Trans?Transfer Case? Strange noise

[Tastlikechikn]

I'm trying to nail down a strange noise. I can't determined the source. It's a 96 4.0 Auto. It shifts fine up and down through all the gears. No slipping or serging, 4x4 works fine. But,while driving at highway speed (above 50) when I let off the gas or the cruise let's it coast, it will start making a fairly loud sound underneath like an airtool or servo/electric motor?RPMs drop to idle, and it makes this sound until you touch the gas again. As long as you don't let it coast it does'nt make the sound. Three different shops said three different things, One says"I never heard a car make that sound before"one says trans the other says TC? Anybody?
What about the shift motor? could it make that noise? I'm asking because I don't know

 

[gmanpaint]

So if your doing 50 mph, let off the gas pedal, the rpm's drop to idle speed? Like around 6-700 rpm's? Then the noise begins while cruising? The noise is like an air tool in operation?

It's def not the shift motor. This only turns around 240* in one direction or the other and only when a signal is sent. This cant be operated until its in Neutral and the brake applied, and only shifts from high to low range and back. You can scratch this off the list.

The rpm drop has me thinking the torque convertor. It locks and unlocks. Does the O/D light come on? Does it ever flash?

The noise has me thinking the trans. Hydro pressure is how the trans works. Faulty pressure release can make a noise.

The 4405 t-case has a pump in it, gears, chain, and has clutches. There is plenty inside that can make noises, but not like an air tool makes. I have seen this t-case do some strange & puzzling things when they go bad.

I would drain/check the fluids for types of particles in the trans and the t-case at home. Both of these are a simple job in the driveway, and can give good clues to whats going on inside.

We have a bunch of great knowledgeable members on here that know these transmissions/t-cases very well. Keep bumping the thread until more chime in and offer more help.

Good luck on the diagnoses!

 

[Tastlikechikn]

Hey g,
The noise only happens while coasting @ cruising speed. If I shift to neutral(going down hill) before the noise starts it doesn't start at all but, if the noise has already started neutral doesn't make it stop.Touching the accelerator always makes it stop.
As for the fluids I changed them a couple of weeks ago(for the first time in years from the looks of it) The TC fluid was Odd (semi thick,grey,crayon smell) might have had 90w added by accident or "doped up" for the auction block. I couldn't tell what it was suppossed to be. I had them look it up at parts store to see what I needed. I have dropped TC fluid again and am going to refill with Mobile 1 synthetic atf this AM and drive it again.(and hope for the best) I can't keep throwing parts and money at it though, and just guessing on trans,transfer case and torque conv. is not an option. I have to Know. It may just end up out in the pasture with the wrecks and parts cars.:
Thanks for the help

 

[gmanpaint]

So I take it the O/D light does not come on or flash. That takes both the trans out of the equation. The t-case can mess up and not trip the O/D light tho.

Could you possibly make a sound clip? Do you have a digital camera that you can make a short video of and post to youtube with a link to this thread? This is one of those problems that is going to be almost impossible to help via the internet without hearing, or experiencing the issue.

I have run down some noises in the past that turned out to be something I would never have thought to be the cause.

Example...the front drive shaft in the 95-96 has a "Ball joint" inside the rear double cardan with a spring in it. That joint failed and made a chirping noise unlike any noise I have heard before. Talk about hard to find. It sounded like it was coming from everywhere.

It wasn't until the rig was lifted on stands and the drive shafts were turned by hand to pinpoint the noise. This is something you can do as well. Try to narrow it down. Place the rig on stands, (safely!!!!) get it in neutral, and try to move each drive shaft by hand, than turn the wheels by hand, one at a time. Maybe the noise will rear it's ugly head.

 

[Glen4X4]

I agree with gman the rpm drop sounds like it has to do with the torque converter but also sounds like as if the noise is in relation to the torque converter also and I assume you always have over drive on? Have you tried driving it over 50 with the over drive off? Won't cause harm for a trial period just raise the rpm's and will help in the diagnosis of the situation.

 

[Tastlikechikn]

Finally a follow up.


Hey guys I just ran across this old post and did not realize I had not posted a follow up. It turned out to be the transfer case after all. What in particular I can't say it failed a couple hundred miles from home. I had it towed $$$$$$$! to the house and put a junkyard TC in it and it has been fine. The only reason I ran across this post is because now it has transmission issues and I was looking on here for a cheaper fix than a complete rebuild. Again, sorry I didn't follow up.