Rear Differential Going Out, What Can I Do?

M

Meliodas

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Oct 27, 2023
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Ky
Vehicle Details
1994 Ford Thunderbird 3.8L v6
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been having a weird intermittent groan / grind sound coming from my drivetrain and traveling through car, also affects the power , seeming like it doesn’t want to shift or go faster. This happens and then disappears for some time, then resurfaces .

My friendly honest mechanic hopped in (he really is honest and friendly) , heard the noise, and said it was u joints wanting to seize up, so got those replaced and he says “bad news” . Rear end is going out. Said something about being to hard on pinions or something he didn’t explain to well. I asked rear differential? He said yeah it can be called that. How can I acquire these ?

Sunk about 1400 in parts and repairs on this car, it’s unfortunately my only one. It sat garaged for a good bit of time, only 110,000 not a lot of rust. He says it is driveable which is good news I suppose, I just don’t want to royally fuck the car up. He says if it fails I’ll hear a loud clap and car will lose all power to wheels. I just don’t want to drive it till it dies, lovely looking car and I need it badly.
 
First of all we need pictures of this lovely car. Can you’re friendly honest mechanic not help with sourcing and repairmen of the rear diff?
I’d say your best bet is to probably find one at a junk yard, hopefully low miles and swap out the whole pumpkin.
 
I'd get a second opinion from a mechanic who specializes in driveline, a rear differential isn't particularly expensive or difficult to diagnose and rebuild. Frankly 110,000 miles is nothing for a 7.5 or 8.8, unless you were running it without fluid, and the lack of wanting to shift or go faster and even the groaning, and your car being a 94 I'd be inclined to attribute to the transmission, especially being intermittent.
 
Agreed, rear end wouldn't come and go. Certainly wouldn't effect perceived power.
 
Easiest thing to do is to call the local junkards, and see if you can find one. Preferably a locking one, but 3.27 ratio, 8.8 irs diff. It can be changed out in an hour, even taking the time to see if the driveshaft is separated, which takes out the diff.
 
It takes me an hour to get my car high enough off the ground to work under it.
🤣
The worst part is that I have to drive all of my tools and stands and jack's etc across town whenever I need to work on my car.
No work allowed at where I live with my cars.
 
XR7-4.6 said:
I'd get a second opinion from a mechanic who specializes in driveline, a rear differential isn't particularly expensive or difficult to diagnose and rebuild. Frankly 110,000 miles is nothing for a 7.5 or 8.8, unless you were running it without fluid, and the lack of wanting to shift or go faster and even the groaning, and your car being a 94 I'd be inclined to attribute to the transmission, especially being intermittent.
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Good points. Being a 1994 could even be a bad Neutral safety switch causing erratic shifting.
 
supergordo said:
Good points. Being a 1994 could even be a bad Neutral safety switch causing erratic shifting.
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And groan could be a buzzing EPC solenoid, my original 94 transmission did that occasionally, it was a very loud disconcerting sensation, it was doing that when the car had only 60-70 thousand miles on it!
 
The hardest part of changing out a diff is getting the driveshaft off; then the front bolts, the rear bolt, and the two lower knuckle bolts, pull one halfshaft, and yank it out. Nothing tough at all. If an impact fit on the ds bolts, it wouldn't take an hour. Doesn't even mess up the alignment.
 
Grog6 said:
The hardest part of changing out a diff is getting the driveshaft off; then the front bolts, the rear bolt, and the two lower knuckle bolts, pull one halfshaft, and yank it out. Nothing tough at all. If an impact fit on the ds bolts, it wouldn't take an hour. Doesn't even mess up the alignment.
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I think the driveshaft is the easiest part, just 4 bolts and push it forward. I know a few of you had luck taking off just one knuckle halfshaft but I don't know how the hell you guys do it without it being a balancing act and making a mess, like when I tried following that advice. It's easier to just take the extra 20 minutes pulling the other knuckle/halfshaft. If you're using factory rubber knuckle bushings you need to load up the suspension before you torque those bolts down, which adds time as well
 
I dump the oil in my oil change pan, by laying it on it's side in the pan. Straight to the ground is easy, lol. It ain't like it's going to go back in,and it's stinky crap from hell, so I treat it like reactor waste, lol. If you pop the halfshafts with a screwdriver, it's pretty easy. I had one in and out several times while I was diagnosing it. It's replacement was removed in 5 minutes by a jy employee; His only tools were a high lift and a torch. (!). Yes, he was going to cut the halfshafts before I stopped him, lol. Cut the ds, cut the bolts, and let it drop, lol.
 
The smell of diff oil is the worst. It lingers in the nose for days its seems. After an oil change a few days later I'll get a whiff and it's like WTF.
 
so if the OP has a 3.8 v6 N/a I belive its gonna be the 7.5 rear end. (not positive) and also not positive if the 8.8 is a direct swap but I feel like the 8.8 would be easier to find than the 7.5. I've pulled several rear lower control arms and that can be done without altering the alginment so by pulling those would give you all the room in the world to disconnect both half shafts.
 
Definitely get a second opinion. If your differential or u-joint was going bad, you'd have the same symptoms all the time while rolling regardless of what gear you were in.

