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post #1 of 5 (permalink) Old 04-30-19, 07:28 PM Thread Starter
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300k mile old diff fluid

I have a '11 Crown Vic with 300k miles on it and my rear diff fluid looks like this...



Been watching some utube vids on how to change this fluid out but after a visit to the local Advance store I've got some questions. Up til now, forums on the net have led me to believe that all former police cars (which is what mine is) need 75w-140 fluid with friction modifier, but after giving them my VIN at Advance the computer comes back with a fluid suggestion of 80w-90.

Here's the diff tag...




I believe that shows a 3 (L or 1)? 27 which indicates a 3.27 gear ratio, and then an 88 after, which indicates the 8.8 axle. Not sure what that SYN means, unless it means synchronous?

This car is driven VERY sparingly, long distances at 60-65mph but never hard, and operates in a climate that can get up to 95F in the summer.

When the rear is jacked up and one rear wheel is spun, the other moves in the opposite direction.

So my first question is... what weight fluid should I use? Also, I need confirmation if I should use a friction modifier.

Next is how to go about changing the oil out. I'm being lazy here and after watching a vid on how you can extract 80% of the diff fluid out the fill hole by using a flexible hose I'm thinking about just doing it that way. But if you guys think it's necessary to get all that old fluid out I'll take plate off and do it the regular way.

And lastly, if I take the plate off, should I spray all the gears down with brake cleaner to get all the old fluid out, or just let it drain for a while and call it good?

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post #2 of 5 (permalink) Old 05-02-19, 08:55 AM
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Re: 300k mile old diff fluid

It looks like it's a limited slip but the clutches are worn out. You don't have to use the friction modifier if you don't replace the clutches inside. It's best to remove the cover and replace it but you can replace the 80%.

Ford Axle and Differential Identification Tag ID

Ford differential tags are very straightforward, as indicated above. The Month of Manufacture uses the following format:
A = January, B = February, C = March, etc.
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post #3 of 5 (permalink) Old 05-02-19, 07:20 PM Thread Starter
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Re: 300k mile old diff fluid

Thanks for the info and that graphic Burt, very helpful.


How would my rear end perform differently if the clutches are worn out? What performance improvement would I see if I replaced the clutches?
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post #4 of 5 (permalink) Old 05-02-19, 11:28 PM
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Re: 300k mile old diff fluid

When the clutches wear your rear end will act like an open rear differential where when one wheel slips power will NOT be transferred to the other wheel. This results in a one wheel peel. If the clutches are good both tires would spin. You only have to replace the clutches if theyre worn. You could very well be due for it but it depends on how they look when you open up the diff.


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post #5 of 5 (permalink) Old 05-03-19, 06:50 AM
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Re: 300k mile old diff fluid

When you raise the rear and one wheel turns the other way, the clutches are worn. It acts like a regular diff. When I worked on the police cars, we replace a lot of clutches. On mine, I just drove it like a regular diff. I don't live where there's snow or slippery places.
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