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Allison Auto Trans Stuck in First Gear

This Allison automatic transmission is a 2009 model in a Thomas school bus that was running around on it’s route when all of a sudden the transmission became stuck in first gear. Past experience with this condition told me that there was something wrong with either the output speed sensor, the connection or the wiring.

Any time in the past when this condition existed the problem was always the output speed sensor. In the video you see it mounted on the output shaft housing. It reads output transmission speed sending that signal to the transmission control unit. This determines shift points and is important data that helps the transmission run smoothly.

The output speed sensor reads a tone wheel inside the housing. Sometimes the wheel becomes loose if the output shaft nut at the yoke isn’t torqued properly causing the pressure that holds the yoke, bearings and tone wheel to back off. There is a specific torque specification for the output bearings that can be checked once the yoke is removed.

There are tools available to adjust the bearings properly. It’s a two piece device that secures the output shaft while torque is put on the bearing adjustment nut. These procedures are in the Allison manual. This transmission featured is the New World 3000 series. I can say with confidence that most times when this transmission locks in first gear it will be a bad connection to the sensor or the sensor itself.

The sensor in this case was at fault with an ohm reading of 370 ohms. Normally the sensors read 300 ohms. It was enough resistance to allow the transmission to shift normally but act up once in a while. I once had a service call 30 miles out of town with this exact scenario. The driver reported that the transmission was locked in first.

In the back of my mind while I drove to his location I knew where I was going to start my troubleshooting. This happened in the dead of winter so to my surprise when I arrived and crawled under the bus to have a look there was a large chunk of snow attached to the output speed sensor wiring harness. There was enough weight to pull it out of the sensor.

Once I cleaned up the connector harness and plugged it back in to the sensor everything went back to normal. The driver was impressed with how little time it took to fix the problem so it was a great moment for my ego ha ha. That won’t happen too often.