A torque converter replaces the clutch in a machine and transmits drive from the engine to the transmission through oil. With no rigid connection, it allows smooth pull-away under heavy load and protects the engine from overload. When pulling away, it can multiply engine torque by up to approx. 3.5 times. That is why its condition determines whether a wheel loader or dump truck has the power to drive into a stockpile or pull away uphill with a load.
Torque converter – what it is and where it works
The converter is mounted between the diesel engine and the transmission, usually bolted directly to the flywheel housing. A hydrodynamic drivetrain consists of three elements: the torque converter, a mechanical transmission (most often a powershift unit that shifts under load) and drive axles with final drives.
This design is found in wheel loaders, articulated dump trucks, motor graders, high-capacity forklifts and some dozers. In an automatic transmission, the torque converter takes over the role of the clutch: the machine stands in gear with the engine running and moves off when the throttle is applied. We cover the other elements of this chain in our article on drivetrain parts and in our guide to final drives.
Design of a hydrodynamic transmission
A hydrodynamic transmission (the same design under another name) consists of four main components:
- pump impeller – connected to the flywheel ring gear or crankshaft, rotates at engine speed,
- turbine – connected to the transmission input shaft,
- stator (reactor) – mounted on a one-way clutch, redirects the oil flow,
- torsional vibration damper and, in many designs, a lock-up clutch.
The pump and turbine are not mechanically connected. The gap between them is small, usually around 2–3 mm. The stator is what distinguishes a torque converter from a plain fluid coupling: a fluid coupling transmits torque one-to-one, whereas a converter can multiply it.
How a torque converter transmission works – operating principle
Once the engine starts, a gear pump supplies the converter with oil, and the pump impeller accelerates it and throws it onto the turbine blades. The kinetic energy of the swirling fluid is converted into mechanical energy at the turbine shaft. Oil returning from the turbine hits the stator, which redirects it so that it assists the pump. This so-called stator reaction torque is added to the engine torque.
Torque multiplication is greatest when pulling away, when the turbine is stationary and the pump spins at full speed. As the turbine catches up with the pump, multiplication decreases. At a certain point the one-way clutch releases the stator and the converter operates like a plain fluid coupling. There is always slip between pump and turbine: input speed is higher than output speed, and the difference is turned into heat.
Slip is eliminated by the lock-up clutch, a bridging clutch that rigidly connects pump and turbine in certain modes. On some machines the operator activates the function with a button on the console. Engagement itself is automatic, based on factors such as engine torque and the current gear. Lock-up reduces fuel consumption, limits oil heating and improves driving performance over longer distances. Some manufacturers combine the converter with a planetary gearset to increase maximum torque and system efficiency.
Torque converter transmission – advantages and disadvantages
A torque converter transmission provides automatic and continuous torque transfer, smooth pull-away and refined operation. It also effectively damps torsional vibrations. The absence of a rigid connection means that when resistance suddenly rises (the bucket hits hard material), neither the engine nor the rest of the drivetrain is overloaded. It excels where smoothness and tractive effort matter more than responsiveness.
The drawbacks are higher fuel consumption than with a mechanical connection and a large amount of heat generated, especially during prolonged operation with high slip. Add to that the cost of repairing the converter itself. That is why a properly working transmission oil cooler and the correct oil are so important.
Oil in a torque converter transmission
The converter oil circuit is linked to the transmission oil circulation. A gear pump draws oil from the transmission sump through a suction strainer, and the oil is cooled in an external circuit with a cooler. Oil requirements are higher than in hydrostatic systems: the right viscosity, resistance to oxidation and foaming, good lubricating properties and suitable friction characteristics for the clutches.
The most common choice is ATF or another transmission oil meeting the specification in the service manual, usually dyed red or green. Practical rules:
- check the oil level under the conditions specified by the manufacturer (temperature, idle, machine position),
- change filters together with the oil and inspect the element – metal particles are a warning sign,
- clean the transmission oil cooler; a clogged cooler and low oil level are the main causes of overheating,
- after a cold start, warm up the oil before heavy work; holding the machine on the service brake speeds this up, but do it briefly and in line with the machine's operating manual.
Wear symptoms and torque converter diagnostics
A converter wears over time. The following should prompt you to check its condition:
- lack of power when pulling away and pushing into material while the engine runs normally,
- engine speed rising without a proportional increase in travel speed – a "slipping clutch" feeling,
- fluctuating engine speed during steady travel,
- frequent transmission oil overheating, especially during roading,
- whining, howling or knocking that varies with engine speed,
- dark oil with a burnt smell, metal particles in the filter, leaks between engine and transmission.
The basic test is the stall test: with the machine braked, in gear and at full throttle, engine speed is measured and compared with the manufacturer's value. The test is carried out for a few seconds, as instructed in the manual. Speed that is too high indicates slip in the converter or transmission clutches. Speed that is too low indicates a weak engine or a slipping stator one-way clutch. So the engine must be ruled out first – our guide on why a machine has no power will help. Diagnostics are completed by measuring converter inlet and outlet pressures, oil temperature and checking the cooler (see also our article on engine overheating, as both coolers often sit in the same package).
Repair, rebuild or replace the converter
When bearings, seals, the stator one-way clutch or lock-up discs are damaged but the impellers and housing are in good condition, repair is usually worth considering. It requires disassembly, inspection of components and balancing. With cracked or chipped blades, seizure or a damaged housing, replacing it with a new or inspected used converter is more often cost-effective. Rebuilding is a relatively expensive service, so in practice replacement is common.
Whatever you choose, the root cause of the failure must be eliminated: flush metal particles from the cooler and lines, change the oil and filters, and check the charge pump. Otherwise the new converter will soon share the fate of the old one. You will find drivetrain parts in our construction machinery parts shop. On-site diagnostics are billed per mechanic labour hour: PLN 230 net per mechanic. Travel is charged additionally: PLN 3 net per km, counted both ways. When fitting parts purchased from WIBAKO, the labour rate is PLN 190 net. If your machine is losing power or overheating its transmission oil, contact us – we will help determine whether the converter is to blame.
Frequently asked questions
How does a torque converter transmission work?
The engine drives the pump impeller, which accelerates oil and directs it onto the turbine connected to the transmission. The stator redirects the oil at a favourable angle, so torque is multiplied when pulling away. There is no rigid mechanical connection between engine and transmission.
What role does the torque converter play in an automatic transmission?
It takes over the role of the clutch: it allows the machine to pull away and stop in gear without stalling the engine. It also multiplies torque when pulling away and damps torsional vibrations.
What is the difference between a torque converter and a fluid coupling?
A fluid coupling has only a pump and a turbine, so it transmits torque without multiplying it. A torque converter also has a stator, which allows output torque to be several times higher than input torque.
What are the symptoms of a failing torque converter?
Typical symptoms include lack of power when pulling away or pushing into a stockpile, engine speed rising without an increase in travel speed, transmission oil overheating, and noises or knocking that vary with engine speed. Metal particles in the filter and dark oil with a burnt smell are further signs.
What oil is used in a torque converter?
Usually ATF or another transmission oil to the specification given by the machine manufacturer. The correct oil and change intervals are listed in the service manual. The wrong oil can lead to overheating and damage.
Is it better to rebuild or replace a torque converter?
It depends on the condition of the impellers and housing and on parts availability for the model. Worn bearings, seals or a one-way clutch can be repaired. Cracked blades or seizure usually mean replacement. It is worth discussing the specific case with a service workshop.
