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Allison troubles - eggtrucks - 11-23-2020
We have two identical straight trucks, 2012 Peterbilt 348 with Paccar PX8, with Allison transmissions. The one runs great, the other one has gone downhill in regards to its shifting. It will shift up to the next higher gear way to soon. Example coming into a hill with the throttle all the way down, it will shift from 3rd to 4th at 14-1600 rpm, same from 5th to 6th. (Its a 6 speed), and feels like its not even trying to pull...when rpm drops to maybe 12-1300 it will downshift a gear, and will pull decent until 14-1500, them shifts up, and then we start losing speed again.
With 6000 lbs loaded any moderate hill, it will only go 30-40 mph up the hill. The other truck(thats working properly) has no probllem pulling 50-60 the same grade.
Had it to the allison dealer and they replaced speed sensor and wiring from sensor up to the tcm, and undated the software. That didnt help a thing. I swapped tcm with the other truck, still no difference. Replaced trans filter and fluid, no help.
Did fuel rail leak test, thats good, turbo boost is good, monitored fuel rail psi vs commanded. No problem there.
Any ideas while its acting like this? If use the button to manually shift, it has plenty of power...Not sure where to turn next....
RE: Allison troubles - DVT873 - 11-24-2020
You didn't say but I'm guessing this is a 3000 series? TCM's are a problem but since you swapped and it is still acting up that is probably not it. Had one this spring. Started shifting oddly/badly. Customer thought they could "drive the problem away" No. Trans was trashed internally. Do you have a computer and a connection device like an Insite Inline 6 or a Nexiq? Codes can also be accessed in the shift selector. Post a picture of the shifter. You should be a Gen4/5 shifter. Could still be Gen 3. Code access is a little different. Should be able to find something on you tube or google about that.
These are pretty well protected but they do have internal solenoid and wiring issues. Did the Allison dealer find any codes? What were they if they did? I would try and check codes. are you getting a trans warning light? Solenoid and wiring usually provide some helpful codes.
Another thing that may be happening is the TCM may be getting bad information. Like there is an ABS or traction control event when there isn't. This usually just affects torque converter lockup. To see all that would require Allison DOC or an equivalent scan tool that can see live data.
But. The surest bet is to drop the pan and look inside. There are two filters in the bottom of the pan. Odd thing is these don't always show a lot of trash in the fluid or in the filters. This is because there is a third filter on top of the valve body. It hides under a cap. It does a GREAT job of holding trash back. To see it you have to drop the valve body. Not too bad a job, make sure you unplug the harness. Careful it is heavy. Once it is down there will be three bolts and the cover will come off. If I had better pics I can't find them.
You can't really see it but there was a 2 inch mound of metal and powdered metal inside.
If you are in NE Ohio there is a very good Allison shop in Youngstown. Some good long term guys including a father and son team. Other than them I don't know. Like everything else ability to figure out what is wrong then fix it correctly is lost. Most don't give a shi#t.
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RE: Allison troubles - eggtrucks - 11-25-2020
(11-24-2020 )DVT873 Wrote: You didn't say but I'm guessing this is a 3000 series? TCM's are a problem but since you swapped and it is still acting up that is probably not it. Had one this spring. Started shifting oddly/badly. Customer thought they could "drive the problem away" No. Trans was trashed internally. Do you have a computer and a connection device like an Insite Inline 6 or a Nexiq? Codes can also be accessed in the shift selector. Post a picture of the shifter. You should be a Gen4/5 shifter. Could still be Gen 3. Code access is a little different. Should be able to find something on you tube or google about that.
These are pretty well protected but they do have internal solenoid and wiring issues. Did the Allison dealer find any codes? What were they if they did? I would try and check codes. are you getting a trans warning light? Solenoid and wiring usually provide some helpful codes.
Another thing that may be happening is the TCM may be getting bad information. Like there is an ABS or traction control event when there isn't. This usually just affects torque converter lockup. To see all that would require Allison DOC or an equivalent scan tool that can see live data.
But. The surest bet is to drop the pan and look inside. There are two filters in the bottom of the pan. Odd thing is these don't always show a lot of trash in the fluid or in the filters. This is because there is a third filter on top of the valve body. It hides under a cap. It does a GREAT job of holding trash back. To see it you have to drop the valve body. Not too bad a job, make sure you unplug the harness. Careful it is heavy. Once it is down there will be three bolts and the cover will come off. If I had better pics I can't find them.
You can't really see it but there was a 2 inch mound of metal and powdered metal inside.
If you are in NE Ohio there is a very good Allison shop in Youngstown. Some good long term guys including a father and son team. Other than them I don't know. Like everything else ability to figure out what is wrong then fix it correctly is lost. Most don't give a shi#t.
Thanks for reply. No, there is not codes showing, Allison coudnt see any either with their software. the truck does have an abs light on, i wouldnt have thought that would affect trans performance. Ill resolve that and also drop valve body to see if that reveals anything.