Hello everybody, I am a new member here and I have some questions about the rear diff. 1998 F-150 4.6 V-8 Automatic 3spd 2wd 140,000 miles
#1. I have never had to work on a truck before so I'm not real familiar with them. (newbie)
#2. How can I tell what size gears/diff are in there ?
#3. Can I swap gears with another F-150 that would be a straight swap without additional modding ?, If so What would those f-150 models be ?
#4. Where can I get the Friction Modifier ?
I did try searching the forums, however I still am not sure of the "fix".

Here's what's up. On the truck after starting to drive 30mph and up I hear this "whining" coming from the rear, it's kind of like a whistle while accelerating.
I am assuming it's the rear diff. The rest of the truck drives perfectly fine.
I'm hoping I don't have to replace the entire diff. I have read that some folks here add some "friction modifier" to theirs and the noise goes away. I did change out the gear oil in the rear diff and it seemed to quiet the noise, but it did not go away, used 75w90 synthetic, I don't think the previous owner ever changed it as it was black and grey and verry stinky, there were no shavings or pieces of anything in the oil. Any help is appreciated.

 

when you changed out the oil you should have put friction modifier back in with the oil again.....thats why it's yelling.....get it at a ford dealer and put it in the same place you put the new gear oil in

 

Is there any special weight or anything for the friction modifier to ask for ? or is it all the same ? How much of it do I need ?

 

it's a small bottle......not sure what the price is but it's not that expensive.....put the whole bottle in

 

you can get a 7 ounce tube of the stuff at Auto Zone or NAPA. it has the FORD spec on the side. just looks like gear lube to the bare eye. put all 7 ounces inside the differential.

 

You need to use 75w140 synthetic. Unless you are looking for more noise, 90 weight is not heavy enough to do the job.

 

Hi ViolentB and welcome to the forum

It's important to use the correct oil and friction modifier if it has limited slip but neither will cure the howling noise. Friction modifier will help keep the clutches (if it has limited slip) from chattering during low speed corners.
It sounds to me like you've got bad bearings which isn't uncommon on these trucks.
Does the noise change when you're on and off the throttle or is it just speed related?
On the VIN tag on your door it will have an axle code. If you search in the forum here you'll find info on decoding it to tell you what gears you have and if it has limited slip.

 

No it just starts at around 30mph and continues really until you stop and park it or slow under 30mph.
Sorry, I'm retarded with trucks,,,,if it is bearings are there any tech articles or instructions you could point me to ?

 

Replacing bearings in the rear end can be a pretty big job with some tools that you might not have access to such as a press and bearing pullers.
I think I'd start by trying to find a good tech to go for a drive with you and give you his opinion.
Noises can be almost impossible to diagnose on here.

 

Probably not the bearings anyway.

More likely it's the mesh between the ring and pinion gears.

Steve

 

Hey racerguy, what is your opinion about what 7700 rob said? Do you think that 75/140 is the way to go in a rear diff instead of 75/90? What about on the front diff if it is a 4x4?

 

Ford says to use 75W140 in the rear and 75 or 80W90 in the front.
Some people don't want to spend the money to buy the 75W140 and use 80W90 instead and don't seem to have problems because of it.
I'd still rather use what Ford calls for though.