There's a difference between a mechanic and a parts installer. Anyone can install parts, but it takes someone with diagnostic knowledge to accurately identify the problem and propose a fix. I'm sure your friend is a decent and honest guy, but it sounds like he's throwing out solutions to identify the problem. When the solution doesn't fix the problem, a new solution is suggested. All that does for you is waste your time and money.
 
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97teebs said:
so if the OP has a 3.8 v6 N/a I belive its gonna be the 7.5 rear end. (not positive) and also not positive if the 8.8 is a direct swap but I feel like the 8.8 would be easier to find than the 7.5. I've pulled several rear lower control arms and that can be done without altering the alginment so by pulling those would give you all the room in the world to disconnect both half shafts.
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Not always. my 95 3.8L NA came with an 8.8
 
There's no functional difference, either will fit. I have a 7.5 in the garage I ran for a bit, because it was a 3.08, but then I found a good 8.8 3.08 ratio. It gave me a better cruising speed. :) Conventional wisdom says a v8 car needs the 8.8, tho. My dead stocker in the tbird was grinding when I got it; It had spit out the shims, in chunks. There are apparently both steel and bronze shims in the carrier somewhere.
 
Grog6 said:
The hardest part of changing out a diff is getting the driveshaft off; then the front bolts, the rear bolt, and the two lower knuckle bolts, pull one halfshaft, and yank it out. Nothing tough at all. If an impact fit on the ds bolts, it wouldn't take an hour. Doesn't even mess up the alignment.
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The driveshaft is the easy part. Those four bolts only take 10-15 minutes. I'd say the most difficult part is wrestling a one-piece exhaust system out of a car with a SC rear bumper cover.

I've done the one halfshaft method before; you really run the risk of tearing up the side seal on the side that you leave the halfshaft installed on. I always pull both sides.

RTV needs an hour to set up enough to torque the diff cover bolts, and it really shouldn't be filled for at least a few hours.

Taking time to do things right, torque all fasteners to their specs, swap the ABS sensors over, clean and seal the cover, fill with gear lube, test drive the vehicle prior to returning it to the customer, and so on, it's a 5+ hour job.
 
Only time I fussed with mine was when I swapped in the 3.73 TL pumpkin I got from rednsilver94sc 13 years ago now. :eek:

I remember doing it in an afternoon. It was fully assembled and all I needed to do was drop/install then fill. The part I hated the most was fussing with the bolts through the subframe, which were stuck. Oh, and the smell. :zpuke:

A couple guys came to grab the 3.27 out of the FIL's sport while I was parting it out. They were smart and brought electric impacts to take care of the mounting bolts. They had it out in about 15 minutes. :)
 
theterminator93 said:
A couple guys came to grab the 3.27 out of the FIL's sport while I was parting it out. They were smart and brought electric impacts to take care of the mounting bolts. They had it out in about 15 minutes. :)
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No exhaust in the way or halfshafts installed? Already up on a lift or stands?
 
Yeah I had it on stands. Exhaust was down too (I previously removed it so I could get the trans out). All they needed to do was pop the knuckles and half shafts, unbolt it and let it fall down.
 
racecougar said:
The driveshaft is the easy part. Those four bolts only take 10-15 minutes. I'd say the most difficult part is wrestling a one-piece exhaust system out of a car with a SC rear bumper cover.

I've done the one halfshaft method before; you really run the risk of tearing up the side seal on the side that you leave the halfshaft installed on. I always pull both sides.

RTV needs an hour to set up enough to torque the diff cover bolts, and it really shouldn't be filled for at least a few hours.

Taking time to do things right, torque all fasteners to their specs, swap the ABS sensors over, clean and seal the cover, fill with gear lube, test drive the vehicle prior to returning it to the customer, and so on, it's a 5+ hour job.
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I agree with all that. If you're swapping one complete diff and removing another you can expediate the process prefilling the diff and dropping the old one without draining it first, a set of the good rotunda halfshaft plugs help prevent any slosh spillage, not that there should be any if you're dropping straight down with both halfshafts out. Still no way I'm getting that done in an hour but possibly under 2 if I hustled

My exhaust is fairly easy to drop too, I ditched the factory style hangers for strap style ones, so I just unbolt them from the rear and it drops down, it's also 2 piece with ball/sockets behind the H pipe
 
Yeah, a one piece exhaust on a car with a SC rear bumper cover adds a solid hour to R&R when you work alone, especially when it's all still hot from an hour long drive. Agreed on dropping without draining. That's easy to do.

When I bill my time, it includes the test drive, clean up, putting away the tools, etc. as well. A legitimate start-to-finish. This one took 6 hours to swap from pulling onto the lift to putting away the tools after the test drive. That includes moving the LPW cover from one diff to the other, torquing all fasteners to spec (not running things together with an impact), coming up with a new vent arrangement, and cleaning up the old diff. Doing all of that in one hour, alone, just doesn't compute.



IMG_2728.JPG
 
It takes forever to clean up that crap; after the first time, I wear a tyvekdressout suit and a respirator, heavy gloves, etc. Anythingyou get that crap on will be ruined. Do Not put it in the washing machine, it will make everything else smell, lol.
 
BTW; when you do the exhaust on your car put a joint to disassemble it near the factory location, because dropping the exhaust as one piece sux!
 
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