 255/55/18's, big sway bars, custom brakes. Soon to be A4WD with XP8 bumpers and wing.

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Re: Rear Differential

Postby robertbweltzien » Mon Jul 11, 2016 5:31 pm

Thanks, Don.

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Re: Rear Differential

Postby robertbweltzien » Mon Jul 18, 2016 12:31 pm

Looks like I'm relatively lucky, Don. The right rear axle bearing has apparently gone south. More noise when locking the left rear axle in place than when locking the right in place. Plus there appears to be a little grease/oil present on the shaft. Maybe this is also causes my amber "Check Antilock Brake" light to come on intermittently.

Also Advance Auto has a repair kit (National, p/n RP-5707) which appears to be the bearing, seal and repair sleeve. Any thoughts on this part? Will I need a puller to get it out of the axle tube?

I originally thought the pinion bearing went since the yoke loosened up a few years ago and had to be locktited/tightened.
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Re: Rear Differential

Postby CDW6212R » Mon Jul 18, 2016 2:37 pm

If the ring, pinion and diff all look good, do the axle bearings and see how it does. The axle bearings do have to be pulled. If the axle(bearing surface) looks good, you can use the normal bearings which should be $10 or close. The "repair" bearing kits are about $25 each, and those are for reusing a bad axle, as it relocates the special bearing inboard from the original location. I prefer to now use those, a new axle isn't that bad to buy.

For the bearing puller, rent the slick tool from a parts store, and a slide hammer. The trick tool slips inside the bearing and holds it securely. It'll come out quickly with a slide hammer.
Don
1991 Red Special Edition, my 2nd SE, dead paint, DWS 255/45/17's on 95 Cobra wheels, and soon; DVD/Navi/backup camera, OBDII and the 347, before custom paint.
98 Mountaineer with Continental DWS 255/55/18's, big sway bars, custom brakes. Soon to be A4WD with XP8 bumpers and wing.

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Re: Rear Differential

Postby robertbweltzien » Mon Jul 18, 2016 4:40 pm

Thank you much, Don! I'll go with the repair kit and rented puller.

Bob
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Re: Rear Differential

Postby robertbweltzien » Tue Oct 18, 2016 2:58 pm

Update on that last post, Don. I have the "axle saver" replacement bearing and seal from National Bearing but now (three months later) the rear is quiet with no more whine. The fluid level is normal and there aren't any leaks. The axle flange doesn't move around but strangely that right rear wheel is the only one that locks up when the ABS decides to malfunction. Meanwhile, I think I'm going to wait on that bearing replacement until I hear noise again and start the ABS fix with first diagnosing the brake sensor at that wheel. Most work is easy but yeah boy, these cars sure can throw some curveballs at you!

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Re: Rear Differential

Postby CDW6212R » Tue Oct 18, 2016 6:51 pm

Try what John suggested about the one right tire locking up first. I agree that one of the calipers is probably sticking or the line's clogged on one side. This ABS system is a 3-way type, only one rear brake line, so at the limit, if the brakes aren't perfectly balanced, one tire is likely to get just a hair too much brake pressure. I think a 4-way system would have the finer control needed to react to one tire locking up better. That's a guess though, I have only my one 4-way 95 Crown Vic to go by for some experience.
Don
1991 Red Special Edition, my 2nd SE, dead paint, DWS 255/45/17's on 95 Cobra wheels, and soon; DVD/Navi/backup camera, OBDII and the 347, before custom paint.
98 Mountaineer with Continental DWS 255/55/18's, big sway bars, custom brakes. Soon to be A4WD with XP8 bumpers and wing.

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