Help with M817 transmission diagnosis?

[Rebar]

Okay. I am feeling pretty cocky after you guys helped me get the old M817 restarted. The only other problem on this truck is transmission-related. Have driven it enough to know the following: Shifting out of first is sometimes nearly impossible (have to rapidly double-clutch, which works) but other times is easy. Shifting into 2nd is likewise sometimes nearly impossible (double-clutch again works), but other times is easy. Shifting into 3rd always goes easy, but always gives a tiny little gear grind. Shifting out of 3rd, into and out of 4th, and into and out of 5th is flawless. Being in low range definitely makes everything work a bit better(?). I obviously have some work-arounds like just starting in 2nd gear (since the big old Cummins thinks that is just fine), but obviously limits my ability to do any heavy pulling.

Also, of note, the PTO shaft to the front winch was broken and has been removed (drive shaft being re-tubed). Some have suggested that this might cause a problem since the pto tends to spin a little (you can stop it with your hand) despite the fact that the winch shift lever is in the off position. And, the dump lever is locked in its upright position (already checked that).

So, is there something easy that can be done, or does the transmission need a big work-over? The truck is functional now as it has been retired as a "fun truck" to the farm. It is not seeing any commercial use. But, it would be nice to smooth things out a bit.

As always, any help and suggestions are very much appreciated.

Rebar

 

[EMD567]

You don't say what the temp of the tranny is when you are having problems shifting. My M814 is a bear to shift 1-2-3 when cold(under 100º). As long as the tranny isn't making strange noises, or popping out of gear, it should be fine.

 

[MyothersanM1]

Mine is cantankerous coming out of first to neutral. It does get better once warm. I am in the habit of double clutching all shifts as my fourth likes to mash its teeth a little up and down.

As a side note...I no longer use the LO called for 80W-90 gear oil. I am using Spicer recommended SAE 50W. It definitely likes the 50W better. Lots of threads about this discussion.

 

[doghead]

You must always start in 1st. If you ever change from reverse to 2nd, your going to damage your front drive sprag.

1st is not syncronized.

You need to confirm your pto is not engaged. (with it not running, can you freely spin the output shafts?) Both front and rear outputs?

 

[MyothersanM1]

You must always start in 1st. If you ever change from reverse to 2nd, your going to damage your front drive sprag.

Once the shift lever is moved from the Reverse position into the Neutral position or any other gear position the sprag will engage for forward movement. The sprag will not change position until Reverse is selected. Then, will the sprag engage for rearward movement. DO NOT allow the vehicle to roll forward in reverse gear or back in any forward (or neutral) because things in your transfer case will not be happy with you.

As a side note...removing all air supply to the declutching (or sprag) cylinder will cause it to default to a neutral position. The sprag will not engage in either direction and the aforementioned warning does not apply. However, you will not have power to your front axle when needed.

 

[topo]

I would check the PTO it could need a spacer . If it's to tight it can make shifting a problem ?

 

[73m819]

Your 2nd/3rd synco is worn

 

[Neophyte]

Double clutch is the way to go. I don't believe these old transmissions had synchroniztion. No down shifting possible (god knows I've tried).

 

[Rebar]

Thanks for the great replies. Being basically truck-stupid, I need a little more help. Just like any truck-stupid person, I am looking for easy things that might work. For instance, I am all over changing the transmission oil and checking the transmission temp.

The other issue has to do with making sure that the pto's are not contributing to the problem. It intrigues me that when the engine is running, the front winch pto spins--once again, it has no shaft on it to supply any resistance to it spinning (it is off getting retubed). It definitely will spin by hand when off. And, when the engine is idling and the front winch pto is spinning, I can reach down and stop it with my hand. I am assuming that the question of needing a "spacer" was mentioned because of this spinning. Is there any chance that this spinning pto may be causing the shifting problem? Will putting the re-tubed drive shaft back on fix this problem? Will putting a spacer on solve the transmission shifting problem?

Once again, your comments have been great and are very much appreciated.

Rebar

 

[73m819]

Double clutch is the way to go. I don't believe these old transmissions had synchroniztion. No down shifting possible (god knows I've tried).

WRONG, the trans has 2nd/3rd and 4th/5th syncos, the transfer has H/L syncos, even the early 39 series

 

[73m819]

Thanks for the great replies. Being basically truck-stupid, I need a little more help. Just like any truck-stupid person, I am looking for easy things that might work. For instance, I am all over changing the transmission oil and checking the transmission temp.

