# only reverse in transmission



## duncandiesel

Hi Alision man. I have a IHC truck s1900 with a MT653 5 speed, with only reverse in all gears. It has no noise and pulls great in reverse only, no neutral. I dropped the oil pan, and it is very clean,and no metallic or clutch parts found in the oil pan. Filter is normal,shift linkage works correct, the oil does not smell burnt. What should I be checking out next?


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## C Nash

Re: only reverse in transmission

Welcome to the forum Terry.  I think Danny is in China and dont remember when he said he would be back. keep checking your post.


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## USMC

RE: only reverse in transmission

I'm not a transmission repair person but when I was younger and had 3 and 4 speed transmissions I had a couple go out on me and when they went out I always had reverse and some times I had reverse and high, and the fluid was always still good, in my case it was from racing that caused the transmission to go out.

If it were your clutch that went out you would have a problem shifting into any gear, so I would guess that you will have to rebuild or replace the Transmission.


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## dbarton291

Re: only reverse in transmission

I'm in China, but have a good internet connection where I am now.

If fourth clutch was burnt, you'd have reverse in reverse, reverse in neutral and it would want to lock up in drive. Is that what it does?


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## duncandiesel

RE: only reverse in transmission

....again, this is a allison MT653 5 speed automatic in a IHC S1900 gravel truck 1987.   The truck was working perfect, no slipping , no noises, nothing was wrong.  Driving in town, stopped at the stop sign in forward gear,  and when pressed on the accelarator to take off all by itself the tranny went into Reverse and that is the only gear it has now.   It doesn't matter which gear you select with the shifter, you only have reverse gear. It does not slip, pulls hard in reverse , but no neutral or any other gears now.  Linkage is good,  pan is whislte clean inside, oil has no filings or any debris in it.  Oil smells normal.  Oil filter is very clean.   So basically the tranny went into reverse with no indication of any problems and it will not shift out of reverse.  Even with engine idling for an hour with max brake on , there are no noises from tranny or anything out of the ordinary...yet tranny is in reverse and will pull strong with no slippage.   Now before I pull this tranny out of the truck is there something that I can check to see if it can be done right in frame, such a valve body problem or ?????  ,    any ideas appreciated on this odd problem that is not in the books  !


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## Triple E

Re: only reverse in transmission

We have a great Allison man on this site.  He may be traveling, but he will help you soon.  Sorry about your problem.  I bet you said Oh Sh--!!!!! a few times.


 :8ball:


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## dbarton291

Re: only reverse in transmission

Most likely candidate is a valve body problem.  First thing I would do before dropping the valve body is look at the selector shaft and cock's comb.  Have an assistant move the shifter in the cab while making sure the selector valve and cocks comb are moving in the transmission.

If that's okay, I would drop the valve body and look for broken springs, stuck valves, modulator body still tight (that's the little valve body that is bolted on the underside of the main control valve body with three bolts and holds the separator plate in palce.

Do you have a parts catalog?  If you do, check to see if this transmission is supposed to have a reverse priority check ball.  I'll bet it does.  It's a little check ball that sits in a round seat in the worm tracks of the valve body and is held in place by the separator plate.  If that little ball wears out and gets sucked up into the main case, you could also have this condition. 

My opinion, and it's worth what you're paying for it, is that the cause of this is the reverse priority check ball worn out, sucked (or pushed) through the separator plate and now lost up the reverse priority passage in the main case.


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## USMC

RE: only reverse in transmission

I think the ball might be the problem like he said and any little speck of dirt in a transmission can cause havoc, also have you checked the bell crank housing and cables?trouble with bell crank will mimic transmission problems at times but I don't think it would cause your situation but it wouldn't hurt to check it to make sure it isn't a combination of problems.


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## duncandiesel

Re: only reverse in transmission

....thanks dbarton291 and USMC for the reply.  I'll tell you what we did today with this mystery problem.  We dropped the valve body and totally inspected it for problems.  This  653 must be a later model than what you decribed as it does not have a reverse priority check ball , and the book we have states that newer models do not use the check ball.......so that theory is out.   We could not find any broken springs or sticking valves,  the body is clean and just looks perfect.  The only thing that I felt may be odd is that the one and only check ball that it does have is a fiber ball, not steel.  It was not stuck in the veins but to me the ball is not round to the touch and under a magnifying glass you can see the pores etc.  Does it seat properly, I don't know.  The shop manual doesn't even tell what this ball does.  I want to swap it for a steel ball ,  as I took a turbo 350 apart a couple of years ago with a shift problem and it had fiber balls in it with one stuck, so I swapped with steel balls which 99% of tranny's have and that fixed it.  But again, I am abit lost here with this allison .  So the valve body looks very good as far as we can tell.  Like this thing only has 142K miles (230K Km ), it can't be worn out.   I took out the governor and it looks perfect as well as its  drive gear.  The oil strainer next to the governor is clean also.  So overall the valve body looks ok,  everything is clean and no filings or debris anywhere in the tranny.   Linkage is OK.   Now, the other suggestion we had is from someone who has worked on many allison's over the years and he said the 4th gear clutch splines are stripped and seized causing it to be engaged all the time thus giving reverse gear only.  Hmmm,   well that don't make sense to me.  If splines stripped then where is all the filings etc that are gone now ? , the oil is clean, a magnet catches nothing, no burnt smell. no oil discoloration,  filter was cut open and is clean,  screens are clean.   So if the clutch is seized and the shaft turns all the time , then  no oil pressure is needed to give this reverse, right ??  if tranny is turning then it should go backwards with no oil pressure  ???? ,   so we figured we'll start the motor and see if the tranny will want to turn with no valve body / oil.   Nothing happened,  engine running and no indication of it wanting to pull in reverse.  So how can the clutch be siezed ?   Does this sound logical to you ?  The troubleshooting in the book doesn't discuss anything for a problem like we have here.   Anyways we can't see what could be wrong.  My guess is the fiber ball is maybe a possibility,  but gee wiz,  to throw the tranny in reverse all of a sudden and stay locked in reverse is weird.  That is as far as we went today.  Need a pan gasket to put back togeether if we are going to just try it again with just changing the ball to a steel one.  But hoping someone can maybe give us another thing to try or check out here.  I still am hoping that we don't have to pull the tranny out to get into the clutches that I just can't see why they should be at fault at a snap of a finger when the it went into reverse by itself and won't shift out.   Any ideas appreciated, THANKS.


