Telma vs Voith Retarder: The Difference

Telma (electromagnetic) retarder

A Telma retarder works through the magnetic field generated between a rotor mounted on the driveline and a ring of fixed coils around it; as current is staged up into the coils, an opposing eddy current forms in the rotor and slows the shaft. It has no wearing parts in the conventional sense — no lining, no pad, no friction surface — and because the braking effect is purely electromagnetic it responds within a fraction of a second of the driver requesting it, with no mechanical lag. Most installations need no separate cooling circuit of their own, since the heat generated is radiated directly from the rotor rather than carried away by a fluid loop. Because there is no mechanical wear to track, performance over the unit’s life comes down almost entirely to electrical-system and coil health: clean connectors, intact insulation and a control unit that is reading and switching current correctly.

Voith (hydraulic) retarder

A Voith retarder creates its braking resistance through oil flow between a rotor and a stator enclosed in a sealed housing; when engaged, the rotor’s vanes accelerate the oil toward the stator, which redirects it back against the rotor, and the resulting turbulence in the oil is what absorbs the vehicle’s kinetic energy. This design offers high braking force relative to its size, which is one reason it is common on heavier trucks and buses running demanding mountain routes, and it is usually plumbed directly into the vehicle’s existing cooling system rather than relying on its own radiator, since the heat generated has to be carried away continuously during a long descent. Because performance depends on the oil doing its job under sustained heat and pressure, oil quality, correct fill level and the condition of the seals that keep the circuit closed matter most when it comes to maintenance.

Which is better?

Neither design is universally better than the other; the right choice depends on the vehicle it was specified for, the application it runs — long mountain descents favour the high absolute force of a hydraulic unit, while stop-start urban routes benefit from the fast, precise response of an electromagnetic one — and whatever hardware and control wiring is already installed. Switching a fleet from one brand to the other is rarely a simple parts swap, since mounting points, cooling connections and driver controls differ. What matters more than the brand debate is using the correct original parts for whichever system is fitted and having maintenance carried out by a team that understands that specific design, since a retarder assembled with the wrong torque, air gap or oil fill underperforms regardless of which principle it uses.

KMQ Otomotiv supplies original and remanufactured spare parts for both Telma and Voith retarder systems from stock, so either brand can be kept running without waiting on a special order or a long lead time. Workshop repair and full overhaul, however, cover Voith hydraulic retarders only — for Telma units the service stops at supplying the correct part rather than extending to disassembly, repair and reassembly on site. That distinction is worth confirming with any supplier before a unit is stripped down expecting a repair service that is not actually offered on that brand, since the two scopes of work are easy to assume are identical when they are not.

Frequently asked questions

Is a Telma or a Voith retarder more powerful?

Both offer high braking force; the result depends on the vehicle and application. Correct installation and maintenance determine performance.

Do you supply parts for both systems?

Yes. KMQ Otomotiv stocks original and remanufactured spare parts for both Telma and Voith retarders, with same-day shipping.