To rebuild a Sauer-Danfoss (Sundstrand) Series 90 motor, pull it from the machine, replace the shaft seal, loop flushing shuttle and motor charge relief from the outside, then take off the end cap to reach the valve plate and rotating group, fit the overhaul seal kit for your frame size and build date, and torque everything to the Danfoss chart. A fixed motor (90M or the 90K cartridge version) has no control to service; a variable motor (MV) adds displacement limiters, a two-position control and its orifices.
Before you start, know where the factory draws the line. The Series 90 pumps and motors service manual covers adjustments and minor repair only, and Danfoss states that removing the end cap voids the warranty on a Series 90 pump or motor. So you get full procedures and torques for everything bolted to the outside, and parts manuals plus service bulletins, not a step-by-step, for the rotating group.
What this page is based on. The Sauer-Sundstrand Series 90 Axial Piston Pumps and Motors Service Manual (BLN-9947, copyright 1987-1998), and the Danfoss Series 90 motor parts manuals for the 42 cc (520L0847), 55 cc (520L0890), 75 cc (520L0892), 100 cc (520L0894) and 130 cc (520L0896) motors, January 2016 revisions, including the service bulletins printed in them. None of these documents publishes wear limits for the cylinder block, valve plate, slippers or swashplate, so this page does not invent any. For the pump side of the loop, see our Series 90 pump rebuild guide.
Which Series 90 motor do you have: fixed, cartridge or variable?
Start with the nameplate. Danfoss's own example motor code is 90M055 NC 0 N 8 N 0 C6 W 00 NNN 00 00 24, and the model code is what defines the unit for parts. The serial number carries the build date: in A-91-26-67890 the letter is the plant (A is Ames, Iowa; N is Neumunster, Germany; the 55 cc parts manual also shows C for China), 91 is the year and 26 is the week. That year and week decide which parts apply, because the parts lists give the same item more than one part number by date code. Our Sundstrand and Sauer-Danfoss identification guide walks through the date code rule, and the Series 90 model code decoder covers the code fields.
The 1998 service manual lists six fixed frames and two variable frames:
| Frame | Max displacement | Min displacement | Rated speed at max displacement | Theoretical torque | Weight, SAE flange |
|---|---|---|---|---|---|
| 030 MF | 30 cm³ (1.83 in³) | - | 4200 rpm | 0.48 N·m/bar | 11 kg (24 lb) |
| 042 MF | 42 cm³ (2.56 in³) | - | 4200 rpm | 0.67 N·m/bar | 15 kg (34 lb) |
| 055 MF | 55 cm³ (3.35 in³) | - | 3900 rpm | 0.88 N·m/bar | 22 kg (49 lb) |
| 075 MF | 75 cm³ (4.57 in³) | - | 3600 rpm | 1.19 N·m/bar | 26 kg (57 lb) |
| 100 MF | 100 cm³ (6.10 in³) | - | 3300 rpm | 1.59 N·m/bar | 34 kg (74 lb) |
| 130 MF | 130 cm³ (7.93 in³) | - | 3100 rpm | 2.07 N·m/bar | 45 kg (99 lb) |
| 055 MV | 55 cm³ (3.35 in³) | 19 cm³ | 3900 rpm | 0.88 N·m/bar | 39 kg (86 lb) |
| 075 MV | 75 cm³ (4.57 in³) | 26 cm³ | 3600 rpm | 1.19 N·m/bar | 44 kg (98 lb) |
Source: BLN-9947, size specific data, page 19. Rated system pressure is listed as 420 bar (6000 psi) and maximum 480 bar (6960 psi), page 18.
Is it the motor, or something else in the loop?
