With a production run of over 20 years, the S-Matic finally finished in 2019 and appeared in several tractors. Specialist firms such as Kretec have the know-how to overhaul a worn or faulty transmission. We find out what is involved.

KEEPING IT BRIEF

  • The S-Matic transmission was developed by Steyr, and later refined and produced by ZF.
  • Power is transmitted up to 50% hydraulically and up to 100% mechanically across four auto-shifting ranges.
  • The compact cartridge can be pulled out of the transmission housing without too much drama.

First introduced by Steyr in 1999, the S‑Matic gearbox was initially installed within the Steyr CVT and Case IH CVX models. From 2001, ZF continued developing the CVT and producing it through to 2019, which saw it fitted to other tractor brands including various Deutz‑Fahr and Claas models.

Its four auto-shifting travel ranges mean a high proportion of the engine power is transmitted mechanically. In addition, the design proved particularly robust in the 150–200hp bracket. This said, there are components that eventually need to be overhauled or replaced preventively.

Michael Krenn from Salzweg near Passau in Bavaria knows these transmissions inside out. At his company, Kretec Antriebstechnik, he refurbishes the complete range of ZF transmissions, including S‑Matic and Eccom units from John Deere 6010 and 6020 series tractors.

Planetary gears

The engine drives what’s known as the dumbbell shaft. This is designed as a hollow shaft, with the drive-through to the pto inside. Mounted on the shaft is a pinion gear that drives the first planetary set, along with another pinion which drives the hydrostat. The hydrostatic and mechanical sides are integrated by a double set of planetary gears (P1 and P2).

The hydrostat drives the central sun gear on the input side, while the engine directly drives the outer ring gear also on the input side. At standstill, the hydrostat is fully swashed (–17°) and turns the sun gear synchronously against the ring gear, holding the planet gears in place.

As the tractor pulls away, the hydrostat moves towards the 0° position, slowing the sun gear. The planet gears begin to rotate and drive the output side of the planetary sets via the so-called planet carrier. As the swash angle decreases, the mechanical part of the power transmission increases until it reaches 100% at 0°. Accelerating further sees the angle move to +17°, reversing the sun gear direction and altering the ratio again.

Four ranges

This is repeated in each of the four ranges which are shifted hydraulically via dog clutches:

Range 1: 0–8km/hr

Range 2: 8–14km/hr

Range 3: 14–30km/hr

Range 4: 30–50km/hr

Each subsequent dog clutch engages at no load when the input sun and ring gears spin in synch. This means the first range engages when the pump is in its –17° position, the second when the pump is in +17° and so on. These range shifts are controlled electro-hydraulically courtesy of five speed sensors, with the torque in each range transferred through planetary gearing to the output pinion.

Behind the pressure filter housing sits the speed sensor, which must be removed before you can remove the cartridge.

In the top range, the hydrostat only swashed through a certain section of its potential range of the maximum speed of 40 or 50km/hr. This boosts the overall efficiency on the road, because it increases the percentage of power that is transmitted mechanically.

Removing the transmission

The S‑Matic is built as a compact unit that is designed to handle engine speeds as high as 5,000rpm, which expose components such as pinions to lower torque peaks so these can be designed smaller. 

A notable advantage is that this compact gearbox can be removed as a cartridge from the side of the housing. Once out, it can be shipped as a complete assembly for repair.

To remove it, you should clear the way first by removing the right rear wheel, the battery box if required, and on Deutz‑Fahr models, the right-hand tank. Transmission oil must then be drained and the left speed sensor removed.

Through the opening, you can access the output-side sleeve. This slides off the output shaft after you punch in the internal roll pin. Next, all bolts are removed and the cartridge is pulled out from the housing. Specialist workshops use mobile racks for this job.

Repair and costs

The cartridge is then sent to Kretec, where it undergoes initial inspection and complete strip-down. All of the issue points are checked systematically and repaired as required, ideally as shown on the following pages.

Depending on the damage, refurbishment takes about two days. Kretec benefits from its proximity to the ZF plant in Passau for fast parts supplies and the option of using their test bench — although such a test run will cost a four-figure sum. In this case, the issue was pressure fluctuations, the cause of which was traced down to a worn swash plate pump. So, costly bench testing wasn’t necessary.

