Differential Chatter with Amsoil 80w-90

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ConfederateTyrant

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I replaced the gear oil in my 1995 Grand Marquis, which was Castrol 80w-90 Hypoid C gear oil, with Amsoil AGL 80w-90. I am unsure if I have a limited slip differential or, as Ford calls it, a "trak-lock" differential.

Now I'm not sure if this is differential chatter but before I changed the fluid (both Amsoil and Castrol) I would hear some clunking when turning at speeds above 7 mph. After installing the Castrol gear oil, the clunk went away for a couple of days, then after replacing with Amsoil the clunking stayed away for a couple of weeks, I even gunned it while doing a 180 in a parking lot and it never clunked during that time. But now it's back, I added about half the bottle of Slip Lock additive, but the clunk seems to be more often. I've only got about 7,500 miles on the gear oil.

I did the change over myself, there is no leak, and the gears were not damaged at all, nor were there chunks of gear in the fluid coming out, it was all silky smooth.

What is the probability that this is a coincidence with changing the fluid?
 
quote:
After installing the Castrol gear oil, the clunk went away for a couple of days, then after replacing with Amsoil the clunking stayed away for a couple of weeks,
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So are you saying the clunk came back after a couple days with the Castrol? Then you changed the fluid to Amsoil and it went away, only to return in a couple weeks?
 
i ended up with non-LSD oil in my Nissan (don't ask...not my fault).

It worked fine for about 10-20km, then really chattered in low speed work, like car-parks etc.
 
I changed out the factory gear oil at 90,000 miles with Castrol 80w-90, then at 98,000 miles I filled it up with Amsoil 80w-90. I removed the cover plate both times so I could check the gears, instead of using a suction pump.

The noise went away for a couple of days with Castrol, and it went away for a couple of weeks with Amsoil.
 
I'm not a differential specialist, but in my opinion that doesn't sound like a fluid problem.

Can you are assure your car has a controlled slip diff?

What fluid does the owner's manual call for?

Here's what my book says:

Controlled Slip Differential, Rear .....GL-5*[3][4] 80W-90
Severe Gear 75W-90

3. IDENTIFICATION: see upper left differential tag, code on plate automatic transmission driver's door.
4. Traction-Lok axles add 4oz of friction modifier Ford part No XL-3 or equivalent meeting Ford spec EST-M2C118-A.
 
Grandparents gave me the car, but the owner's manual was not in the glove box. I have a Hayne's manual which states any GL-5 80w-90 fluid.

Back when I changed it to Castrol I took the ID tag off and asked a friend of mine, who worked in the parts department of a local Ford dealer, and he entered in the information on the tag and said I needed 80w-90 and the friction modifier. I am not 100% sure if it is a limited slip. I tried a simple test by spinning the rear wheels when off the ground and they both spun in opposite directions, and that should be an indicator that it is not a limited slip. Limited slip diffs should spin in the same direction, to my understanding.

I just thought it was odd that the clunk went away each time I changed the fluid, but returned soon after. The suspension does not appear to be a problem either, as I have checked it multiple times and pushed on every moving part, and nothing clunked.
 
quote:
I tried a simple test by spinning the rear wheels when off the ground and they both spun in opposite directions, and that should be an indicator that it is not a limited slip. Limited slip diffs should spin in the same direction, to my understanding.
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That is correct, you have an open diff, and not trac-lock.

I'd suspect you have something loose or worn inside the diff that's clunking. A loose pinion nut would also let the pinion move back and forth and make a clunk. (It's the big nut behind the U-joint that requires superhuman strength to loosen.)
grin.gif
 
You may have a situation in which the fluid is being heated and or sheared after a few thousand miles, whereas the new fluid was unsheared and offered a cushioning effect.

Assuming it is not the Universal joints making the noise, I suspect your pinion/ring gear pre-loading has changed due to worm shims, or pinion and carrier bearings going out.
 
