Re: Allison Transmission Questions
Danl - 6/4/2009 12:31 AM
The unit has 69000 miles. The trailer weighs about 8500 lbs. Rpm flares 500-800 RPM. It happens almost every 3-4 shift once the trans warms up when pulling and sometimes when not pulling. Had a code for the oil level sensor but I had it replaced. No codes now. Can the tcm be reprogrammed to act like a tow/haul mode on a gm truck? Instead of having economy mode reprogram to shift later while towing? What I have been doing is putting the trans in 3rd and shifting manually to 4th at a higher rpm. This seems to work about 90% of the time. Thanks in advance for your help
Dan
I'm assuming you're at wide open throttle when this happens.
Assuming wide open throttle, a 500-800 RPM flare on the 3-4 upshift
that is consistent is not normal. By adding 8500 pounds, you change the gross vehicle weight the transmission's adaptive controls have adapted the shift quality to and that will cause a flare. But, after making the 3-4 upshift (at the same throttle position) several times, the shift should have re-adapted and the flare should be gone. That also indicates you probably are operating with an economhy shift schedule because if the upshift point was near engine governed speed, there isn't 500 - 800 RPM left in the engine's speed range to flare into.
The TCM has in it the ability to have two shift schedules. In many motorhomes, the pressing of the mode button changes the shift schedule from economy to performance or vice versa. The performance shift schedule should upshift closer to the engine's full load governed speed, but you must be at or near full throttle.
If you don't have any original documentation, you can take the vehicle to an Allison outlet that has the DOC software and determine which shift schedules you have in which mode (they're called primary and secondary). If you don't have a performance mode, with a shift schedule that has its wide open throttle upshifts near the engine governed speed, it can be reprogrammed, but you may have to shop around on price.
This still begs the question as to why the transmission is flaring and not correcting itself. You
may have an internal transmission issue. But because of the 8500 pound GVW change this vehicle gets, my personal opinion would be to try to address it with calibration first.
