No reverse on my M923A2

So we arrived at the Graves Mtn. Rally and after checking in, went for a short spin until the driveway I pulled into and could not back out. Fluids are fine. Levels good, smells/tastes good. Just no reverse. Had several other 5 ton owners take a look, but no go.
Now Sunday arrives and I pack up and head out ( Have to work tomorrow ). After getting out of the country roads, now have no 5th gear. Stopped several times to both manually vs. auto shift but no go. When I made it home it did shift into reverse but as soon as I accelerated it seems to pop out as if in neutral? Tried this several times as I back it into a space but could not get it to move enough to get it in. Looked at some old threads plus the tm.
Ant thoughts? Only obvious thing I was hoping was an o-ring or clogged port? But now with no 5th?
Could the selector shaft come loose inside?

Simp? 74M?

Did you put the transfer case in low range and see if it would run thru all the gears forward? Could be a pressure problem with the kicking out in reverse. in Low range it doesn't take as much pressure to shift.

Check your shift cable attachment at the transmission to ensure it is not loose or come loose from its bracket. Any play could cause it to stick in between gears and not fully lock into a gear. Usually when the allison's go out they do absolutely nothing.

I assume the temps are showing ok?

You would need to check for pressure at the rear test port on the allison in reverse and thru all the gears. This can be done with the transfer case in neutral.

simp5782 said:

Did you put the transfer case in low range and see if it would run thru all the gears forward? Could be a pressure problem with the kicking out in reverse. in Low range it doesn't take as much pressure to shift.

Check your shift cable attachment at the transmission to ensure it is not loose or come loose from its bracket. Any play could cause it to stick in between gears and not fully lock into a gear. Usually when the allison's go out they do absolutely nothing.

I assume the temps are showing ok?

You would need to check for pressure at the rear test port on the allison in reverse and thru all the gears. This can be done with the transfer case in neutral.

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temps are good. Saw a YouTube video of a guy checking pressure but that was for checking no forward gears. Those seem normal accept reverse and now 5th.

I’d also suspect pressure issue from your description, of which there are external pressure test ports to check. On one like this, I would arrange to have an Allison tech look at it for at their place. Several people have had good luck in doing so. They may be able to find a quick fix vs changing out the trans. Offer to pay 2 hours of diagnostic time up front ($100/hr).

Sounds like the clutch pack piston seal is not holding pressure. On my M35A3 the frictions and steels got so hot they literally melted the piston that holds pressure against the steel/friction clutch pack of the 4th and reverse clutch. I suspect the operator kept thinking higher RPM was going to make it better...which it may have for a mile or two. Of course what makes the A3 unique is the locking torque converter and that all of the trannys will experience the problem if the OEM seal was not replaced. New seals have solved the problem. I have not heard of it being a 5 ton issue, but your problem sounds a lot like what I have with my A3 Allison.

74M35A2 said:

I’d also suspect pressure issue from your description, of which there are external pressure test ports to check. On one like this, I would arrange to have an Allison tech look at it for at their place. Several people have had good luck in doing so. They may be able to find a quick fix vs changing out the trans. Offer to pay 2 hours of diagnostic time up front ($100/hr).

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will make that call tomorrow as no one open today.

tobyS said:

Sounds like the clutch pack piston seal is not holding pressure. On my M35A3 the frictions and steels got so hot they literally melted the piston that holds pressure against the steel/friction clutch pack of the 4th and reverse clutch. I suspect the operator kept thinking higher RPM was going to make it better...which it may have for a mile or two. Of course what makes the A3 unique is the locking torque converter and that all of the trannys will experience the problem if the OEM seal was not replaced. New seals have solved the problem. I have not heard of it being a 5 ton issue, but your problem sounds a lot like what I have with my A3 Allison.

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so does that trash the trans?

So I had the pressure check done today at Western Branch. The big Allison people by me. They of course had the truck first thing this morning and left me a message as they were closing. Could not get tech on the line but was told pressure was good. Guess I will find out in the morning what they feel should be next. Guess the only way to know is drop the pan.!

If you need a core transmission to have a rebuild done I've got one you can just have. I can probably drop it off on my trip to the Northeast this month sometime

CARMAN said:

so does that trash the trans?

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Not necessarily, depends on the wear on the remaining parts. Trash is a relative term since about every individual part can be had for a price.

tobyS said:

Not necessarily, depends on the wear on the remaining parts. Trash is a relative term since about every individual part can be had for a price.

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But there has to be a point where replacing the entire trans. Out weighs chasing the demons? With the way the trans is acting it seems more like a blockage over burned up. Just need the shop to be honest with thier investigation and not just tell me it needs a trans. The other Allison shop I called said they did not even know the model I gave them. And I guess Allison no longer services these trans as well at the 740. These Legacy models we sold off to Williams Distrabution in Ohio I was told.

CARMAN said:

But there has to be a point where replacing the entire trans. Out weighs chasing the demons? With the way the trans is acting it seems more like a blockage over burned up. Just need the shop to be honest with thier investigation and not just tell me it needs a trans. The other Allison shop I called said they did not even know the model I gave them. And I guess Allison no longer services these trans as well at the 740. These Legacy models we sold off to Williams Distrabution in Ohio I was told.