A Series 90 motor gets blamed for a lot of pump and valve problems. Work the Danfoss troubleshooting checks before you pull it:
| Symptom | Motor-related check (per Danfoss) |
|---|---|
| Works in one direction only | Measure charge pressure in forward and reverse. If it decays in one direction, the motor loop flushing valve may be sticking; inspect and repair it. |
| Will not drive either way | If charge pressure is fine in neutral but low in stroke, the motor charge relief valve or a system relief may be set wrong. |
| System running hot | Monitor motor case flow with loop flushing defeated (defeat spool installed). If the case flow is excessive, Danfoss says replace the motor. |
| Low output torque | System pressure measured at the motor, a variable motor stuck at minimum displacement, and internal leakage. |
| Wrong output speed | Pump flow and stroke, and whether the variable motor control is actually shifting. |
The case flow check is the one that tells you whether the rotating group is worn. Our case drain flow test guide shows how to set it up, and the hydrostatic transmission troubleshooting guide covers the rest of the loop.
What can you service on a Series 90 motor without opening it?
These are the minor repairs Danfoss documents. Clean the outside first, replace every gasket and O-ring you disturb, and lightly lubricate O-rings with clean petroleum jelly.
Shaft seal and shaft
- Stand the motor shaft up. If the unit lies on its side with the shaft out, the cylinder block can move out of place and the shaft becomes hard to refit.
- Remove the three or four screws holding the retainer plate and seal carrier: 10 mm hex on 030 and 042 units, 5 mm internal hex on 055 through 100, and 6 mm internal hex on 130. Some early units use a one-piece retainer and carrier.
- Spring force on the shaft may push the carrier out about 5 mm (1/4 inch). If it does not move, pry it out or lightly tap the shaft end with a soft mallet.
- Remove the carrier O-ring, press the old seal out on an arbor press, and check the carrier, new seal and O-ring for nicks. Danfoss allows a light coat of sealant on the seal OD to guard against a damaged carrier bore.
- Inspect the shaft sealing area for rust, wear or contamination. To change the shaft, pull it with its roller bearing, move the bearing to the new shaft and refit the retaining ring.
- Wrap the spline or key with plastic film, lubricate the carrier O-ring and seal lip, install, and torque the screws in a sequenced pattern, then recheck: 9.5 N·m (7 lbf·ft) on 030 through 100 motors and 22.5 N·m (16.6 lbf·ft) on the 130.
Loop flushing shuttle and motor charge relief
Every Series 90 motor end cap is built to take a loop flushing valve: a spring-centered shuttle that connects the low pressure side of the loop to a charge relief valve in the motor end cap. Remove the shuttle hex plugs with a 1-1/16 inch wrench, take out the springs, shoulder washers and spool, and note the washer orientation (shoulders face out, toward the springs). Early motors used a small diameter spool and later motors a larger one. Reinstall and torque the plugs to 41 N·m (30 lbf·ft) on 030 through 100 motors or 68 N·m (50 lbf·ft) on the 130.
For the charge relief: on shim-adjustable valves, remove the plug with a 7/8 inch hex, do not alter the shims or swap parts with another valve, and torque the plug to 68 N·m (50 lbf·ft). On screw-adjustable valves, mark the plug, lock nut and housing first, loosen the lock nut (1-1/16 inch on 030 through 100, 1-5/8 inch on the 130), and unscrew the plug. Inspect the poppet and its seat in the end cap, reinstall aligned to your marks, and torque the lock nut to 52 N·m (38 lbf·ft).
Old and new end caps do not mix. Service bulletin SB-1989-020 added the externally adjustable charge relief (10 to 32 bar range) and enlarged the loop flushing spool at the same time. The change came in at serial A-89-38 on 55 cc, A-89-44 on 75 cc and A-90-20 on 100 cc fixed motors, and it is non-interchangeable: the adjustable valve cannot be added to an older end cap.
Variable motor (MV) limiters, control and orifices
- Displacement limiters: measure the screw length to the seal lock nut before removal, loosen the lock nut with a 19 mm wrench and back the screw out with a 6 mm hex. One turn changes displacement about 5.6 cm³ (0.34 in³) on the 055 and 7.1 cm³ (0.43 in³) on the 075. Final adjustment belongs on a test stand. Torque the seal lock nut to 54 N·m (40 lbf·ft) and fit new tamper-resistant caps.