I can see the fluids shearing in a few thousand miles, but this was in less than 100 miles with the Castrol fluid, and about 600 miles with the Amsoil fluid. I don't do much driving, so it takes me a while to gather up the miles. When I was upgrading my exhaust, I had a friend of mine, local and respectable mechanic, check the U-joint and he said it did not look worn or damaged, so it should be in working order.

I just want to make sure we all are noting that this clunk only happens when turning at about 7+ mph, and not while driving straight down the road, or in gradual turns such as highway turns.

One more observation, when I drained the Castrol fluid, it only had about 8,000 miles on it, and it came out with a color mixture of a green and greyish color. That could just be it's reaction to heat, as I drove around to get it warmed up. The fluid was smooth, not gritty and the magnet had normal amounts of shavings on it.
 
CT check the lower mount bolts on your shock absorbers.. It should be a grade 5 bolt or better. tighten them up a bit with a breaker bar and socket.I had this problem on a 86 t-bird and had the rear end rebuilt to no avail. the rebuilder suggested I try the shocks.300 dollars and 10 minutes later the combo wrench solution fixed the clunk! this will work only if the 100 mile phantom theory is inaccurate.no kidding!
 
I don't know for sure what diff/options your car has, but most limited slips will act like an open diff if one tire (or both) is/are off the ground. A limited slip, particularly a clutch type (factory Ford Track-Lok) needs some amount of resistance to lock. Checking with both tires off the ground is not a good test, with the inspection cover off roll the carrier around. If you can see inside and all you have are the spiders, it's an open diff. If you can't see inside or see some type of clutches or springs (factory LS) then it is a LS. The True Track will have a solid carrier case with several sets of gears in the case "wall." They will look similar to a worm gear but are actually part of a planetary gear set.

The clunk could be excessive back lash. This condition is only adjustable by moving the ring and pinion in relation to each other. It could also be excessively worn carrier bearings, but this is usually noted by low speed growling/high speed howl. If it is a LS, it may be the clutches locking or unlocking. When does the noise appear? If going from reverse to drive or coast to power it is probably a bearing/backlash issue.
 
Noise ONLY appears on turns, sharper turns at lower speeds, I notice it most from 7-30 mph. Going 45 mph down the road and making a gradual turn with the road or at a fork in the road is quiet, or the road noise prevents me from hearing it. Shifting from reverse to park, or to drive does not make a clunk.

I thought about what Lewk said, and I also notice a similar clunk when it goes over bumps, which could indicate the shocks are out of it, which I believe they are, these roads around here are so crappy, they'll tear a suspension apart in no time. However the bumps are usually located in turns, plus it throws the steering off so I have to turn the wheels which bring on the clunks.

When I drive to school, I usually travel at 20-25 mph on the campus roads and I hear it everytime I turn, clunks when I turn and when I go back to straighten up the wheels. I do not believe it is anything to do with the front suspension as I have checked it numerous times and everything is in working order, no excessive play or clearance issues.
 
I figured it wasn't a limited slip differential, I just find it strange how the fluid changes make it go away for a short time. Is there any logic behind this that I am missing? Could water contamination be an issue by breaking the lubricity properties which would in turn let things hit together? I used a gasket, no sealant, and it does not leak, so I'm not sure how a leak could have happened, but I know anything is possible.

I appreciate all the responses.
 
Not sure if I'll put a 75w-140 in there, but I assure you that I'll keep on using synthetic. I am going to try Amsoil's Long Life 75w-90 or the Severe Gear 75w-90 in the future.
 
It sounds like you have a LS and that it is locking and unlocking at lower speeds. Clutch type LS's are adjustable to some degree and can lock/unlock at different speeds depending on setup from factory. I wouldn't worry too much about it. The 74-140 shouldn't cause any problems; I use that in everything almost. With gear oil thicker is better and you have enough HP you aren't going to notice a difference most likely. My .02
cheers.gif
 
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