M50D noises

[Steve777]

I have a 94 B4000 (4.0l V6) with 5sp manual tranny, 4x4. It is fairly new to me and when I got it, the PO had just had a new clutch put in (with slave cyl). I beleive it has an M5OD tranny.

It has been running very well, with one problem: every so often there will be a bearing noise (squeal) from the tranny area (or perhaps the clutch or xfer case). It is not always there, and seems to be worse in cool weather and when driving uphill and when rpms are over 2500.

I checked the tranny fluid and when I got it, it was way overfilled??? (perhaps the PO was trying to cover up this tranny noise). Anyway I drained and flushed the tranny and refilled with new Mercon atf and a bit of some BG tranny additive.

I've tried some "experimenting" when the noise is present to try and isolate it, but it has not been that consistent. Just pushing in the clutch does NOT change the noise. If you hold the clutch in (in neutral) for a bit but still roll along at speed the noise slows then stops (front input shaft?). Sometimes if you let up on the clutch as that point all remains quiet, sometimes not. I've tried shifting into 4th, and again sometimes that eliminates the noise, sometimes not; however that may be more due to rpms going down than it being that center shaft bearing.

I've done some some reading on this topic, and it seems that the front input shaft bearing and mid shaft bearings are the likely culprits. But I suppose it might be a badly installed throwout or clutch. Pilot bearing seems unlikely as the noise usually starts when in gear clutch out and no motion at the pilot.

I'd like to nail down what is causing the noise. If anyone can ID this from my descriptions, please let me know. If there are some tests I can do to nail it down please let me know about them.

Also, I am hoping that since most of the time this noise isn't there, perhaps some change in lube might improve or fix the noise. A more or less viscous fluid maybe? (I once had the ford tranny mechanic at the local ford dealer tell me to put 5-30 in my f250 tranny to quiet it down) Again, any suggestions here are welcome.

Other than this intermittent noise the tranny is great, smooth shifts up and down, no popping out of gear and no obvious leaks, so I don't think I need to replace/rebuild it, other than for this bearing noise. Do people just replace the input shaft bearing?

Thoughts/comments?

TIA

 

[Shran]

Transmission bearings would be just about the last thing I'd suspect... and even if bad they would likely make more of a "whine" than a "squeal."

I would rule out belt noises first - far more likely culprits could be AC compressor clutch or bearings, idler pulley, power steering pump. U-joint, wheel bearing, etc... lots of possibilities.

 

[Steve777]

Thanks Shran. Bad word choice on my part. Squeal was probably not the right word, whine would be more accurate. It definitely sounds like a bearing noise not a belt noise. Seems to be coming from under the transmission hump, but that could be tranny, clutch or xfer case (my hearing is not good enough to pinpoint its location).

Assuming it is tranny, are there any tests I could do to narrow down which bearing is making the noise?

 

[Shran]

About the only meaningful tests you could do involve the following:

Drive and see if the noise occurs in a specific gear, multiple gears, etc. Worn countershaft bearings would make noise in every gear except 4th, I'd imagine it would almost always make noise with a worn front input bearing.

Remove the shifter and floor plate from the truck, reinstall just the shifter, and drive it around so that you can listen through the hole in the floor.

You could try changing fluid out for 5w-30 engine oil, I found that this helped a lot in one I had years ago, seemed quieter and definitely shifted better. However I have tried it in a couple others and it made no difference.

I don't recall that the front input shaft bearing can be replaced easily - seems to me that a full teardown is necessary to get it out, but it has been a couple years now and I may be wrong.

 

[Lyle]

Does your engine have a cam sychronizer? On the 3.0 engine these have been known to chirp and squeal when they are starting to fail. They are located near the rear of,the block too so that may sound like a transmission or release bearing noise. Just sayin'.

 

[Shran]

His profile says Mazda B4000 which would have a 4.0, but 1994 4.0's also had a cam synchronizer, so that is possible.