M1078 transmission hesitating shifting

Forgive me if this is somewhere else on the forum. I could not locate it doing a search.

Just got our LMTV so we are green and still learning the systems. When I drive the truck on surface streets it seems that I must give the truck full throttle to get the RPMs to rise enough for the truck to shift. I have the optional TAC and it runs above 2500 RPMs then sticks at the top of the revs and eventually shifts. I notice this a lot when going from 5th to 6th.

The previous owner put some kind of ATF in the transmission instead of plain old engine oil.

All of my trips have been just moving the truck on surface streets. I have not gotten the truck warm enough for the fan to come on.

Aernan said:

Forgive me if this is somewhere else on the forum. I could not locate it doing a search.

Just got our LMTV so we are green and still learning the systems. When I drive the truck on surface streets it seems that I must give the truck full throttle to get the RPMs to rise enough for the truck to shift. I have the optional TAC and it runs above 2500 RPMs then sticks at the top of the revs and eventually shifts. I notice this a lot when going from 5th to 6th.

The previous owner put some kind of ATF in the transmission instead of plain old engine oil.

All of my trips have been just moving the truck on surface streets. I have not gotten the truck warm enough for the fan to come on.

Click to expand...

it is an UPGRADE to use "the correct" ATF over engine oil in the tranny. Numerous threads on that in here. How use to trucks with Diesel Engines are you?

Wish I had a tach.

Is the speedo smooth or jumpy? Any stored codes on the keypad? Did the previous owner use allison atf or the cheap stuff?

My m1088 had to have pretty high rpms to shift also, but it improved when I did the fluid swap to tranny fluid (Napa synthetic dex III). I have an intermittent problem with mine that seems worse now. When I am doing lot work with it, in short forward and reverse trips, it wont shift into reverse and hesitates going into drive. if I shut it down and let it cool, it is fine but gets worse the warmer it gets. Also, sometimes it gets stuck in 3rd and will not up or downshift while I am driving. The local Allison shop wants 250 just to diagnose.

You might try adjusting the throttle linkage & idle on top of the engine. That seems to affect shift points

Maybe the tps needs to be aligned. I believe that controls kick-down.

I also have a 1078 "A0" and it's on the original engine oil. It shifts perfectly fine. It thuds abruptly in the first couple of gears when it's cold but it's not remarkable when it warms up after a little bit of driving. I'm telling you this because although there's probably merit to an ATF change, these transmissions can/do work properly in their stock configuration. And personally I like pouring engine oil in it rather than pricey ATF - this saved me a big way when I developed a crack in the inter-cooler line on a week long multi thousand mile trip.

What I'd be curious about is does it shift on command properly if you hit the buttons, or when you manually do it, does it fail to find the gear. It it manually commands the changes fine but is failing to do it automatically I'd be looking seriously at the speed sensor and the voltages / wiring to the control modules. Frankly though if you can find an Allison guy willing to touch the truck, even for $250, that might be worth it to get a definite answer. Ironically if you search around the Internet you'll see that Allison likes sending RV guys with these transmissions to stewart & stevenson for repairs - they actually have repair shops throughout the country. None in California but we have one out here in Phoenix. But a guy willing to work on the truck is a blessing, doubly so if they fish out the weird adapters necessary from the basement to get their scan tools onto the truck. If the guy is flexible, he can reprogram the transmission to work however you'd like.

Mine has some shifting issues too, I believe it is a tps issue but it hasn't set any codes. I changed tps adjustment it helped a little but still shifts, in my opinion at the wrong times, like when im just giving it small amounts of throttle driving around town.

Ok this gives me a bunch of stuff to check and research. Sounds like this is a common area of trouble for many folks.

1. I have driven another truck that had the engine oil in the trans. The first couple of gears shifted really hard. I don't recall as much sticking at the top of the revs before shifting so I believe this is unique to my truck.

2. It sounds like the throttle position sensor in the cab can be adjusted. I'll go figure that out. I know there is some wacky wiring in my dash so I'll go sort that at the same time. The other truck I drove had a very sticky throttle. Mine does not.

3. Sounds like I may have the wrong fluid in the trans. Is there an easy way to identify trans fluid brand/type?

4. I have heard that it can take a bit to warm up to get smooth shifts. I'm going somewhere by highway this weekend so I'll see if it behaves better after some highway miles.

5. Looking at the troubleshooting section of the TMs the fist thing to check is the speed sensor in the transmission via a ohm meter. I will go follow the TM to see if I don't have a bad sensor or dirty connections.

6. Command shifting. I tried pushing the up/down button and nothing happened. Do I need to be in a mode to make that work? I will go check to see if any fault codes are displayed.

7. I have basically no experience with large diesel vehicles. I had a mercedes 300TD wagon and it did not behave like this. I did have the transmission rebuilt because it was shot and I'm pretty sure the turbo was shot too.

You use the arrow keys to select the max gear it is willing to shift into, just like a car. Pick one under a gear that delays, like if you have a problem with 5->6, pick 5, run it up against the governor and hold the speed for a little while, and then press the arrow to allow 6. It should shift right away.

The diagnostics for reading the codes is in TM -20-4 if I remember correctly. Basically you hold both the arrows and you can go through a deal to get the codes. Then you look them up to see the meaning. IMO this is your first stop.

Just so you know on the A0, the engine is mechanical (will run without electricity if the fuel solenoid is otherwise held open). So the transmission and electronics are a self contained system, it's not like a more modern vehicle where they are all networked. So far as I know, the transmission electronics and the fuel solenoid are the only things that actually need power for the truck to run and drive once started, but maybe someone with more experience will correct me on that.

TPS is on the engine IIRC and exists solely to give info to the transmission. If it's dodgy you can have shift problems like this.

Pulled the codes from the transmission. It was D1 21,12. Looking up this quick guide.
http://www.beamalarm.com/Documents/allison_transmission_codes_manual.html
It looks like throttle position sensor.

I pulled the error codes. It looks like D1 21,12 Throttle Position sensor low position.

Voila. I wonder where those TPS sensors come from (allison, cat?)

Make sure you checked all the codes...it will store up to 5. D1...D2...D3...etc.

The TPS can be sourced from your local ALLISON dealer. It cost me somewhere between $300-400 for a new one; installation took all of 5 minutes, and then you have to follow the TM to set the 0% and 100% throttle positions with the foot pedal. Its all in the TM. Very simple.

I noticed a remarkable improvement in shifting once I replaced my TPS. It would shift too late, shift roughly, spend too much time out of gear when shifting between 4th and 5th, and the list goes on and on. I replaced the TPS, set the 0/100, and it ran like a beast, shifting just like you would expect from a modern passenger car.

Parts arent cheap for these vehicles, but small fixes like this make a HUGE difference.

Same with the output speed sensor on the trans. If its a little off it may not throw a code but I have found replacing it improves shifting. Although its a non contact sensor it seems to wear out over time.

Aernan said:

Looking up this quick guide.
http://www.beamalarm.com/Documents/allison_transmission_codes_manual.html

Click to expand...

Thanks for posting the guide. Anyone know if our transmissions have the oil level checking function described in there?

Sorry to bring up a dead thread, but anyone have the actual part number from Allison for the TPS? I may be needing one soon for a new ride. The part that should be the sensor in TM does not resemble the actual one in real life. I hope someone has changed it out and kept the #. Thanks!