Help! Metallic "Crunching" Noise When Releasing Clutch In 3rd-5th?

Its been a long time since I posted. Mostly because I took everyone's sage advice and use the TMs for repair/maintenance. However, I have something that doesn't appear to be in the TMs.

I took my 1970 M35A2 for a drive today and noticed an unusual sound when releasing the clutch in 3rd, 4th, and 5th. When I release the clutch when in 3rd-5th there is a metallic "crunching" sound. Soldier B said it sounds like someone rattling a toolbox full of wrenches/metal. I reviewed the TMs and didn't find anything specific.

It doesn't happen in reverse, 1st or 2nd.

It doesn't happen when I move the shifter into gear. Only when I release the clutch and add throttle.

It only happens when I am spooling up. If I am downshifting/engine braking it doesn't make the noise.

I have all gears and there are no unusual noises coming from the transmission when in operation.

Any ideas on where to start?

I'd pull the inspection plate off the bottom of the bellhousing first and inspect the throwout bearing, fork, and clutch as well as you can. If you don't see anything obvious then pull the transmission tunnel and pop the top cover off the transmission and have a look inside.

Maybe take a good half hour under the hood and under the truck carefully inspecting all your linkages and lines too. A loose or missing line clamp can allow lines to rattle against other things at different harmonics. Same with worn linkages or linkage ends that were missing their original pin and the pins were replaced with a nut and bolt.

Stranger said:

Any ideas on where to start?

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Does it do the same when the TC is NEUTRAL ? And in LOW ?

What Jeep Sinker said is spot on, and to add to that I would drain the transmission into a pan and get out my magnet first .

Takes very little effort, and if there's no problem you can just screen the oil and put it back in .

You need the drain the oil first anyway if you're going to look down inside, otherwise the bottom gears are submerged and you can't see them

Check the jackshaft, that all bolts are tight.

Based on what I learned yesterday from studying the transmission TM diagrams (thanks again @gringeltaube ) and the problem you're describing, I'm wondering if the problem might be here:



That shift sleeve and synchro need to be engaged 3rd gear onward. Perhaps it's broken and slipping?

HDN said:

Based on what I learned yesterday from studying the transmission TM diagrams (thanks again @gringeltaube ) and the problem you're describing, I'm wondering if the problem might be here:

View attachment 930920

That shift sleeve and synchro need to be engaged 3rd gear onward. Perhaps it's broken and slipping?

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Usually when that happens it just rings all the teeth off and you completely lose gears. That's been my experience anyway. Still, worth a look. May be worn and getting ready to come apart.

Jeepsinker said:

Usually when that happens it just rings all the teeth off and you completely lose gears. That's been my experience anyway. Still, worth a look. May be worn and getting ready to come apart.

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That makes sense. If that sleeve isn't there or working at all the rear shaft won't spin 3rd gear and up.

HDN said:

If that sleeve isn't there or working at all the rear shaft won't spin 3rd gear and up.

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Not sure if I'm understanding what you mean by "3rd gear and up".......... ??

Just in case....... What you call "sleeve" (encircled in your diagram, post #6) is the shift collar of the 2nd-3rd synchro unit.
Shifted to the front would be 2nd gear position (only). All the way to the rear, 3rd gear is engaged. Only 3rd (!) And when in the middle, that is NEUTRAL, by default.

It should be clear that before you could shift into any of the other gears (R, 1st, 4th or 5th), that collar needs to be in the N-position.

gringeltaube said:

Not sure if I'm understanding what you mean by "3rd gear and up".......... ??

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I meant 3rd through 5th gear by that.

I think I understand what you're saying now. I originally thought that shift collar stayed engaged after 3rd gear and never considered it beingin a neutral position.

I wonder then if the noise starts in 3rd gear if that shift collar is related to that? Maybe something that's involved in the shift to 3rd gets hung up the rest of the way up the gears and causes that noise

I'm looking forward to seeing what's actually happening!

I changed out my deuce’s transfer case because I was sure it was the source of significant metal to metal noise. This was after months of diagnostics involving removing drive shafts for test drives. When I removed it, I found the rubber bushings supporting the t-case were toast. Nothing wrong with the t-case at all.
My point is, it could be something not inside your tranny causing your noise.

You could pull the doghouse/floor out of it and test drive it. You might be able to pinpoint things a bit better. Is it speed related, like only happens above a certain speed? If not, low range and short shifting could make your noise easier to find due to lower speed, less road and truck noise.

As old as these trucks are, pretty much all of them need new engine and transfer case mount bushings anyway. Probably not the noise in this case, but not a bad idea to replace them. They're cheap.

I have run into a seemingly similar sounding issue and it was the transfer case trying to fall out of high gear under load. I'm guessing the bushings are shot, I also have found there is zero detent retention on the transfer case shifter. This only occurs in higher gears and hence more torque load on the transfer case.

When it tries to fall out of gear but is barely engaged it sounds like a metal tool box full of wrenches in a paint shaker. Unloading the driveline with the clutch would result in the gears meshing again generally and everything sounding normal.

734 said:

I have run into a seemingly similar sounding issue and it was the transfer case trying to fall out of high gear under load. I'm guessing the bushings are shot, I also have found there is zero detent retention on the transfer case shifter. This only occurs in higher gears and hence more torque load on the transfer case.

When it tries to fall out of gear but is barely engaged it sounds like a metal tool box full of wrenches in a paint shaker. Unloading the driveline with the clutch would result in the gears meshing again generally and everything sounding normal.

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There is a detent in the t case that holds the shifter in the position selected. If the spring broke it will flop around and try to come out of the selected range. Since high range is lever up, the lever will try and move down. Gravity.

WillWagner said:

There is a detent in the t case that holds the shifter in the position selected. If the spring broke it will flop around and try to come out of the selected range.

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This is what I concluded as well. Aside from finding mentions of there being a detent around here, I haven't gotten into the TM's to see where it is and how to get it sorted. Based on my transfer case trying to engage the front drive under moderate braking I'm almost afraid to dig into it. I assume there's a spring of some kind holding the air shift gear in the disengaged position, or there should be, and my spring isn't functional currently.

Getting at the detent isn't bad. It's probably just gummed up. This video is kinda long but shows how to do it.