I work for the state DOT, we have a 2014 Mack Granite GU713 that is set up as a snowplow. It has an Allison automatic. About a year ago, it started losing power in all gears and surging in 4th gear. This only happened when pulling a hill or pushing heavy snow. Our mechanics were unable to find anything wrong. It continued getting worse until it now surges between 1000 and 1200 RPM in 4th gear, so hard it lifts the plow off the ground, about a 50% loss of power in other gears (under load or climbing a hill) from what it had before. If you let off the throttle and let it get down to 1000 RPM or below it starts backfiring through the exhaust (only in 4th gear, other gears lug down to 800-900 RPM just fine). It upshifts at 1250 RPM instead of 1400-1600 like the rest of the gears. It went to the Mack dealer, and they ran the rail, camshaft was bad and some valves were too. All were replaced, problem did not go away. If it is run on flat ground unloaded, it works fine, as soon as you start an incline or start pushing snow, it falls on it's face power wise. It will surge and backfire on level ground in 4th if you are pushing heavy snow. Our mechanics have run diagnostic on it, only thing they find is error codes for some sensors, mainly oil pressure. It has also gone through 4 fuel level sending units, as the shaft breaks. They had left some of the pieces in the tank, but we convinced them on the last one to pull them out. No change.
Now it is starting to surge in 3rd gear between 1400-1600 RPM, not as bad as in 4th, but starting out like 4th gear did. Transmission diagnostic shows nothing, no error codes. Our mechanics replaced transmission fluid and filter, no change.
This thing is driving me nuts, any help would be appreciated.
