Specifications, Service and Repair of Farm and Lawn Tractors


Dyna VT Transmission for Massey Ferguson 7624 Tractors

Operating principle of the DYNA VT transmission

The Dyna VT transmission of MF 6616, 7624, 6615, 7620, 7622 tractors is a continuously variable transmission in both forward and reverse positions.

Hare/Tortoise range switching with synchronisation is incorporated into the transmission. The Tortoise range allows ground speeds of 0 to 32 kph. The Hare range allows ground speeds of 0 to 50 kph, and the maximum standard speed is electronically limited to 40 kph.

The Tortoise range is intended for heavy traction work at low speeds of less than 12 kph. The Hare range is intended for road driving (transport). At 50 kph, the transmission ratio is controlled electronically according to engine speed.

Power transmission can be hydrostatic OR mechanical or hydrostatic AND mechanical.

In simple terms, we can state:

- Slow forward position = Power transmission: majority = hydrostatic / minority = mechanical
- Fast forward position = Power transmission: minority = hydrostatic / majority = mechanical.

Power transmission hydrostatic circuit

The Massey Ferguson 6616, 7624, 6615, 7620, 7622 Dyna VT transmission unit is flexible suspended in the transmission housing. This latter also serves as an oil tank for the hydrostatic transmission.

Filling: OilTerrac Extra or Terrac Tractran 9/Fluid 9, or any other oil complying with standard CMS.

The lube oil pump sucks oil through the suction strainer. The temperature sensor monitors the transmission oil temperature.

In brief, if the transmission oil is cold, little oil passes through the cooler and most goes through the by-pass valve. This valve opens at a differential pressure of approximately 3.5 bars. The hydraulic oil temperature is monitored by the temperature sensor.

The service pump supplies the system pressure to the Dyna VT control spool valves and comfort control solenoid valves. The system pressure of approximately 18 bars is limited by a pressure-limiting valve with throttling port.

Two different pressures are present in the system.

- Low pressure for the MF 7624, 7615, 6616, 7620, 7622 tractors Dyna VT transmission control and auxiliary pressure for rear PTO clutch, differential block and universal joint brake. The measuring point of this pressure is approximately 18 bars.

- High pressure in the Dyna VT transmission. Pressure measuring point, approximately 500 + 20 bars.

The oil filter clogging is monitored by a pressure switch according to the transmission oil temperature. When this temperature is lower than 50C, oil filter clogging is not monitored.

Cooled transmission oil inlet into the high pressure circuit is performed in alternation by two non-return valves. Hot transmission oil outlet from the high pressure circuit occurs through the pressure relief valve.

The high pressure circuit comprises : a variable displacement pump and a hydrostatic motor, two non-return valves, two high pressure limiting valves, one pressure relief valve, one coupler function solenoid valve, one clutch function controlled valve and one check connector.

The pump and motor screw cylinders are controlled by two 4/3 spool valves. These 4/3 spool valves are mechanically controlled by the cam channel drive shaft.

Rotation of the drive shaft is initiated, according to the need, by the control unit which thus defines the hydraulic capacity and hence the hydraulic power.

The variable displacement pump and hydrostatic motor pivot proportionally. In backup position, the screw shaft is manually activated from the driver’s cab. In limp home position, the transmission automatically blocks at approximately 30 kph after engine start-up.

If the clutch pedal, parking brake or neutral switch are used, the high pressure circuit is automatically unloaded by means of two high pressure limiters. The coupler function is controlled by the pressure limiter.

The control unit


The control unit drives the cam channel drive shaft, which changes the Dyna VT transmission ratio.

The Massey Ferguson 7624, 7616, 6615, 7620, 7622 tractor Dyna VT control unit comprises the following elements:

- Limp home operation control (required in the event of electronic control failure).

- Coupling of the incremental rotational angle sensor with a digital resolution of 8,000 pulses per revolution.

