Allison Transmission issue

[ARYankee]

I was on my way to a car show this weekend in the 923 and it seemed the transmission wasn't shifting correctly on the top end. I started out fine, it was shifting great, but after it got up to temp and I was half way there I was going up a slight incline and it seemed that I started losing power. It downshifted but then it seemed it didn't want to shift back up and gain speed. I was stuck doing 45 mph like it was governed. Now I pulled over a couple of times and started from a dead stop and it would shift up fine but the second I would hit the slightest incline it would act up again. I'm thinking that I may need to change the fluid and filter on the transmission.

Any input about this would be great. I'm really curious about how the transmission operates, like what determines when it shifts. How the governing works?

 

[wsucougarx]

Man, 45 MPH on an incline? I wish mine did that. I find that these allisons don't really like to downshift on their own up hill. Or at least I don't like how low the RPM's get on inclines. I typically downshift to keep my speed up.
The governor is RPM dependant. Most are set around 2100 or so. Question about the transmission and shifting? Check out this video
[media]http://www.youtube.com/watch?v=leCEmJA0WsI[/media]

 

[ARYankee]

This was a slight incline that it never had an issue with before. Once I was on a flat straight away it wouldn't shift up to 5th and stayed at 45 mph. I downshifted to 4th then tried to get it to go to 5th but it seemed it just didn't want to, I could downshift lower but it just didn't want to hit 5th. It really lugs down on any hill. It started acting up even on flat runs.

I'm hoping it is a fluid/filter issue. I would just like to know more about these transmissions.

 

[FireFighterHill]

Does this have a mode button. We had a firetruck with an allison that did something similar and the problem was someone hit the mode button which locked out overdrive.

 

[wreckerman893]

I find that I can maintain speed better if I shift the transmission manually in the M927 when pulling hills.

I'd check all the adjustments and the fluid change might be an option.

Is there an Allison repair shop close by that can give you some insight?

 

[ARYankee]

Does this have a mode button. We had a firetruck with an allison that did something similar and the problem was someone hit the mode button which locked out overdrive.

I don't think I have that setup on mine.

 

[ARYankee]

Is there an Allison repair shop close by that can give you some insight?

I'm not too sure but I plan on checking to find out.

 

[rrrr]

Have you changed over to DEX 3 fluid ? I would do a flush and change out the fluid and two filters.

 

[jcappeljr]

I prefer manually shifting the 900 series trucks on the hills around here.It maintains its speed better.

 

[ranchhopper]

When mine is cold it shifts too soon for my liking so I manually shift it until it warms up and it does not like to downshift going up hills so I manually shift it there too.

 

[ARYankee]

Have you changed over to DEX 3 fluid ? I would do a flush and change out the fluid and two filters.

I checked my fluid and it looks really thin. Not too sure if it has DEX 3 in it. I figure that its 30wt. I'm not too familiar with what DEX 3 looks and smells like. My plan is to do a fluid change and go to DEX 3. I got a couple of numbers to shops that work on Allison transmissions so I'm going to call them for some quotes.

 

[wsucougarx]

DexIII type is red in color. If it's thin yellow stuff then probably its 10wt

 

[rrrr]

There is also a shift linkage that goes up to the IP that can impact the shifting of the tranny. Do a search. I had to do some adjusting on mine. There is also a cable adjustment on the shifter. Use the search on this also. Good luck.

 

[quickfarms]

I would have Allison service it and check and adjust the shift points. I have heard a lot of the 900 series trucks that are not set up correctly.

 

[Beerslayer]

I would have Allison service it and check and adjust the shift points. I have heard a lot of the 900 series trucks that are not set up correctly.

Of the 7 or so 939 series trucks I have driven [not very many, but more than one], all have the shift points set differently. Just like the ladies, they all handle a bit different.

I too find that the downshift on hills is better managed manually, otherwise you are going to get a hard downshift when you least expect it. I usually back off the throttle a little at just below 1500 and shift down, it goes nice and smooth that way.

Changing the transmission fluid will help, and so will having the shift points set.

 

[rrrr]

Do the work yourself first. Its not that hard and can save you a ton of money. I think you'll find that the linkage to the IP is off. I had an early shift into 3rd and made the adjustment and got it about 80% corrected with less than an hours work. The first thing I did was change to DEX 3. The TM's in this case are not too bad.

 

[quickfarms]

Since you say it is acting like the truck is governed have you checked the fuel condition and air filter and, replaced the fuel filters recently? Diesels are temperamental about there fuel and air.

 

[ARYankee]

Since you say it is acting like the truck is governed have you checked the fuel condition and air filter and, replaced the fuel filters recently? Diesels are temperamental about there fuel and air.

I don't think its the fuel because it shifts down to 4th and stays there, no matter if I manually shift, let off on the pedal, shower down on the pedal. I would think if it was the fuel or air filter it would not idle well. I could be wrong, that is why I posted it up here so I could get some info.

 

[ARYankee]

Do the work yourself first. Its not that hard and can save you a ton of money. I think you'll find that the linkage to the IP is off. I had an early shift into 3rd and made the adjustment and got it about 80% corrected with less than an hours work. The first thing I did was change to DEX 3. The TM's in this case are not too bad.

I'd like to know more about how that linkage effects the transmission. I'll have to go find that thread you had mentioned.

 

[quickfarms]

Diesels will idle just fine with a partially restricted fuel or air filter or dirty fuel, to a point. What the filters and fuel can affect is a lack of power. They will hit a wall and not cross it.