Skid Steer, Wheel and Drive Motors: How to Identify, Troubleshoot and Source Parts

A skid steer, wheel or drive motor is almost always one of three types: an orbital (gerotor or Geroler) motor such as a Char-Lynn or Parker Torqmotor, a radial piston cam-lobe motor such as a Poclain MS, or an axial piston motor running in a closed hydrostatic loop such as a Sundstrand or Danfoss unit. Identify it from the tag and the shape, then check brake release, charge pressure and case pressure before you buy parts, because most "bad motors" on these machines are a blown shaft seal or a dragging brake caused by something upstream.

The motor is the last thing in the drive line, so it gets blamed for everything. Spend twenty minutes with a couple of gauges before you pull a wheel. It is cheaper than pulling it twice.

What kinds of hydraulic motors drive skid steers and wheeled machines?

None of the factory documents we checked names a particular skid steer make and model with its motor, so we do not either. Your machine's parts book is the only reliable source for that. What the motor's own tag says is what we go by.

How do you tell which drive motor you have?

1. Find the tag. A Char-Lynn product number starts with the series prefix (103- is S, 104- is 2000, 109- is 4000, 162- is W, and so on), and our Char-Lynn decoder reads the rest. A Parker Torqmotor code reads like TF0080AS010AAAB: series, displacement, mount and port, shaft, rotation, options; see the Torqmotor page. A Poclain plate carries a model code, a part number and a serial number, and Poclain needs both the part number and serial number to supply spares; see how to identify a Poclain MS motor. Sundstrand and Sauer-Danfoss tags are covered in our Sundstrand identification guide.
2. Look at the shape. An orbital motor is a short stack: flange and shaft housing, then the rotor set, then an end cap, all held by long bolts, with the two main ports on the side or the end. A radial piston wheel motor is a large round unit, often with the wheel studs on a rotating housing. An axial piston motor is a long body with an end cap carrying the main ports and a case drain port, sometimes with a loop flushing valve or a two-speed shift port.
3. Count the ports. Two main ports only usually means an orbital motor with no case drain. A third, smaller port is a case drain. A small port near the flange on a wheel motor is often a brake release. A fourth pilot port can be a two-speed shift.
4. Note the mount. A wheel motor has its mounting flange near the middle so part of the motor sits inside the hub and the load lands over the bearings. A bearingless motor has no shaft or bearings at all and plugs into a gearbox spline. A standard motor has the flange next to the shaft.

No tag at all? Our guide to identifying a pump or motor with no tag walks through measurements and casting numbers.

How does a hydrostatic drive loop affect the motor?

On most compact machines the drive is a closed loop: the pump's two main ports run straight to the motor's two main ports, either line can be the high-pressure side, and the pump swashplate sets speed and direction. The Sauer-Sundstrand Series 40 and Series 90 service manuals describe the rest of the loop, and every part of it touches the motor.

• Charge pump. Keeps the loop full and pressurized, feeds the controls, and makes up the oil lost to internal leakage. The Series 90 manual says charge pressure must hold under all driving and braking conditions or the transmission gets damaged. On the Series 40 M25 it must be at least 7 bar (100 psi) above case pressure, and the charge relief is usually set 13.8 bar (200 psi) above case.
• Check, relief and bypass valves. Check valves feed charge oil into the low side, high-pressure reliefs dump across to the low side, and bypass (tow) valves join the two sides so the machine can be moved. Open bypass valves also mean no hydrostatic braking.
• Loop flushing. A shuttle valve and a charge relief in the motor end cap pull hot oil out of the low side for cooling and cleaning. A sticking flushing valve can make charge pressure fall in one direction only.
• Case drains. The pump and motor cases carry hot oil out. In the Series 40 layout the motor is drained from its upper port, that line runs through the pump case, then a cooler, then the tank.
• Holding. A closed loop leaks internally, so it cannot hold a load over time. That is why drive motors on slopes carry a static brake.

Orbital motors run in closed loops too. Eaton offers a shuttle valve option to send some low-pressure oil through the motor for cooling in closed-loop use, and says any motor with a shuttle valve must have its case port plumbed to tank. For the pump side of the loop, see hydrostatic transmission troubleshooting, which has the charge pressure specs by series.

Why do drive and wheel motors blow shaft seals?

Case pressure. The shaft seal is a lip seal holding case oil in against the atmosphere, and every maker puts a limit on it.

Look at the spread. An orbital motor's standard seal is rated for many times the case pressure a closed-loop piston motor's seal will take, which is why you cannot treat them the same. Danfoss lists skid steer loader attachments among the jobs for its high-pressure Char-Lynn seal, noting loaders often have no case line available, but still prefers the standard seal with a case port for the longest life. The Char-Lynn W wheel motor calls for a case drain in its standard form, allows 103 bar (1500 psi) without one in the optional version, and wants the case flooded whenever it runs.

The fixes are plumbing fixes: an unrestricted drain line to tank, not teed into a busy return, a clean drain filter where one is fitted (Poclain lists high casing pressure from the drain line and filter as a cause of external leaks), and no capped drain ports. Then fit the seal kit. Our shaft seal leak guide and the case drain flow test go deeper.

What causes bearing failure on a wheel motor?

Side load, and a lot of it. On a wheel motor the radial load on the bearings is the wheel's share of machine weight combined with the tractive force, and Eaton has you add those as vectors when sizing the motor. Danfoss's current Char-Lynn catalog gives a maximum continuous side load of 635 kg (1400 lb) for spool valve motors and 4500 kg (10,000 lb) for disc valve motors, and Eaton's 2006 catalog put VIS motors at 8640 kg (19,000 lb). Parker rates its TE wheel motor at 1600 lb (727.3 kg) maximum at the center of the output shaft.