The other issue has to do with making sure that the pto's are not contributing to the problem. It intrigues me that when the engine is running, the front winch pto spins--once again, it has no shaft on it to supply any resistance to it spinning (it is off getting retubed). It definitely will spin by hand when off. And, when the engine is idling and the front winch pto is spinning, I can reach down and stop it with my hand. I am assuming that the question of needing a "spacer" was mentioned because of this spinning. Is there any chance that this spinning pto may be causing the shifting problem? Will putting the re-tubed drive shaft back on fix this problem? Will putting a spacer on solve the transmission shifting problem?

Once again, your comments have been great and are very much appreciated.

Rebar

This pto spinning is normal for a unloaded pto, will not cause your shifting problem. just the internal movement along with the viscosity of the gear oil cause this. You DONOT need a spacer added. very simple test for pto clearance, With motor off, put trans in gear, put pto in gear, move shaft by hand, If you can feel a bit of play back and forth, your good, to much play, you will here a whine, being that the pto is a old , existing install, there is no reason to think it "CHANGED" clearances.

How did the winch drive shaft get tore up.

One more thing NEVER I REPEAT NEVER GRAB A SPINNING PTO SHAFT WITH YOUR HAND if you value your fingers, use a piece of wood to press againest the shaft, ptos have been known to engage on there own due to viberation.

 

[Rebar]

Thanks for comments. Very helpful. I have no idea how the winch drive shaft got torn in half. Just found it in the back of the M817 dump bed when picked up at GL. I also noticed that somebody had torn up the hinged safety restraint for the winch lever. Maybe somebody moved the lever with the front winch not released and popped the clutch??? Have no idea. Surely there is a broad range of experience in the Army operators of these trucks--kinda like the post-GL operators of these trucks. That is what makes this so fun. It like a TV mystery series--a new mystery every week. Also appreciate the comment regarding pto drive shaft. Having been around farm tractor pto drive shafts all my life, I could not agree with you more. You can lose more than your fingers! I actually did exactly what you suggested--pushed it with a piece of wood.

Thanks. Great comments.

Rebar

 

[Neophyte]

WRONG, the trans has 2nd/3rd and 4th/5th syncos, the transfer has H/L syncos, even the early 39 series

Thank you for the insight. I responded from experience, not understanding the technical breakdown of the transmission. How do you downshift without waiting for the trans to synchronize? I did a lot of driving for gravel in my M817 and was not able to down shift until the trans synchronized at lower speed? What would you suggest I look at if they have synchronization in the higher gears?

Thank you

Bob

 

[73m819]

Still needs a double clutch to down shift, from/to any speed that is NOT the matched speed (shift point), this is the problem a lot of owners have, trying to down shift with either to much road speed or to much rpm, it is more KNOWING your truck and how you drive it. It really helps to be experienced in driving older BIG TRUCKS, remember practice, practice, practice, this = learning how to drive your truck, gives you a reason just to take it out on the road, if your going to put money into the truck, might as well put it into fuel instead of a transmission.

I drove the m819 about 40 miles without a clutch (would not disengage), shifting from 2nd to 5th and back down quite a few times with very little gear grind. The trick in shifting these trucks is KNOWING each gears shift point both up and down, shifting early/late is very hard on the trans. With the proper shift point, the gears will just about fall out or fall into gear

These transmissions ARE NOT fullers, eatons, 15 speeds, close ratios, ect, ect, they ARE old and slow not in a hurry transmissions

 

[Rebar]

73m819 THAT is incredibly helpful. I know this sounds stupid (it is), but it was like a light bulb went on when I read your post. I had no idea that the shift point was that crucial in these old trucks. In my modern day manual transmissions, I can shift wherever I want depending on my temperament at the moment. I have been spoiled by the new trannies and have gotten sloppy--I do whatever I want. For sure, the more that I have been driving this truck, the more my experience has been exactly as you have said. I am learning the shift points and the shifting is definitely getting better. This is so great.

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[Rebar]

Ron (73m819),

Just wanted to give you a little more feedback. Got out to the farm this last weekend and started "learning" my m817's tranny. You were spot on correct. Finding the shift point is the key. Simply can't treat this transmission like I treat a modern day transmission where anything goes. Once I started "respecting" her, all of the gear grinding went away. Just a matter of figuring out where she wanted to be shifted. It is really fun to drive this old truck because it combines this really fun old manual transmission with that big old torquey Cummins that lugs through about anything I do to it. Thanks for your help. You just saved me a bundle of cash and made that truck even more fun than it already was. I love this website because you guys are so willing to share.

Rebar