M923 Transmission Assistance Please!

[zout]

Try putting the old headlights and burned out bulbs - see if the trans goes back to working right.
If it does you may need a Catholic Priest to do an exorcism on the truck..

Interested in what you find out on the trans when your able to get it into a shop - I know nothing about automatics except just the fluid end of them.

 

[swbradley1]

If you understand that these trucks are expensive then why wouldn't you pay to get it done correctly? I carry several credit cards when I'm driving my trucks just to make sure I can get them towed home or someplace safe. It runs around $15-20/mile in this area. I've done it three times.

 

[M35A2-AZ]

I would not tow the M923 with a tow strap, you would have not brakes and with out are you would to cage the brakes and that is not a
good thing.
There has to be a SS guy in Socal with a 5 ton that could give you a tow.

 

[Tow4]

If the engine runs, why can't he let the engine idle to pressurize the air system so he has brakes and power steering? Obviously it's better to tow it with a big enough truck and tow bar or a wrecker. If he doesn't have far to go and no appropriate tow vehicle available; you do what you have to do.

 

[jw4x4]

OP has stated that he is new to the 5 ton world. With little or no experience in a 5 ton (full air brakes no less), even with the engine running to provide air for the brakes, it would be a white-knuckle ride for whomever is in the tow vehicle. Might be do-able with a very experienced driver behind the wheel. Towing is a far better (safer) option.

 

[o1951]

Doomsday, if you work on engines and man trans, do what you said in post 16. Don't let guys scare you. I've rebuilt auto Trans, not that big a deal. There is a possibility that the selector is not moving the valve spool to the right position, and oil is not pressurizing. In any event, you are not going to make it any worse by pulling the pan and checking things out. You will either fix it, or be in the same position you are in now. If you can't fix it, and can get the proper equipment to safely remove and reinstall it - no jury rigging - too heavy to take a chance- a good used one might be lowest cost option. I second the negative opinion on using a tow strap. Even with engine running (which, depending on what is wrong, could mess up trans more), it is risky.

 

[rickf]

Also pay close attention to what is in the pan when you take it down. Small metal pieces are bad, dark fuzz stuck to the magnet is normal. Not real familiar with the Allison but most autos use more oil in reverse than in the forward gears so that would tell me the pick up and pump are good and it is mechanical in nature, hopefully just a dropped linkage.

Rick

 

[Doomsday]

Thanks for the help guys, the tow strap idea was gonna be tried with truck running and just my buddy pulling it to a shop in neutral, but yeah maybe not a good idea, who knows what could go wrong. Gonna hopefully get a chance this weekend to drain fluid and pull pan for inspection. Im sure its just something mechanical, at least I'm hoping but yeah, if nothing works then exorcist is next. Too weird how it just happened out of the blue and was running fine before.
I'll for sure keep updated as I'm on the honey-do-list today unfortunately.....

Joe

 

[rickf]

You know that Blackstone can analyze transmission oil too. Might not hurt to send in a sample to see what it has in it. I am thinking mechanical linkage somewhere, inside the valve body most likely. The pump is working and the mainshaft is connected since you have reverse. A broken band strut or similar would still leave you with some forward gears. A broken converter would leave no reverse.

 

[FP1201]

Here's what I would do: First check the lever on the side for operation; remove the cable and the lever should "click" between ranges. IF you're comfortable with it; set the brakes, chalk the wheels where it CAN MOVE and with the linkage disconnected manually shift it to see what happens. All good? Next some transmissions, (and I may be confusing them with the larger HMMT transmissions) had a 24V solenoid on the side that diverted fluid, but I don't remember what for or why; if it has one, you'll see it, it's about the size of a couple cans of Vienna sausages stacked on top, two wires, line in, line out. That may have failed especially given it's electrically operated and you were playing with the vehicles electrical components. If those two are a wash, there are ports on the side where pressure gauges can be installed to see if the clutch packs are pressurized. Of course having the correct amount of fluid is a given. Draining the fluid and dropping the pan is about the last thing you want to do; that said, the valve body could have gotten some water in it and one of the pistons rusted/stuck, but somehow I just don't think so. About six years ago we were rebuilding those transmissions and the larger 7400 series? for the HMMT's but that's been quite a while back and we don't do them anymore.....maybe more will come to me later, but before getting too carried away I'd check the simple, testable things first.

 

[emr]

Just an fyi from what I have heard and agree with on this site, And what we say in the MTA, Never put anyone at any risk with our vehicles anywhere/anytime, never Put the Hobby at risk either, Anyone who moves an MV on the road or anywhere else becomes one of us. It is important to never hook an MV up on a public road with anything but official gear. Never use a tow strap, if you or anyone does , It may sound silly to those with less experience getting on trouble. You will be harming us all. With that said i am sure you have thought it through and will take the good advice you are getting. Its a great site. Good luck and i hope it is an easy fix, allot of times it is.

