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Discussion Starter · #1 ·
I just rebuilt my differential from a factory 2.41 open to an Eaton Detroit Truetrac limited slip

with Yukon 3.23 ring and pinion. During my initial 20 mile break in, I noticed a humming that wasn't there before.

It doesn't do it at all speeds, and doesn't sound overly loud, but I can hear it. Wondering if that slight hum is normal for this type of posi? Anyone else experiencing this with a Truetrac?

Other than that, the new setup feels great, huge improvement, even during my soft 20 mile break in run.
 

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I'm guessing you rebuilt the entire rear end? All the bearings/seals, etc. What weight lube, and how much additive did you put in?
 

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Shouldn't need additive sine a truetrac does not have clutches. I am curious to see the responses as I plan to do the same thing shortly.
 

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Discussion Starter · #4 ·
Yes, all new bearings and seals, used lucas non synthetic 175w40 , 2 quarts.
Correct, no additive was required per instructions
 

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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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Who set the gears up - pinion depth, backlash, contact pattern?

Assume you meant 75W140.
 

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I had a Detroit TrueTrac installed on my '71 Firebird a few years ago. I also hear a humming sound, I'd say between 35-50 mph (approximately). I had the shop look at it a few times and they called the supplier to see if anything could be done. According to them, if i want to keep it as 'strong' as possible I should keep it this way. If not, they could adjust it but it would be weaker. And they they also stated that in a 40 year plus car these things could happen.
 

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The 3.42 gear that i swapped to in my red bird hums a little bit. Most of the time, original delco brand gears won't make a noise if they are installed properly, but aftermarkets will.
 

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Discussion Starter · #8 ·
Yes, sorry meant 75w140. Good to hear others are experiencing this same noise. I had a buddy of mine install it that has been doing this kind of work on the side as a second income. I trust he followed all the correct specs, he is pretty meticulous. I know nothing about this kind of stuff.

I just wanted to make sure this wasn't anything out of the ordinary, seems to be very common from my searching. I am very happy with the performance upgrade. Seems to be performing well. Thanks for the input all!
 

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'70 Esprit TA clone Pontiac 400
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It's probably not the posi unit itself. Usually (but not always) it's the meshing of the teeth on the pinion and ring gear that will give you a hum. as Eric noted some brands of gears are a bit more noticeable than others. Slight may wear in with little noise after break in.

When setting up the new gears you need to get the contact patch correct by marking the teeth and observing the pattern and making adjustments as needed. If it's off you may get a hum which is usually more noticeable under normal acceleration (not full throttle) OR while coasting after letting off throttle during normal driving, rarely both. In pic below you can see the pattern created. Gears are 3.73's in a 12 bolt I did for someone a couple years ago. Yukon gears also if I remember correctly. I usually take a couple pics to show customer.

003-6.jpg
 

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Discussion Starter · #10 ·
It's probably not the posi unit itself. Usually (but not always) it's the meshing of the teeth on the pinion and ring gear that will give you a hum. as Eric noted some brands of gears are a bit more noticeable than others. Slight may wear in with little noise after break in.

When setting up the new gears you need to get the contact patch correct by marking the teeth and observing the pattern and making adjustments as needed. If it's off you may get a hum which is usually more noticeable under normal acceleration (not full throttle) OR while coasting after letting off throttle during normal driving, rarely both. In pic below you can see the pattern created. Gears are 3.73's in a 12 bolt I did for someone a couple years ago. Yukon gears also if I remember correctly. I usually take a couple pics to show customer.

003-6.jpg
Thanks John. I will check that with my buddy who installed. I am getting close to 100 miles. Hopefully its less noticeable after a little more break in. If not I will have him recheck.
 
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