Disgusted with Cat 7155

[NDT]

Having never operated one of these transmissions I had high hopes they would be something like a Powershift in a Cat construction machine. NOT! After driving the Allison in the 915A1, I do not like this Cat no I dont. Lots of lurching, non-smooth shifts, chirping tires on downshifts, and way to much driver focus on the shifter and not the road. And all this hissing and clunking going on under the floorboard. What am I doing wrong? I am shifting at moderate throttle, skipping 2 gears going up and down. And don't be slow about getting in neutral at stops!

 

[velociT]

Haven't driven one, but good to know for when I do...

 

[jatonka]

Yeah, I'm pretty glad I bought the 915A1 with the Allison, It is unbelievable how much power it puts to the ground. Got the SF 97, now it can pull my lowboy, good night Freightliner. JT

 

[bulldog_mack13]

I drove one a few weeks ago , there really is a bit of driver focus on the tranny and the shifts really arent that smooth but I didnt have any chirping tires or grinding noises. I figured par for the course on a old Army very heavy equipment hauler. I really see where your coming from, Jay

 

[Burgerboy13]

Swapping in a 13 speed or an Alison wouldn't be too difficult, would make it a lot more appealing for resale as well.

 

[westfolk]

I still have a few that have the 7155. One by one though I am pulling it out and putting an E/F 18-speed back in. Minor work to do the swap. Being able to hit 80MPH instead of maxing out the RPMs at 55 is an added bonus.

 

[Andydieselpower]

The two 7155's we have (916 and 920) shift good in my opinion. They take getting use to thats for sure. Make sure you let off on the throttle in between shifts and let the transmission go through its brief shift sequence and then resume with the throttle. You should have clean shifts- make sure your air pressure if over 90psi.

 

[m16ty]

I haven't driven a 915A1 but I have driven a civilain truck with a auto that acted like that. To be honest, I'd just as soon have a stick if I constantly have to be fooling with the throttle and the shifter to make it shift right.

I guess it could be that I'm just not used to driving a auto in a big truck. Just give me a stick so I just shift it, hit the gas pedal, and go.

 

[DieselBob]

Give me a 13 spd any day in a road truck. Although I do have a soft spot for the old Mack quartoplex twin stick. Learned to drive on that.

 

[bulldog_mack13]

Yeah , talk about truck driving, drive an antique Mack. Shift up right , split gears shift down right at the correct RPM , dont get both sticks in neutral at the same time , never lug your engine etc,etc,etc. Nothing like driving a MV.

Now the 7155, Just like driving a manual tranny you still have to hit a certain RPM to shift correctly so theres no more paying attention to that than driving a OTR truck in my opinion. It is semi auto and not a Auto tranny to clear that up. Andy's dead on with what he said. Anyone should take that advice and really read up on them.
Just like A2's and A3's we all have out likes and diss likes. -Jay

 

[cranetruck]

....... Make sure you let off on the throttle in between shifts and let the transmission go through its brief shift sequence and then resume with the throttle. You should have clean shifts- make sure your air pressure if over 90psi.

Just curious, my '69 8x8 Allison is linked to the injection pump governor, which has a "torque link" to alleviate shift shock (quote below from 1974 TM 9-2910-226-34), do the newer 5-tonners have a similar set-up or was this another piece of technology lost in the annals of history?

"(1) The governor on code A injection pump (fig 1-1 7) is the same except a torque control link ((D), fig. 1-1 has been added.
(2) The control link retards governor action during automatic transmission shift and does not allow engine speed or torque increase during the change of gear ratios within the transmission. This action alleviates shift shock to the transmission."

 

[LanceRobson]

I am shifting at moderate throttle, skipping 2 gears going up and down.

It's been a whhile but I think I remember that yuo can skip gears only when lightly loaded. I also seem to remember that you can skip one gear at a time up to 6th or 7th and then are supposed to hit the rest. These were designed for use on secondary and unimproved roads and I don't think quick accelleration was something the army cared about. I'd be willing to bet it was just the opposite.