- Two-position control valve (electric types NA, NB, NC, ND or hydraulic type PT): torque the valve into the housing at 47 N·m (35 lbf·ft) and no more, because over-torquing can make the spool stick. The solenoid nut takes 5 N·m (44 lbf·in).
- Orifices: the control supply orifice and the minimum displacement cylinder orifice both come out with a 1-1/16 inch hex and go back at 37 N·m (27 lbf·ft) with new O-rings and backup washers. Do not swap the two. The maximum displacement orifice check valve goes in at 15 N·m (11 lbf·ft) with its seal lock nut at 34 N·m (25 lbf·ft).
What changed inside Series 90 motors over the years?
Danfoss does not publish a rotating group procedure, but the bulletins in the motor parts manuals tell you what you cannot mix:
| Bulletin | Applies to | What it means at rebuild |
|---|---|---|
| SB-1988-023 | All Series 90 pumps and motors | Only welded pistons are sold. When welded pistons replace hollow ones, install them symmetrically in multiples of three (all units have nine pistons). |
| SB-1988-018 | 75 and 100 cc fixed motors | Dowel pins went from 1/4 to 3/8 inch, changing the end cap, housing and valve plate holes (and the 100 cc thrust plate). Order by the date code on the nameplate. |
| SB-1994-024 | 55 and 75 cc variable motors | New swashplate hold-down pin design, interchangeable; change it at first repair and torque to 27 N·m (20 lbf·ft). |
| SB-1999-036, SB-2003-010, SB-2004-015 | 100 cc, then 55 and 75 cc fixed motors | Housing ribs strengthened, then housings changed to take longer end cap screws. Match screws to the housing you have. |
| SB-2001-011 | 55, 75 and 100 cc fixed motors | End cap gasket material changed at A-01-19 / N-01-19; new gasket numbers. |
| SB-2005-018, SB-2006-012 | 75 cc motors | Cylinder block spline process change (three ID grooves) from A-06-17, and a new interchangeable lip seal and seal assembly from A-06-18. |
| SB-1996-003, SB-1996-011, SB-2000-038 | 42 cc cartridge motors (90K042) | One-piece slipper guide retainer, two-piece seal carrier and retainer plate, and a housing and swashplate with a retaining screw. Mixed old and new parts only work as noted in each bulletin. |
In our shop we mark the end cap to the housing, keep the valve plate with its end cap, and keep each rotating group together. A scored block or valve plate face, a loose or worn slipper, or swashplate tracks you can catch a fingernail on is a parts decision, not a seal kit job. Danfoss gives no figures for those calls, so they are made by eye and experience.
Which overhaul seal kit does a Series 90 motor take?
The overhaul seal kit is item Q210 in each motor parts manual. These are the kits as listed, with the first date code each applies from:
| Frame and type | Overhaul seal kit | From date code | Parts manual |
|---|---|---|---|
| 042 fixed (and 90K cartridge) | 11134051 | 92-40 | 520L0847, pages 15 and 21 |
| 055 fixed and fixed cartridge | 11134053 | 87-02 | 520L0890, pages 25 and 39 |
| 055 variable and variable cartridge | 11141343 | 90-34 | 520L0890, pages 56 and 74 |
| 075 fixed | 11134054 | 87-02 | 520L0892, page 23 |
| 075 fixed cartridge | 11141342 | 93-07 | 520L0892, page 37 |
| 075 variable and variable cartridge | 11141344 | 93-07 | 520L0892, pages 54 and 70 |
| 100 fixed, standard | 11134055 | 87-07 | 520L0894, page 39 |
| 130 fixed, standard | 11134057 | 89-04 | 520L0896, page 28 |
| 130 fixed, special (double lip seal carrier) | 11134059 or 11134060 | 89-04 | 520L0896, page 28 |
The 130 cc row is the trap: three different kits for one frame, depending on which seal carrier the model uses. Send us the full model code and serial number and we will match the kit. Kits we carry are in the Sundstrand seal kit collection, and the Sundstrand seal kit guide covers the other series.
