Gear Pattern

[spdeangelo]

I have a 2008 Saleen S281 Supercharged Mustang that I bought new with 140 miles on it. It now has just under 3,500 miles and I’ve noticed what appears to be gear whine coming from the rear. The noise starts at approximately 60 mph and gets somewhat louder as I approach 80. The noise only occurs under load or acceleration and disappears when I let off the gas. The reason this concerns me is that the car came with the optional 3.73 gears installed by Saleen.

I’ve completely disassembled the rear and using Permatex gear marking compound derived a contact pattern. I’d like you to look at the attached photos and help me understand weather or not the pattern is acceptable. The drive side appears to be centered root to top but looks a little toe heavy. The coast side appears to be pushing the top but centered heal to toe. The backlash is within specification (.010-.011). Any help or advice you can provide would be greatly appreciated.

[accu tech]

Actually, the drive side looks a bit low and the coast side looks a bit high to the root, which is a bit unusual because if this is was a new gearset it would be indicating conflicting pinion depth adjustments.

A gearset with several thousand miles may not even be able to correct for improper setup, as the pattern will have already altered the tooth contact area. It could actually make it worse.

Also, the setup only will affect the initial sound of a differential assembly. If it is quiet after installation & the bearings remain preloaded properly the only thing that will cause it to get louder over time is by wear. Premature wear is usually caused by a lack or breakdown of lubrication, or by the gearset not being broken in properly. Occasionally a gearset that does not have the proper heat-treating from the factory can wear prematurely also, but this is very rare. We mention the premature wear, because your gear teeth do not appear to have a mirror-like finish yet. A gearset with several thousand miles will usually have polished in a reflective surface by now, but in the pictures it appears to still be dull and hazy.

A great guide to patterns is found at the Yukon Gear website, and is the only guide that I have seen that has the proper depth and pattern theory for checking a new setup.
http://www.yukongear.com/Downloads/Tech ... ctions.pdf

-Accu Tech
AAC/AAP

[spdeangelo]

Thank you very much for your reply. Based on the information presented, I’ve decided to replace the gears with new. I’ve purchased all of the components and will be tackling the project this weekend. I'll let you know how it turns out... Thanks again for your help!

[spdeangelo]

In your forum topic “What’s that noise” you mention the following; the most common clunking noises are from the differential internal spider gears and/or (if the vehicle has a limited-slip) the side gear clutch assembly. Is this normal?

I completed the gear installation and set the backlash to .010”. Holding the axles secure with the emergency brake, I rotated the pinion flange back and forth. Doing this I noticed some play in the spider gears allowing the flange to rotate well beyond the .010” backlash. If I rotated the flange with some force I also noticed a pretty significant clunking noise. Is there a specification for angular rotation of the pinion to determine if this additional play in the Trac-Loc is acceptable?

The clunking noise was definitely prevalent before changing the gears and most noticeable when getting on and off the throttle. This probably wouldn’t be so annoying if I had an automatic transmission but I have the 5 speed manual. Any help or advice would be greatly appreciated!

[accu tech]

The slight clunking sounds commonly associated with used factory limited-slips are generally more of a nuisance than an urgent problem. As long as the integrity of the spider gear teeth and clutches are ok then it is fine to continue use.

If the noise is uncomfortably loud or if there is noticeable wear or chipping on the pinion teeth or clutch tabs, either replace the clutches and spider gears or the entire posi differential. We offer several factory and aftermarket units with varying levels of aggressiveness. We list and sell the above kits and units as listed for your vehicle at the www.accuautoparts.com website.

-Accu Tech
AAC/AAP

[spdeangelo]

Well, I installed the new gears and this is the pattern. Per the manufacturers recommendations I’m breaking the gears in slowly and planning to change the differential oil at 500 miles.

While I had the differential apart I replaced the driveshaft with a new one piece unit from Roush. The gears do not make any noise under acceleration and only a very slight whine on deceleration. (Barely audible.) I do, however, have a new noise that I’m concerned about. The noise is a high pitch whine similar to that of a supercharger. It remains constant under acceleration or deceleration and gets higher in pitch as the vehicle speed increases. It’s hard to tell while driving where the sound emanates from but it is definitely in the driveline and not the motor. Could pinion bearings or universal joints make this kind of noise?

I also checked the differential temperature after an 80 mile road trip and found it to be quite hot to the touch. How hot is normal for a differential? I have an infrared thermometer and will check it again next road trip.

As always, thanks in advance for your help and advice.

[accu tech]

Pattern looks acceptable, and should have felt very smooth when rotating by hand.

With newer gears, light driving is the key to a good break in. Unless finished or polished, the contact surface of the teeth are rough. Until the contact area of the teeth have worn-in has a smooth and mirror-like surface, there isn't a lot of surface tension for the gear oil to try to lubricate well. This creates heat that if it gets hot enough, can break down the gear oil and cause the new gears to wear.

Also with new gears, your ears tend to be more sensitive to noise. Often you are so focused on any noise that you hear small normal noises that you have never noticed before. Assuming a proper installation and that all the bearings are preloaded properly, as long as there isn't any new vibration and the noise is less than normal transmission operation noise it is most likely fine. I can imagine even a driveshaft may have a different harmonic than the old that may create a 'new' noise.

Take it easy and change that gear oil when it is time. As long as any of the very slight sounds you gear do not get progressively worse, it sounds like it will be ok. Good luck and hope it all works out well for you.

-Accu Tech
AAC/AAP