Why Does My Allison AT545 Slam? The Science of the “Dart Kick”

If you’ve ever driven or travelled on a classic Dennis Dart, you know the sound: that signature whine followed by a decisive, sometimes violent “BANG” as the gearbox grabs the next gear. While it might feel like the transmission is trying to exit the chassis, that “slam” is actually a characteristic of 1990s hydraulic engineering.

Today, we’re diving into why the AT545 behaves this way, especially when paired with a Telma retarder and an air throttle.

1. No Computer, Just Pressure

Unlike modern gearboxes, the AT545 has no electronic “brain.” It makes decisions using a tug-of-war between two hydraulic pressures:

• Governor Pressure: Tells the box how fast the wheels are turning.
• Modulator Pressure: Tells the box how hard you are pressing the throttle.

2. The “Telma Trap”

The most violent slams often happen right after using the Telma retarder. When you use the retarder to slow down, the Telma is braking the driveline, but the gearbox “thinks” you are just coasting.

Because the AT545 is slow to downshift while coasting, it often stays in a high gear until the very last second. When you come off the retarder and hit the air throttle, the engine revs up quickly, but the gearbox is still finishing its “late” downshift. The result? A massive surge of pressure that slams the clutch packs together to catch up with the engine.

3. The “False Neutral” Phenomenon

Have you noticed that when you stop, the bus feels like it’s in neutral for a second, then “nudges” forward?

1. The Telma slows the bus faster than the hydraulics can react.
2. Once stationary, the fluid finally moves to engage 1st gear.
3. This delay creates a “False Neutral” where the engine is disconnected from the wheels for a brief moment before the gear grabs.

4. The Air Throttle Lag

On a Dart, the air throttle adds another layer. There is a split-second delay between your foot hitting the floor and the air signal reaching the transmission’s modulator. In that split second, the Cummins 6BT has already started to rev, causing the gearbox to “panic” and jack up the line pressure to maximum.

Is the Slam Damaging the Box?

Generally, no. In the world of old-school hydraulics, a “firm” shift is a healthy shift. It means the clutches are biting hard and fast. A “soft” or “sliding” shift actually causes more heat and wear.

As long as your ATF stays red and doesn’t smell like burnt toast, that slam is just the sound of your Allison doing exactly what it was built to do!