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## dbarton291

Re: only reverse in transmission

1987 is NOT a late model MT653.  In 1987, the MT653 still had the reverse priority checkball, which was the one and only check ball that transmission ever had except for VERY old ones that may have had a governor check ball.  That little fiber ball you see IS the reverse priority check ball.  If it's not round, it's not seating.

It sits in a half round machined area in the valve body, between the valve body worm tracks and the separator plate.  Was the ball you found in such a place?

A steel ball will eventually bang its way through the separator plate on that transmission.  I've done it.  The latest reverse ball used in those was made of DuPont SP-21 and is brown.  The service part now may be a different color.


If fourth clutch was stuck on due to being stripped and welded, you'd have reverse in neutral, reverse in reverse and in drive it would try to lock up, because then you'd have forward, fourth and first clutches all on in drive.  In an Allison, one clutch on is neutral, it takes two clutches for movement, and three clutches on locks the gear pack. 

I may be wrong, but I still think the next logical step is to replace that reverse check ball with the current Allison service part(make sure it's in the right spot in the valve body) and check the separator plate seat it seals against for sharp edges or burrs.  If it's lasted all these years, the seat is probably good, but should be checked.


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## duncandiesel

RE: only reverse in transmission

looking at the diagram in the service book,  the check ball  is not in the same place as the one I got here.  Yes it sits in a hole like you described and it is a brown ball.   But this one sits more closer to the middle of the body, not in the corner like shown in the pic.  The plate seems very smooth as does the body surface.  You can see the gallery black lines all over the plate some darker some very light so I don't think there is a problem there.  I'm going to try take a picture of it and post here just to show where the ball is to verify.  This is the only place that a ball can sit so it is in correct location in the body.  Yes I thought it strange that if a clutch was seized and selecting different gears that there has to be some conflict in there where it wants to go,  but like I said, strong reverse only in any gear selected and no nuetral either .   Will get back here asap,  we got other stuff on the go and just doing this on the side.  Thanks 4 now.


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## dbarton291

Re: only reverse in transmission

A check ball off in one corner is the old governor check ball.  Those haven't been used in an MT since the early to mid 1970s.  Must be a very old picture in whatever kind of service book you are using.

Those black lines in the separator plate are normally just very fine clutch material that gets trapped between the valve body and the separator plate.  That stuff gets embedded in the metal and may not come off in the parts washer.  I've seen guys use scotch brite pads on them to clean up the separator plate.  I don't think it's really necessary, though.


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## duncandiesel

Re: only reverse in transmission

shoot, I don't see attachment option on this board , so I can't post a picture of the valve body.  I will have to order a new check ball.  Am surprised at what you claim that the steel ball will bang its way through in time. How many miles did it take for that to happen ?   This book could be outdated then,  I just took it for granted that the early ones had the ball in the corner and later ones had no ball like the book says,  and this body has a ball far from the corner.  Anyways will have to wait till we gets the parts and then try it.  I am convinced also that the ball is the problem cause I just can't see anything else wrong here.  Very hard to mic the ball for any difference in shape,  but to me just rolling in the fingers it feels out of round.  Yet how can just a very slight leak past the ball create such a crazy problem ?  Guess we'll  see soon.  Thanks again for your professional input ....


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## dbarton291

Re: only reverse in transmission

You were asking for what you can do without removing the transmission from the truck.  

Well the ball is out of round.  So we know that will leak fluid into the reverse feed passage to fourth clutch.  That has to be fixed.  If the linkage, selector valve, valve body etc. show no other out of spec conditions, that ball should be fixed to see if it fixes it.  I know the ball's a problem, I don't know it's the only problem.  

The more I think about it, what I don't understand 100%, is if the linkage, selector valve, valves, springs, cock's comb etc. is all okay based on your inspection, why doesn't the transmission try to lock up when you go to drive.  I gotta think about that some more.  

What I'm used to seeing with a failed reverse priority checkball is a burnt fourth clutch.  Maybe forward clutch in this trans is slipping just a bit and as a result, fourth has more capacity resulting in a strong feeling reverse when the selector is in drive, but that's pure speculation on my part.

The steel balls we tried in MTs didn't last very long. It was a long time ago and I don't remember the mileage.  Remember 10,000 miles in a dock spotter or some other commercial vehicle with an Allison, is not the same as 10,000 miles in a car.  An Allison in a truck is spec'd to work, not loaf, like a transmission in a car that has a comparatively high power to weight ratio.


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## dbarton291

Re: only reverse in transmission

After you get that ball fixed, I suggest putting a gauge in the reverse pressure tap.  Fire it up and see what the gauge reading is in neutral, drive and reverse.  

If the ball fixes it, that's a good checkup on the trans.  If the ball doesn't fix it, that will probably yield some valuable information.


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## dbarton291

Re: only reverse in transmission

Any update?  I'd like to know what fixed it, or if you're still looking.


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