Lance

 

[wreckerman893]

The first couple of gears (1st thru 4th if I remember correctly) are the "LOW SIDE" you can skip a gear between if you are loaded....once you get the hang of it you can "slap shift" it by bumping the stick with your hand sort of cupped from slot to slot.

Accelerate......ease off throttle....slap.....accelerate...ease off throttle....slap.....repeat as necessary.

I love that tranny as long as I don't have to pay if it tears up....most Cat dealers have no one that can work on them.

 

[eldgenb]

I still have a few that have the 7155. One by one though I am pulling it out and putting an E/F 18-speed back in. Minor work to do the swap. Being able to hit 80MPH instead of maxing out the RPMs at 55 is an added bonus.

so you must be talking about swapping that into a 915 right? I can't believe they make a tranny with enough overdrive to overcome the deep gearing in the 916's and 920's considering the 7155 is an od tranny already.

 

[Old Lincoln County Iron]

Some of the 916's that came up on GL in California had Allison auto's in them. I am
not for sure but I believe these were Marine Corps Trucks. Again not for sure but I
heard that they only run about 45mph? Those trucks brought a higher price than most
of the 916s I've seen go with the Cat 7155.

 

[eldgenb]

Some of the 916's that came up on GL in California had Allison auto's in them. I am
not for sure but I believe these were Marine Corps Trucks. Again not for sure but I
heard that they only run about 45mph? Those trucks brought a higher price than most
of the 916s I've seen go with the Cat 7155.

you are correct, those were marine corp trucks. The military did not order them that way. Even at that the allisons are direct drive 1 to 1 ratio in final. The cat is an overdrive, the gearing in the 916-920 is like 6.71 gear ratio I think the 915's were in the 4.30 range if memory serves me. With that being said a 915 converted to an overdrive tranny could reach 80mph, I do not belive you could get a 916-920 up to that speed without changing out the axle gears to 4.88 (tallest you can go without replacing carrier) as Soni did to reach those speeds, even with 16's on the 920. Just my .02

 

[Floridianson]

Im not going to rake you over the coal's but yes this is a trans. you must drive.
Drive it more and you will get the feel for what gears you can wind out to 2100 and one you can shift at 1850. I only use 7 gears on mine from stop to start empty.
They like to be really warmed up to shift a little better. As soon as you have the air pressure to hold in the shifter lockout in push it in and let warm up for ten min. and it will raise the trans temp. 100*
If you can when you are makeing hard turns roll as long as you can all the way through the turn and no fuel. Then down shift one at a time and wait a second then down shift again one more. I can roll through most turns a a good speed and only have to drop to 13th and then hit the fuel and rock and roll. Don't be in such a hurry aint no body going to push you around anyway. The one with the biggest tires and weight is boss.

Where is you Gov. set. 2100 or 2400. I shift at 22/23

I love mine.

 

[Floridianson]

And don't be slow about getting in neutral at stops!

What are you saying here. It will sit in 16th no problem at a stop or you got something going on.

 

[NDT]

What are you saying here. It will sit in 16th no problem at a stop or you got something going on.

James thanks for your comments, I was hoping you would chime in. This is where I am confused. Coming off of speed, I have been downshifting all the way to 1st, keeping the engine speed over 1400 like the manual says to. Right before the stop sign, I pop it into N. Are you saying to stop from high speed, you use leave the trans in 16th, let the engine idle down, and apply the brakes all the way to a dead stop? That sure would be easier than what I am trying to do. I just thought it was a big no-no to leave the trans in gear under 1400 rpm. The manual is not specific about any of this.

 

[cundupa]

I does take a little practice. Once you get the hang of it they are kinda fun. If you come to a complete stop, you can lift the shift handle and pull it down into N, you don't need to ratchet it all the way down.