View Full Version : Possible Explanations for Noise from Rear End Needed Inside

01-28-2010, 06:03 PM
I recently installed a used 3.73 pumpkin into my Lincoln and it's making a speed-sensitive howling noise that comes on during acceleration, prominently around 30mph, and is less evident while coasting and decelerating.

The previous owner declared the rear end made no noise, so I'd like to rule out ANY other possible explanation before I tear down the rear end.

Thoughts include rear wheel bearings, halfshafts, driveshaft/u-joint issues. For reference, there was NO evidence of this sound previous to install.

I'd appreciate it if you guys could throw some possible explanations out there so I can rule them out.
Thanks as always,

01-28-2010, 06:11 PM
Well if the sound was not evident prior to the install, thats what I would bet it is.

01-28-2010, 06:33 PM
Noise under accel only and not under decel means something is wrong with the way the gears are meshing. If it was a bearing noise, it would be there all the time, or possibly only under decel if it was a pinion bearing. Before you installed the gears, did you inspect them thoroughly to make sure there were no cracks, chips, etc.? How did you set up the rear? Did you measure backlash? Did you use the yellow grease in the install kit to check the tooth pattern? Either way, the diff definitely has to come apart to check it all out, and I wouldn't drive it until then cause if it was set up improperly, the gears themselves might still be alright, but if you keep driving it, it definitely will destroy them.

01-28-2010, 08:58 PM
The gears were set-up by the previous owner, I installed the pumpkin as a sealed unit.

Well if the sound was not evident prior to the install, thats what I would bet it is.

I'm figuring this, however since I had to pull the previous pumpkin out I'm just making sure a mistake on the subsequent install of the new pumpkin isn't responsible for the noise.

I'd like to be able to point out the problem when I have it rebuild, is improper backlash most likely responsible for this?

01-28-2010, 09:56 PM
The independent rear suspension (IRS) found on 1989-1997 Thunderbird's and Lincoln Mark VIII's are different from the well known solid axle's found on Mustangs (FOX or SN95), and other rear wheel drive Fords. I will refer to the MN12 Thunderbirds differential here only. The differentials for the MN12 Thunderbirds come in two sizes 7.5 and 8.8 (diameter of the ring gear). All Thunderbird SC's came with 8.8 differentials and depending on the transmission (automatic or manual) either 2.73, 3.08, or 3.27 ring and pinion gear set. If you decide on replacing or upgrading your Traction-Lok Limited Slip there are some important things to keep in mind. When upgrading the 8.8 Traction-Lok unit you will find a lot of 8.8 units that are made to fit the solid axle cars but, will not fit the IRS cars. The MN12 8.8 and 7.5 differentials use a 28-spline shaft. The difference between the standard solid axle and the IRS differential is a "Step" (grove) in both differential side gears in order to retain the Circlip (C clip) that holds the Halfshafts in place (see diagram). A "Step" is a grove that will allow the Circlip (C-Clip) to slip over the Halfshaft and secure the Halfshaft in the differential. The differential has precise tolerances and should not be disassembled or assembled without the proper tools and experience. When replacing or adding a Traction-Lok Limited Slip to your SC that may or may not have one installed the easiest ways is to get a Traction-Lok Limited Slip unit from Auburn Gear Inc. 219-925-3200. High Performance unit for standard OE replacement part# 542079 or Pro Series unit part# 542080 for better traction and faster engagement. The average street price for the High Performance unit is $340.00 and $410.00 for the Pro Series. Torsen also sells a limited slip differential through Ford Racing part number M-4204-T28 unit is $450.00 through Ford Racing By, Rich Thomson

this might help

01-28-2010, 10:27 PM
It could be improper backlash, or bad tooth pattern, or the gears themselves are worn out, or maybe something is broken inside. Once someone gets it apart and checks it out, it should be pretty easy to tell what is going on. The first thing I would do is pull it out, take the cover off, clean everything, and check the tooth pattern. A gear install kit comes with a yellow grease that you paint onto the gears and you spin them over a few times, and as it tranfers around it makes a clear impression of where the ring and pinion gears are meshing. You want them to be dead center of the ring gear tooth. If they are too shallow or too deep, then the carrier shims need to be adjusted. If they are too far towards the center or outside of the ring gear, then the pinion depth needs to be adjusted. If the tooth pattern is good, double-check the backlash, preferably with a dial indicator, but if you don't have one of them, a dial caliper and an extra set of hands can get you pretty close. Basically what you want to do is hold the pinion gear still, and measure the total distance you can move the ring gear without moving the pinion. It I remember correctly it should be around .012"-.016". If all that checks out, then remove the carrier from the housing, and thoroughly inspect both the ring and pinion gears for any cracks, chips, pitting, or any other kinds of imperfections on the surface of the gears. Also once the carrier is out, make sure the pinion gear spins with just a little bit of resistance, and that there is no play. If you do all of that in that order, you will find your problem.

01-29-2010, 11:11 AM
You may want to check the carrier bearings for wear also. I had a mustang with the same symptoms you describe, the carrier bearings were worn to the point where the gears would not hold a consistent backlash setting. Had new bearings pressed on, re-installed the 3rd member, and the gears were silent for as long as I owned the car. I realize that was in a mustang and not a 'bird, but if you have to take it apart, might as well have a look:D A quick glance at the races and rollers will tell you if they are good or bad. Mine were in horrible condition.

In this situation a dial indicator with a magnetic base is your best friend. It is pretty straightforward to measure backlash with one of these, they can be had for pretty cheap or maybe even rented from a local parts store.

Best of luck, hope you find a solution!

03-01-2010, 08:35 PM
The pumpkin and diff were removed. The noise was more than likely improperly shimmed carrier/backlash like most of you suggested, if I remember right it the backlash was .025". While apart I replaced both the carrier and pinion bearings and shimmed the carrier to .010". I believe factory spec is .008-.015".

Noise is gone around town, what a difference such a small distance makes. I can still hear some whine under highway acceleration but it's only audible to someone who knows what to listen for and more than likely acceptable considering the difficult nature of trying to set-up the ring and pinion to wear exactly as it had before teardown with used gears.
Thanks for the help guys,

03-01-2010, 09:37 PM
Three things to know here. Always set pinion depth first. Set backlash last. Never use a used ring and pinion set and expect them to be quiet. .025 is way too much backlash and will lead to failure if the car is beat on at all.

03-02-2010, 01:53 AM
They either got the pinion depth or the backlash off. I actually have good luck with used gear however you have to allow higher backlash with used gears. Pattern is the most important thing on the whole build no matter what gears you are using.

03-03-2010, 02:01 AM
what kind of gears are you using?

03-03-2010, 03:03 AM
Ford Racing gears

03-03-2010, 01:07 PM
I had some richmonds back in the day and the rearend shop told me they hum because of their gear geometry