2016 Mack Pinnacle - Grinding into 9th and 10th Gear - mDrive Transmission

Question

I have a Mack Pinnacle with the mDrive automated manual. Recently, it's started acting up during the shift from 8th to 9th and 9th to 10th. It feels like it's hesitating, then I hear a physical 'grind' before it finally slams into gear.

It only happens when the transmission is up to operating temperature. If I'm light on the throttle, it's less noticeable, but under a full load, it's violent. I'm worried the synchros are gone, but I was told these units are usually bulletproof. No fault codes on the dash, just poor mechanical performance.

ScannerSandy · Accepted Answer

✅ Confirmed fix: This is almost certainly the split cylinder or the range piston o-rings. The mDrive uses air actuators to move the collars. If the air seals are leaking, the actuator doesn't have enough force to push the collar over the synchro quickly enough, causing the grind. Rebuilding the top-shelf air actuator assembly with new o-rings fixed mine completely.

MackMarty · Answer

Check your air dryer. If you have moisture in your air system, it'll get into the transmission solenoids and cause slow shifts. If the air is 'wet', the synchros won't engage in time with the engine rpm drop.

2016 Mack Pinnacle - Grinding into 9th and 10th Gear - mDrive Transmission

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