- Epicyclic gear train transmission i = 192: 1 (screw shaft motor)

- 12 VDC electric motor, 0.4 to 7 amps, vacuum control unit 4,500 rpm

- Friction clutch 2.5 to 3.5 Nm, using the limp home operation socket wrench, 4 to 5 Nm.

Following ignition, the control unit searches for the reference point (approximate neutral point between forward and reverse operation).



Engine sensor (TR23): measures engine speed. This sensor measures engine speed. If it fails, only the limp home operation can be used.


Summing shaft (TR19) and drive pinion (TR11) Hall sensor: measures engine speed or rate and recognizes the rotational direction.


High pressure sensor (TR17) transmits the instantaneous oil pressure in the high pressure oil circuit to the electronic system.


Throttle pedal sensor (CAB 44): transmits the position of the throttle pedal to the electronic system and compares it to the engine speed. This position sensor is used for load control.


Clutch pedal progressivity sensor (CAB 12): electronically monitors clutch pedal travel.


Hare / Tortoise range position sensor (TR8): electronically monitors the range selector switch position.


Temperature sensor (TR12): monitors transmission oil temperature. Temperatures greater than 110°C are stored with an error code.


HP filter clogging sensor (TR13): monitors the clogging of the high pressure filter.

Massey Ferguson MF 6615, 7624, 6616, 7620, 7622 Dyna VT transmission operating diagrams

Fig.82. Epicyclic gear train / Power distribution

A - Transmission of mechanical force, B - Transmission of hydrostatic force, C - Power take-off drive, D - Front axle drive, 1 - Epicyclic gear train, 2 - Hydrostatic pump, 3- Summing shaft, 4 - Hydrostatic motor, 5 - Range shifting, A - Planet carrier, Driven by the engine - B: Ring gear, Drives the hydrostatic pump - C: Pinion gear, D: Sun gear pinion - Drives the summing shaft, C: Pinion gear

Fig.83. Dynamic stop

A:Transmission of mechanical force
B: Transmission of hydrostatic force

- The engine drives the planet carrier (A)
- The ring gear (B) turns, driving the pump (2) without any flow
- The hydrostatic motors 4 do not turn
- The sun gear (D) does not turn as it is blocked by the MF 6616, 7624, 6615 tractor wheels via the rear axle bevel gear

Fig.84. Start-up

A:Transmission of mechanical force
B: Transmission of hydrostatic force

- The pump (2) tilts and supplies flow to the motors (4)
- The motors (4) drive the rear axle
- The sun gear starts to turn and the ring gear speed decreases

Fig.85. Average ground speed

A: Transmission of mechanical force
B : Massey Ferguson 6615, 7624, 6616, 7620, 7622 Dyna VT Transmission of hydrostatic force

- The pump (2) angle increases
- The pump flow increases and the angle of the motors (4) decreases
- The sun gear speed increases, increasing the bevel gear speed
- The planet carrier speed is constant
- The ring gear speed decreases

Fig.86. Transport

A: Transmission of mechanical force
B : Transmission of hydrostatic force

- The angle of the motors (4) is 0° and the flow from the pump is blocked
- The pump (2) is thus blocked stationary, as is the ring gear
- The planet carrier turns
- The sun gear turns, driving the bevel gear on its own

Fig.87. Reverse

A: Transmission of mechanical force
B: Transmission of hydrostatic force

- The pump (2) is tilted at the opposite angle, so the flow is reversed
- The motors (4) turn in the opposite direction
- The sun gear thus turns in the opposite direction to the planet carrier and so also drives the bevel gear in the opposite direction
- The ring gear speed increases

Architecture of the hydrostatic components

For conventional hydrostatic components, the rotational angle only reaches 30°. Example of a conventional variable displacement pump with axial pistons.

This system, on the other hand, proposes hydrostatic components specially designed to provide a 45° rotational angle. Example of a special variable displacement pump with axial pistons.

This means:

- Higher intrinsic yield of hydrostatic components with respect to conventional components.

- A 45° rotational angle offers the possibility of increasing the ground speed scope, resulting in a decrease in the number of required mechanical ranges.