Two things shorten bearing life fast. A spool valve motor where a disc valve or VIS motor belongs, and case pressure: on the W Series, Eaton notes that case pressure adds outward thrust on the shaft at 87.1 N per bar (1.35 lbf per psi). Signs of a bearing going are shaft play you can feel by hand, a rumble that changes with load (Poclain's first cause for a steady rumbling MS motor is a worn bearing support), and shaft seals that keep failing after a correct plumbing fix, because a loose shaft chews the seal lip.

Why is the drive motor weak, slow or losing torque?

• Not enough oil getting there. Parker's Torqmotor troubleshooting puts low oil supply (a relief valve or a worn pump) first. On a hydrostatic drive, measure charge pressure in neutral and then in stroke. The Series 90 manual reads low charge in stroke only as a motor charge relief or system relief problem, and low charge in neutral as the pump side.
• Internal leakage in the motor. Parker's fix for high internal leakage is a new rotor set; Poclain's is the cylinder block and distribution valve. On a piston motor, the Series 90 manual's test is case flow with the loop flushing defeated: if it is excessive, replace the motor.
• Worn splines. Parker lists worn splines on the rotor set, drive link and coupling shaft. A motor that slips, then catches, is telling you this.
• Heat. Parker does not recommend running Torqmotors above 200°F (93.3°C); seals harden and crack and leakage climbs.
• A dragging brake. Covered below. It looks exactly like a weak motor.

If it only drives one way, swap the relief or multi-function valves side to side. If the fault moves, the valve is bad; if charge drops only in one direction, check the motor's loop flushing valve. For orbital motors in an open circuit, see Char-Lynn orbital motor troubleshooting.

How do brakes on wheel and drive motors fail?

Integral brakes on these motors are spring applied and hydraulically released: no pressure, brake on. That makes the release circuit part of the diagnosis.

Low or slow release pressure gives a brake that drags, heats the motor and wears the discs. Too much case back pressure on a shared-cavity brake does the opposite and lets the machine roll. Poclain lists a brake that stays engaged as a cause of a motor that will not turn, and says to check the brake pilot circuit. Char-Lynn W Series motors with the parking brake take seal kit 60577-000, and the brake has its own repair manual. See the W Series parts catalog.

Which seal kits and parts do drive motors take?

• Char-Lynn. The Char-Lynn / Eaton brand page covers the whole line. The design code picks the kit, so decode first with the decoder, then go to Char-Lynn seal kits or the seal kit collection. Danfoss's current Char-Lynn catalog lists kits to convert to the high-pressure shaft seal: 60572-000 for the H, 9900098-000 for the S, 60579-000 for the T and 9901109-000 for the 2000. Eaton's 2006 catalog gave 60578-000 for the S and 61329-000 for the 2000, so older paperwork may show those numbers. Series parts: 2000, 6000, W, Delta, T.
• Parker Torqmotor. Parker's charts give SK000092 for the TF and TG (SK000093 for fire-resistant fluid), SK000115 for the TH, SK000212 for the TL, and SK000146 or SK000148 (Viton shaft seal) for the TJ wheel motor. Check the date code too, as the Torqmotor rebuild guide explains. Turf-style Torqmotor wheel motors are in turf wheel motors.
• Poclain MS. Poclain's parts lists include a repair seal kit, a reinforced sealing kit (mechanical face seal), shimming, brake repair, brake disc and brake shaft seal kits. Poclain also recommends replacing piston clips and spacers every time the clips come off, and after 100 hours of running, replacing the complete cylinder block sub-assembly or all piston assemblies together rather than single pistons. Start at Poclain motor parts, or the kits for the MS02, MS08, MS11 and MS18, and the Poclain MS seal kit guide.
• Sundstrand and Sauer-Danfoss. The Sundstrand brand page, Sundstrand seal kits, the seal kit collection, Series 42 parts and the Series 90 model code decoder.
• Eaton heavy-duty hydrostatics. Eaton heavy-duty pumps and motors and 54/64 series parts.

Swapping to another brand? Check the orbital motor cross reference first, and remember kits do not cross between brands. Units built for an equipment maker can also differ from the catalog motor, so compare the old seals with the kit before you start.

What are the signs of a bad final drive versus a bad drive motor?

The final drive is gears or a chain, so it fails like gears and chains: a grinding or clicking noise that follows wheel speed rather than load, a clunk on every change of direction from a slack chain, metal on the drain plug, gear oil weeping from the hub seal, or a wheel that wobbles on worn hub bearings. A drive motor fails like a hydraulic part: it gets weak or slow under load, runs hot, leaks at the shaft seal or shows high case flow. If the gear case or chain case is overfull, or the oil in it looks like hydraulic oil, the motor shaft seal is pushing oil past, and the case pressure checks above come before you fix either one. None of the factory documents we used give final drive gearbox data, so take the gear oil type and fill level from the machine's own service manual.

Should you reseal, rebuild or replace a drive motor?

Reseal when the motor still pulls hard and the only problem is a leak you have traced to a cause and fixed. Rebuild when it is weak, slow under load, noisy, or has shaft play: that is a rotor set, cylinder block or bearing job, and it means a teardown. Replace when the housing, cam or shaft is damaged, or when the rebuild parts cost more than the motor. Our hydraulic motor repair and rebuild page and the Char-Lynn rebuild guide explain what is involved.

Frequently asked questions

Related reading

• Orbital motor cross reference: Char-Lynn vs Danfoss, Parker, White and M+S
• Hydrostatic transmission troubleshooting
• Char-Lynn orbital motor troubleshooting
• How hydraulic motors work
• Case drain flow test
• How to identify a Poclain MS motor
• Where to buy Poclain motor parts
• Where to buy Char-Lynn parts
• RestoPower repair network