Rear Diff Check... | Ford Explorer Forums - Serious Explorations

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Rear Diff Check...


Active Member
March 12, 2009
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City, State
Leeds, Yorkshire
Year, Model & Trim Level
1998 V6
Evening Guys,

I've just had my first ever foray into Diffs, knowing sod all about them but what the haynes manual stated, this happening after i sheared a Wheel Lug Stud off the hub and i've now had to drop the axleshaft out to replace it....:(

I've drained the Diff Oil from the rear and it smells so burnt that even when covered it and put it in the far corner of the garage it still managed to smell the house out!! :D

Guys, Whilst i've got the diff apart is their anything i should be looking for in there or wiggling any parts for play?
(I know the oil is buggered so i'm going to sieve it to see if their are any metal bits floating around, a dead giveaway - I'm wary of just replacing the oil as if the diff itself needs parts replacing i might aswell do it all at the same time):confused:


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Look for a silver or a gold "sheen" to the oil. That is a sure sign of excessive bearing wear and pending failure.
That is more likely than the metal bits if you did not have serious issues with the diff previous to tearing into it.

Then take a close look at the condition of your axle seals and axle bearings. If you see any oil seepage around a seal, now is the time to replace it. Actually, if this is the first time anyone has been in this diff, you may want to change those seals anyway. Get a good look at the axle bearing surface, if there is even the slightest bit of irregularity on the roller bearing surface, replace it.

Check the axle surface where the axle bearing rides, see the shiny part of the axle in the pic below:


You should not be able to feel any grooves in it with your fingernail. You should also not be able to feel any difference in the circumference of the shaft in this area. If you do, you will need to either replace your axle shaft or remove the axle seal and the axle bearing and replace those with an "axle saver bearing". It is an integrated bearing and seal and the bearing will ride in a different area of the axleshaft.
Next would be to check the condition of the spider gears (the small gears inside the middle of the diff) and replace if needed.

As always, ensure you buy a new 9mm "cross pin keeper bolt" so that you will never have to deal with a broken one.

Mounty, Thats exactly the info i was looking for! When my parts arrive i give it a good look over as per your advice. (Looking at the state of it i don't think the oil has ever been changed and with it running 102,000 - i think it's time for some fresh minimum aswell as any other faults found)

I'll update the post with my findings...:D

I'm doing the work on Wednesday so I'll post some pic's!

Diff Oil? As we all know Ford states 75w 140 with ford friction modifier....I've read on the forum:

- If using 75w140 fully synth I don't need the friction modifier?
- can I use 75w 90? And do I need a friction modifier? (1998 V6 SOHC - the Diff Tag says 75w 140 synth)

The old spec is 80W90 + FM
They then changed the spec to 75W140
With the more modern clever synthetic alot of guys don't use the FM but in my mind you would be looking at the high end stuff. The answer may be use some 75 140 and try it, if you have no chatter then all good but if you get any chatter ad some FM.
Let us know how you get on as i need to do my diff fluids.
Check out opie oils for different options. I believe we can get a discount from them that Rich set up.

I've just managed to buy 1 Gallon (3.8L) of 75w 140 Diff Oil for £29.95 including next day delivery from Lucas Oil on Anglesey!

I've also spoken to Rob at Ford's First Parts Solutions...he's located the Friction Modifier and it's instructions for use. They said that it must be smeared over all the internal diff parts on a full rebuild and left to cure for 15mins?

They said that it must be smeared over all the internal diff parts on a full rebuild and left to cure for 15mins?

Uhm, thats a new one on me. Never heard of that one. The only thing I have ever heard of needing a "soak" was the friction plates when rebuilding the clutch pack in the limited slip. You soak them in the diff fluid for a few hours, not solely the friction modifier.
If I am not mistaken, the friction modifier is not needed if you use the Lucas synthetic.
I would only add the modifier if you have clutch chatter when going around a sharp corner at low speeds. Add an ounce at a time and give it a day or two to work its magic. If it still chatters, repeat.

What does the clutch chatter sound like? would this affect a 99 north face? Mine gets a funny noise from underneath when I turn sharp corners at slow speeds, say in a car park. Other than that it runs quietly both in a straight line at any speed and round corners when going above parking speed. Would I have the limited slip diff on mine? I should know the answer to that but I'm still learning - slowly! Graham.

The chatter can present itself differently depending on different factors.
From what you describe, it sounds as though you would benefit from a rear diff fluid change.
To find out what you have for a gear ratio, and whether or not it is a limited slip, find the label on the drivers door jamb. Get the number or letter and number combination under the heading "axle" (circled in red below), then look at the chart below the pic.


Now the chart to decode it:

Door Code/open or limited slip/ratio
41....….…… Open 8.8"…...….. 3.27
42..…..……. Open 8.8".….…... 4.10
43..……..…. Open 8.8"…..…... 3.08
44..………... Open 8.8"….….... 3.73
45..…..……. Open 8.8".….…... 3.55
46..……..…. Open 8.8".…….... 3.73
D1..……..…. L/S 8.8".…...…... 3.27
D2...…..…… L/S 8.8".…...…... 4.10
D4...….……. L/S 8.8".…...…... 3.73
D5..……..…. L/S 8.8".……...... 3.55

I've had a look and there is no label on any of the door jambs or anywhere else.
Do non-US cars have this Truck Safety Compliance Certification label? Thanks anyway. GRoB

There would be a metal tag on the differential itself.
That tag will tell you what the ratio is.

Thanks Mounty. I'll have a look. Sorry for hijacking this discussion Bubble but the mention of noise from the diff hit a chord. Hope all goes well. GRoB.

Hi GRoB, there are 2 tags on my diff (my door jar sticker came off years ago) mine say:

75w140 Synth Oil Only


S613C R. 3L73.88. 7E19

Don't worry about this thread, alot of very useful rear diff info is going on it so post away!

Thanks. I shall have a look tonight if it's not pouring down (no garage!).
Do you know the "translation" for the tag numbers? Anyway, it looks like it would be worth changing my diff fluid sometime soon. Anglesey is not far way so getting the oil should not be a problem.

Bubble. Having looked at the info from your tag and the info supplied by Mounty, I think I have worked out that the 3L73.88 corresponds to D4...L/S 8.8"...3.73 making yours an LSD. Not the brightest bear at times but I have my moments! Hope that the work on yours is going OK today. GRoB.

Axle Destruction...

Right... Problems r us!

R/n/s Axle has been "ate" by the bearing - Now need axle saver bearing.

R/o/s inner oil seal has gone leaving the channel where the axle runs from the wheel to the Diff full of oil manky oil.

However the internals of the Diff all look good - bonus!

Now need to find thread for replacement of oil seals...

I can't find a thread nor an internet page regarding replacing the rear diff inner seals anywhere....

Does anybody have knowledge of doing this including special tools needed or am i better going for a diff rebuild?

In which case has anybody in the UK had this done and how much roughly was it?

Cheers Guys! :D

(PS i've taken some pics which i'll post in the next couple of days)

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Just a thought... It it absolutely necessary to fix the inner seals?

The fluid got into the channel but didn't get past the wheel bearing / axle seal (in fact the bearing was immersed in oil - can't of done it bad!) The fluid on draining couldn't make it back past in the inner oil seal to be drained due to it being locked into the channel by the Diff bearings!