Semi Truck Transmission Repair
Jumping out of gear, grinding on a shift, or a truck that won’t shift at all — the fix depends on whether it’s the transmission itself, the clutch, or the electronics on an automated manual. We diagnose which system is actually at fault before opening anything up.
Symptoms we see
| Symptom | Likely cause | What we do |
|---|---|---|
| Jumping out of gear under load | Worn shift fork, worn synchronizer, or a range/splitter valve issue on Eaton Fuller manuals | Inspect shift forks and synchronizers, test the range/splitter valve |
| Hard or grinding shifts | Clutch adjustment, worn synchronizers, or low transmission fluid | Check clutch adjustment and fluid level before inspecting internal components |
| AMT won’t shift or shifts erratically (UltraShift, Endurant, DT12, I-Shift, mDRIVE) | Clutch actuator fault, shift solenoid failure, or a software calibration issue | Pull codes and test actuator and solenoid function before replacing components |
| Shudder or vibration on takeoff | Worn clutch, or on an Allison automatic, a torque converter issue | Inspect the clutch assembly, or check torque converter function on automatics |
| Allison automatic shifting late, hard, or erratically | Fluid condition, a valve body issue, or a shift quality calibration problem | Check fluid and filter condition first, then test the valve body |
SymptomJumping out of gear under load
Likely causeWorn shift fork, worn synchronizer, or a range/splitter valve issue on Eaton Fuller manuals
What we doInspect shift forks and synchronizers, test the range/splitter valve
SymptomHard or grinding shifts
Likely causeClutch adjustment, worn synchronizers, or low transmission fluid
What we doCheck clutch adjustment and fluid level before inspecting internal components
SymptomAMT won’t shift or shifts erratically (UltraShift, Endurant, DT12, I-Shift, mDRIVE)
Likely causeClutch actuator fault, shift solenoid failure, or a software calibration issue
What we doPull codes and test actuator and solenoid function before replacing components
SymptomShudder or vibration on takeoff
Likely causeWorn clutch, or on an Allison automatic, a torque converter issue
What we doInspect the clutch assembly, or check torque converter function on automatics
SymptomAllison automatic shifting late, hard, or erratically
Likely causeFluid condition, a valve body issue, or a shift quality calibration problem
What we doCheck fluid and filter condition first, then test the valve body
What the job involves
On a manual, we start with the basics — clutch adjustment and fluid level — before assuming internal wear.
On an automated manual (UltraShift, Endurant, DT12, I-Shift, mDRIVE), most shifting problems trace to the clutch actuator, a shift solenoid, or a software calibration issue rather than the gearbox itself.
On Allison automatics, fluid and filter condition is the first thing we check — a lot of "transmission problems" are actually maintenance issues.
If it is internal wear — synchronizers, shift forks, bearings — we open it up and rebuild what’s actually worn.
What drives the cost
What this costs depends on whether it’s a fluid/adjustment issue or internal wear, how many components inside the transmission are affected, and whether it’s a manual, AMT, or automatic — each has a different diagnostic path. We tell you after we look at it, before we start.
Common questions
What transmissions do you work on?
Eaton Fuller manuals (9, 10, 13, and 18-speed, Roadranger), UltraShift and Endurant AMTs, Allison automatics, Detroit DT12, Volvo I-Shift, and Mack mDRIVE.
Why does my truck jump out of gear?
Usually a worn shift fork, synchronizer, or a range/splitter valve issue on Eaton Fuller manuals. We inspect all three.
My AMT won’t shift right — is that a transmission problem?
Often it’s the clutch actuator, a shift solenoid, or a calibration issue, not the gearbox itself. We pull codes first.
Do you service Allison automatics?
Yes — fluid, filter, valve body, and shift quality diagnosis for the 1000 through 4000 series.
What does a transmission repair cost?
Depends entirely on what’s actually wrong — an adjustment or fluid issue is a different job than an internal rebuild. We diagnose first.
Related services
Clutch Replacement
A slipping clutch, a clutch brake that won’t release cleanly, or a shudder on takeoff usually means the clutch assembly has worn past adjustment. We confirm it’s the clutch and not something else in the drivetrain before replacing it.
Learn more Drivetrain
Differential & Driveline Repair
A whine or howl from the rear end, or vibration through the driveline, usually points to ring and pinion wear, a failing carrier bearing, or a driveline balance issue. We isolate which one before opening the differential.
Learn more
Service area
Ready when you are
Call now or send a quote request — we’ll tell you what it takes to fix it.