Front and Rear Diff Fluid Question - Ford Powerstroke Diesel Forum
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post #1 of 3 Old 04-08-2011, 12:46 PM Thread Starter
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Front and Rear Diff Fluid Question

I am getting ready to change both the front and rear diff fluid in my 03 Excursions and I have a question about the friction modifier. I am going to use Mobil 1 75-140 in the rear and 75-90 in the front both full synthetic. I read the Bottle and it does not say to use or not to use the friciton modifer, so I was wondering if anyone has used these oils and can tell me weather I need to add it or not. I appreciate any and all help. Thanks again.
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post #2 of 3 Old 04-08-2011, 06:05 PM
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Don't add it. If the LSD clutches act up, add an ounce, drive it for a few hours. If they still act up, add another ounce. I doubt you will actually need any (I never have).
To check if the clutches are acting up, get the rear diff oil up to temp. That will take about an hour of driving. Come to a complete stop, turn hard right or left and slowly accelerate. If you feel a shutter, add an ounce of the modifier. Drive it for a few more hours so that the modifier has time to soak all the way through the clutch pack, and try again. You are trying to find the balance of just enough modifier, too much and the clutches will just slip and be ineffective, not enough and you will get chatter.
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post #3 of 3 Old 04-09-2011, 04:24 PM Thread Starter
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NYC F-350, Thank you very much for your reply. I replaced them and I don't hear any chatter so I think I am good to go. Thanks again.
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