What are the Series 90 motor torque values?
| Fastener | Frame | Torque |
|---|---|---|
| Shaft seal retainer screws | 030-100 / 130 | 9.5 N·m (7 lbf·ft) / 22.5 N·m (16.6 lbf·ft) |
| Loop flushing hex plugs | 030-100 / 130 | 41 N·m (30 lbf·ft) / 68 N·m (50 lbf·ft) |
| Charge relief lock nut, screw adjustable | All | 52 N·m (38 lbf·ft) |
| Charge relief plug, shim adjustable | All | 68 N·m (50 lbf·ft) |
| Loop flushing defeat plugs | All | 41 N·m (30 lbf·ft) |
| MV displacement limiter seal lock nut | 055, 075 MV | 54 N·m (40 lbf·ft) |
| MV two-position control valve / solenoid nut | 055, 075 MV | 47 N·m (35 lbf·ft) / 5 N·m (44 lbf·in) |
| MV supply and min displacement orifices | 055, 075 MV | 37 N·m (27 lbf·ft) |
| Speed sensor lock nut | All | 13 N·m (10 lbf·ft) |
| O-ring plugs, hex / internal hex: 9/16-18 | All | 37 N·m (27 lbf·ft) / 23 N·m (17 lbf·ft) |
| O-ring plugs, hex / internal hex: 7/8-14 | All | 95 N·m (70 lbf·ft) / 68 N·m (50 lbf·ft) |
| O-ring plugs, hex / internal hex: 1-1/16-12 | All | 163 N·m (120 lbf·ft) / 115 N·m (85 lbf·ft) |
Source: BLN-9947 pages 48-55 and 71-77, and the fitting chart on page 53. Plugs going into aluminum housings take the lower internal hex value for the same size. The end cap screw torques are not in the service manual, because end cap removal is outside its scope; get them from current Danfoss service literature for your unit.
How do you start up a rebuilt Series 90 motor and set motor charge pressure?
- Clean every system component, then fill the reservoir through a 10 micron (nominal, no bypass) filter and fill the inlet line.
- Fill the motor and pump housings with clean oil through the upper case drain port.
- Put a 50 bar (1000 psi) gauge on the pump charge port and disconnect the pump control input so the pump stays in neutral.
- Jog the prime mover until charge pressure starts to rise, then run at the lowest speed until charge pressure is established. Bleed air from the high pressure lines at the system gauge port if needed.
- Bring it up to operating speed, reconnect the control, confirm neutral, then cycle slowly forward and reverse for at least five minutes. Shut down, remove the gauges, plug the ports and top up the reservoir.
Then check the motor charge relief. Put a 50 bar gauge in the motor charge port M3 (the 030 and 042 have no M3, so use the motor system gauge port and add 1 bar to the values below), gauge the case as well, and run with the pump in stroke. Danfoss lists these acceptable motor charge pressures, referenced to case, at 50°C (120°F) for a one pump, one motor system, plus or minus 1.4 bar (20 psi):
| Charge code on nameplate | Motor charge pressure |
|---|---|
| 10 | 8.1 bar (117 psi) |
| 18 | 16.1 bar (233 psi) |
| 20 | 18.1 bar (262 psi) |
| 24 | 22.1 bar (320 psi) |
| 28 | 26.1 bar (378 psi) |
On screw-adjustable valves, clockwise raises the setting at roughly 5.4 bar (78 psi) per turn; lock the nut at 52 N·m. Shim-adjustable valves are changed by shim thickness behind the spring. Danfoss calls for the first fluid change 500 operating hours after start-up, then every 2000 hours or once a year, with cleanliness of ISO 4406 18/13. Our start-up procedure guide and fluid cleanliness and flushing guide cover the general side.