 

[FP1201]

Go to the TM section, find TM9-2320-272-24P-1 down load it and look at illustration 198. The Transmission itself is an Allison MT654 with an NSN of 2520-01-117-3010 (Allison PN# 11669085) There's also a modulator valve on the side that's connected at the throttle linkage; it could be "stuck" in the kick-down position and/or broken, I don't know if that in itself would keep the transmission from going into one of the forward gears or not.

 

[o1951]

I am thinking mechanical linkage somewhere, inside the valve body most likely..

I agree - per my post 26 . .[/QUOTE]
A broken band strut or similar would still leave you with some forward gears ..[/ QUOTE]
NO - low is all forward clutches disengaged, all forward bands engaged. A busted band, ripped off lining or dropped pin would give nothing forward because have to have speed to shift up and engage clutch. A bad clutch pack would give you some gears - could rev up lower gear, take foot off gas and skip bad gear. Been there, done that, and a lot more .

 

[FP1201]

I agree - per my post 26 . .

A broken band strut or similar would still leave you with some forward gears ..[/ QUOTE]
NO - low is all forward clutches disengaged, all forward bands engaged. A busted band, ripped off lining or dropped pin would give nothing forward because have to have speed to shift up and engage clutch. A bad clutch pack would give you some gears - could rev up lower gear, take foot off gas and skip bad gear. Been there, done that, and a lot more .[/QUOTE] That transmission does not have "bands" it has clutch disks & plates in a pack that's compressed by a piston when fluid pressure is diverted to it. "Bands" would be found in a smaller transmission like a CUCV or HMMWV.

 

[FP1201]

Again, look at the TM; I prefer the Parts book when looking at exploded diagrams, makes things much easier than "assembly (10) is attached to (3) outside of (12)..." drives me nuts!

 

[Doomsday]

Here's what I would do: First check the lever on the side for operation; remove the cable and the lever should "click" between ranges. IF you're comfortable with it; set the brakes, chalk the wheels where it CAN MOVE and with the linkage disconnected manually shift it to see what happens. All good? Next some transmissions, (and I may be confusing them with the larger HMMT transmissions) had a 24V solenoid on the side that diverted fluid, but I don't remember what for or why; if it has one, you'll see it, it's about the size of a couple cans of Vienna sausages stacked on top, two wires, line in, line out. That may have failed especially given it's electrically operated and you were playing with the vehicles electrical components. If those two are a wash, there are ports on the side where pressure gauges can be installed to see if the clutch packs are pressurized. Of course having the correct amount of fluid is a given. Draining the fluid and dropping the pan is about the last thing you want to do; that said, the valve body could have gotten some water in it and one of the pistons rusted/stuck, but somehow I just don't think so. o we we those transmissions and the larger 7400 series? for the HMMT's but that's been quite a while back and we don't do them anymore.....maybe more will come to me later, but before getting too carried away I'd check the simple, testable things first.

Thanks FP.....I will look for this solenoid you speak of but I was under the impression these transmissions were hydrologically operated. I have already inspected shift linkage and selector shaft and all seem to b operating fine, I have yet to inspect modulator cable as another just mentioned. Work sucks, just wanna b home fixin truck! Lol

 

[rickf]

I think you meant "Hydraulically" operated. Hydrologics is a little outside the realm of an Allison transmission even as big as it is.

 

[FP1201]

The transmission is a combination of hydraulic pump and mechanical gear box. The Torque converter has two functions; it transmits power to a pump that supplies pressurized oil while rotating gears inside. When engaged, the fluid is diverted to the proper piston which applies pressure to a clutch pack that in turn starts the planetary rotating. I suppose taking time to go over the diagnostics in the 20 manual or even the specific one for the transmission (either the Military TM or Allisons troubleshooting guide) would save a lot of un-necessary work and expense.

 

[Doomsday]

Ok, so what is that black solenoid looking thing next to the selector shaft lever? I can't find that in any of the TMs or parts TMs....? Something tells me that is my culprit.....

Joe

 

[Doomsday]

Ok, found it in the parts technical manual with a little more searching. It appears to be the neutral safety switch but not sure if that would prevent it from going into the forward gears. There are 2 wires, maybe one for forward and one for reverse? I don't know just guessing......if its not that guess I could inspect the governor or the modulator. I never did inspect the governor filter on the tailhousing of the trans. Anyone ever inspect theirs? Is that a filter that needs to be attended to at every service??

One first order of business is get that area cleaned up a little bit, as you can see I only cleaned up the pan when I changed the fluid and filter upon purchase of the truck. lol