These parts amounting to around €7,500 were replaced.

The repair costs came to €9,000, with more than €7,500 spent on parts alone. Prices are high largely because production ended in 2019, meaning there’s a limited availability of replacement parts.

The nitrogen accumulator damps pressure fluctuations during clutch engagement. If problems arise which can be traced back to pressure fluctuations, checking the accumulator is a good first step. The accumulator is usually  replaced during repairs.

Alexander Bertling

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FAULT DIAGNOSIS AND TYPICAL REPAIRS

INSPECTING THE ACTUATOR MOTOR AND LOOM

The hydrostat actuator motor can be checked using a 2.5mm Allen key to turn the adjustment mechanism a few degrees left and right. Each time, it should return to neutral on its own. On older Tier 1 and Tier 2 models, the control claw must be moved to the centre using pliers. A new actuator motor costs about €5,000, but a repair solution is being developed.
The control box is also accessible from outside and is cleaned first. If it is damaged, it can be easily replaced. In this instance it was fine so was not replaced.
Another weak point is the loom that runs from the actuator motor to the control box (pre‑2014 versions). Over time oil can seep into the electronics through the harness. Kretec has an alternative solution, as the connector on the actuator motor is no longer available. The harness remains in place up to the point where it enters the housing. A new connector is then attached, and the corresponding socket is securely mounted in the housing. The cable to the control box is replaced. The intermediate connector prevents any oil leaks.

BROKEN BOLTS AND FAULTY BEARINGS

The main drive’s cast brackets are bolted to the housing. If these work lose, vibration occurs, damaging the central shaft bearings. If the bearing seats in the cast housing are worn, Kretec reconditions them. One tell-tale sign is scratched speed sensor heads, caused by fouling with the sensor discs.

PLAY IN THE SUMMATION PLANETARY SET

Attention here focuses on the double planetary set and the sun gear. The worn tooth tips are clearly visible on the sun gear, which is why it is being replaced before reassembly.
Next, the experts also inspect the double planetary gear set which drives the output-side ring gear to the gearbox which shifts the travel range. Often the problem lies in noticeable play in the planetary gear bearings, which is why it is being replaced in this overhaul. The updated part has reinforced bearing seats for the planetary gears to prevent repeat wear.

SEALS AND CLUTCHES IN THE RANGE-SHIFTING AND SHUTTLE GEARBOX

A total of five dog clutches are used in the S‑Matic: one for forward/reverse and four for the range changes. They are engaged hydraulically with a system pressure of 32 bar. If the pressure is too low, slippage can occur, damaging the dog clutches. In this case, the teeth on one dog clutch ring have broken off, so it is replaced along with its mating ring and ring gear.
Whether the newly installed claw ring is sealed and functioning correctly can be tested by applying compressed air. When the air escapes, it must return to its original position. Two disc springs inside the ring ensure this happens.
If the clutches are not engaging properly, worn piston rings on the central shaft may be to blame. These separate the shift ranges and are replaced and greased during every overhaul. The planetary set and shaft are then reinstalled into the housing, followed by the summation set.

FLUCTUATING OIL PRESSURE DUE TO DEFECTIVE HYDRAULIC PUMP

In this example, pressure fluctuations in the transmission were reported, which manifested themselves as SGR fault codes. The cause was a worn axial piston pump, evident from heavy wear on the pump’s control piston.
A new hydraulic pump was fitted as part of the repair, costing about €3,500. The pump is mounted on rubber dampers to cut out vibrations being transmitted to the housing. These are screwed in and also serve as cable glands.

REPLACING FILTERS AND ACCUMULATOR

Service intervals for the S‑Matic are 1,000 hours for oil and suction filters (right), and 500 hours for the pressure filter (left). After refitting the cartridge, the transmission oil is filled through a pre-filter. Neglecting these service intervals may trigger SGR faults, because the sensors can quickly detect drops in operating or lubrication pressure — for example, due to a dirty filter.