Should you rebuild a Series 90 motor yourself?
The shaft seal, loop flushing shuttle, charge relief and MV controls are fair bench jobs with the torque chart above. Inside the end cap is where it goes wrong: a dowel pin size from the wrong era, a welded piston set installed out of pattern, or a scored valve plate put back because it looked fine. If case flow is high or there was metal in the filter, have the motor torn down and quoted before buying parts, and remember the pump that fed it probably saw the same dirt. Our hydraulic motor repair page explains how we handle motor jobs.
Frequently asked questions
Is there a Series 90 motor rebuild manual?
Danfoss covers Series 90 motors in the Series 90 pumps and motors service manual (BLN-9947), which gives troubleshooting, adjustments, minor repairs and torques. It does not cover end cap removal, which voids the warranty. The motor parts manuals (520L0847, 520L0890, 520L0892, 520L0894, 520L0896) list the parts, seal kits and service bulletins.
What seal kit fits a Series 90 55 cc motor?
Danfoss lists overhaul seal kit 11134053 for the 55 cc fixed and fixed cartridge motor (from date code 87-02) and 11141343 for the 55 cc variable motor (from 90-34). Send the full model code and serial number so we can confirm the kit for your build date.
What is the shaft seal screw torque on a Series 90 motor?
The 1998 service manual gives 9.5 N·m (7 lbf·ft) for 030 through 100 frame motors and 22.5 N·m (16.6 lbf·ft) for the 130 frame. Torque the screws in a sequenced pattern, then recheck.
How do I set charge pressure on a Series 90 motor?
Gauge port M3 and the case, run with the pump in stroke, and compare to the Danfoss table for the charge code on the nameplate, for example 22.1 bar (320 psi) for code 24. Screw-adjustable valves change about 5.4 bar (78 psi) per turn; lock the nut at 52 N·m (38 lbf·ft).
How do I know if my Series 90 motor is worn out?
Danfoss's check is motor case flow with the loop flushing valve defeated. If case flow is excessive, the manual says replace the motor. Low output torque with good system pressure at the motor also points to internal leakage.
Can I add an adjustable charge relief to an older Series 90 motor?
No. Service bulletin SB-1989-020 says the adjustable charge relief and the larger loop flushing spool came with a new end cap and cannot be added to motors built before the change (A-89-38 on 55 cc, A-89-44 on 75 cc, A-90-20 on 100 cc fixed motors).
What is the difference between a 90M and 90K motor?
Both are Series 90 fixed displacement motors. The 90M uses an SAE flange and the 90K is the cartridge flange version. Some frames use different overhaul seal kits for the cartridge motor, such as the 75 cc (11134054 SAE fixed, 11141342 fixed cartridge).
About the technical information on this page. Figures, procedures and part numbers here come from manufacturer service literature and are provided for reference only. They are only as accurate as the factory data, and the revision we have on file may not be the latest for your unit. Always confirm against the current manufacturer documentation for your exact model and follow safe work practices. RestoPower is not responsible for damage, injury or loss resulting from the use or misuse of this information, or from incorrect assembly, adjustment, installation or operation of any unit.
Need the right kit, or the motor rebuilt?
Send us the full model code and serial number from the nameplate, or a photo of it. We reply with price, availability and lead time. If you would rather have the motor rebuilt, the work is done in our shop or through the RestoPower repair network, and we arrange getting the unit to the shop doing the work. The teardown evaluation is free and you get a firm, itemized quote before any billable work. Work done in our shop carries a 1-year repair warranty. In-stock parts ship within 24-72 hours, and in-stock parts carry a 2-year warranty.
Request a quote · (810) 882-1171 · Repair and rebuild service
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