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Replacing a transmission can be done on your own with a Large transmission jack from harbor freight. Luckily its an A2 so the bellhousing bolts are easy to get to. Only real pain in the butt is the upper transmission mount. Everything else is pretty easy. About 90 mins and it should be out and on the ground. Going back in is the real fun part. in fact it just sucks. I would rather pull the cab off and put the transmission in than fight with one on a jack. but that is just me.

You can do it in a day with general hand tools and a decent impact gun even if you are a beginner.

Well.....Simp is just a bit faster than I am.

It is as easy a 4x4 1/2 ton truck if you have the correct jack that lifts to 32.5" -35" in height. You can let the air out of the tires if the jack won't go high enough....BUT get the trans on the jack and underneath the truck first. I had a tough time of getting the trans under the truck with it on the jack. Use a small wratchet strap to secure the trans to the jack.

BigRig379 was kind enough to loan me a jack.

(2)Rear upper trans mount bolts are 15/16".

Be sure to flush your intercooler of old fluid before connecting to new transmission.

It is easier to remove/install the torque converter bolts thru the starter hole with it removed.

Cut the head off of (4) longer bolts and use them in the bellhousing when stabbing the new transmission in. Take a zipsaw and cut a groove where the head was on the bolt so that you can remove the bolts with a flat blade screwdriver when trans is back in.

Also hang the spacer ring on your (4) guide bolts and zip tie it thru one additional hole, so that the ring stays put while stabbing the new transmission.

The torque converter is very very heavy. Do not fool around and let it fall out half way up when installing. Keep jack tilted back until you are almost all the way up.

Remove the dipstick first and put it back on last so that it doesn't get bent during remove/install. Use Permatex thread sealant pn 59235 on dipstick to pan threads.

Be very careful when tightening down bellhousing bolts, go a little at a time, you are screwing a steel bolt into aluminum threads and it is possible to strip the threads by torqueing one bolt all at once when mating trans to bellhousing.

Oh and that other wrong move. Install all your torque converter bolts to snug tight before hitting them with an impact. If you tighten the first one you put in you will have a hard time getting the others to start. A2 trucks are alot easier than NHC250 trucks. If your torque converter does happen to come out you will have to stand the transmission on its tail shaft and then spin the torque converter to lock it down. It has a 3step drop on it

I have no A2 experience. Wish I had an A2.

Think I jacked the front axle up on passenger side 2-3" in order to get the trans/jack combo to roll under the toolboxes. Didn't expect that. Mine is an A1 nhc250 with 14.00-20 supersingles.

Yes definitely get all the torque converter bolts started by hand before tighten any one of them completely.

Have you checked your governor filter on the very back of the transmission?

lindsey97 said:

Have you checked your governor filter on the very back of the transmission?

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have not. Just had a pressure check done and was fine. Problem is intermittent no reverse and no current 5th gear on my drive home.

Is that on the inside or outside?

Ok. Spoke with Allison shop. Answer...pressure test came back at 300. Said the valves are shifting? (did not drop pan). According to trouble shooting guide it means bad clutches? So there answer is bad trans. Fluid seems fin to me. Not convinced.

Take your external filter off (the one under hood passenger forward of trans). Cut it open and inspect pleats in filter for residue/debris.

You can also send a fluid sample off to be tested for content and condition of fluid.

It's most likely the clutches if Allison has tested it.

The governor filter is external, located on the very rear of trans, below output shaft, behind a large 7/8" or 15/16" nut. It is thimble sized filter, cleanable and reuseable.

I had a transmission fail in july 2016. When I bought the truck in july2014 it made sounds like a screech owl when upshifting. Pulled solid with no slipping. Oil was clean and clear, smelled fine. Downshifts were quick, firm, and decisive. So I proceeded to drive the truck 7000 miles before the trans failed in the forward gears one night. Never lost 1st or reverse. I replaced the entire trans with a new rebuilt one out of the can.

Be sure and use the correct oil when going back with a new trans, do not use motor oil. In cold enviroments, the motor oil is too thick and will cause failure of the transmission unless you use a heat pad on the trans pan, and let the transmission warm up 30 minutes before driving per Allison instructions.

lindsey97 said:

Take your external filter off (the one under hood passenger forward of trans). Cut it open and inspect pleats in filter for residue/debris.

You can also send a fluid sample off to be tested for content and condition of fluid.

It's most likely the clutches if Allison has tested it.

The governor filter is external, located on the very rear of trans, below output shaft, behind a large 7/8" or 15/16" nut. It is thimble sized filter, cleanable and reuseable.

I had a transmission fail in july 2016. When I bought the truck in july2014 it made sounds like a screech owl when upshifting. Pulled solid with no slipping. Oil was clean and clear, smelled fine. Downshifts were quick, firm, and decisive. So I proceeded to drive the truck 7000 miles before the trans failed in the forward gears one night. Never lost 1st or reverse. I replaced the entire trans with a new rebuilt one out of the can.

Be sure and use the correct oil when going back with a new trans, do not use motor oil. In cold enviroments, the motor oil is too thick and will cause failure of the transmission unless you use a heat pad on the trans pan, and let the transmission warm up 30 minutes before driving per Allison instructions.

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what did you put back in?