Sauer-Danfoss (Sundstrand) Series 40 Rebuild Guide: M25, M35, M44 and M46 Pumps and Motors

To rebuild a Sauer-Danfoss (Sundstrand) Series 40 pump or motor, identify the frame (M25, M35, M44 or M46) and the date code first, do the outside work (shaft seal, trunnion seals, check and relief valves, charge relief, charge pump and control), then split the housing from the end cap on an M25, M35 or M44, or lift the front cover on an M46, to reach the rotating group. Replace every seal with the overhaul kit for your exact unit, torque to the factory figures below, and start it up with a gauge on the charge port: a standard M25, M35 or M44 charge relief is 14.0 bar (200 psi) and an M46 is 19.5 bar (285 psi), both measured above case pressure.

Know where the factory draws the line before you start. Danfoss calls anything done without splitting the housing from the end cap (M25, M35) or removing the front flange (M46) a minor repair. Opening the unit is a major repair, and the factory says it can affect or void the unit warranty unless an authorized service center does it. The good news for a rebuilder is that the M25 manual and the older M46 repair manual print the full major repair procedure, so this page can give you real teardown steps, inspection rules and torques for those frames.

Which Series 40 unit do you have?

Read the nameplate. Danfoss says the model code completely defines the unit and must be used when ordering parts, and the serial number must be quoted too, because it carries the build date. On Series 40 tags the first four digits of the serial are the date code: year, then week of manufacture. The parts manuals often list two part numbers for the same item split by date code, so a model code alone is not enough to order a kit. Our Sundstrand and Sauer-Danfoss identification guide walks through reading the tag.

Frame	Displacement	Units in the family	Standard charge relief	Major repair starts when you remove
M25	24.6 cm³/rev (1.50 in³)	Variable pump (MPV), tandem (MPT), fixed motor (MMF), "U" style transmission	14.0 bar (200 psi)	The housing from the end cap or center section
M35	35.0 cm³/rev (2.14 in³)	MPV, MPT, MMF, variable motor (MMV)	14.0 bar (200 psi)	The end cap
M44	43.5 cm³/rev (2.65 in³)	MPV, MPT, MMF, MMV (shares kits with the M35)	14.0 bar (200 psi)	No M44 service manual on file; Danfoss describes the M44 as similar to the M35
M46	46.0 cm³/rev (2.81 in³)	MPV, MPT, MMF, MMV, cartridge motor	19.5 bar (285 psi)	The front flange (front cover)

Displacements from model code supplement 520L0635MC; charge relief settings from 520L0635 Rev EJ, page 12; repair boundaries from BLN-10056, BLN-9991 and 11026743. The M25, M35 and M44 use a direct displacement control: the control shaft is tied straight to the swashplate, so pump neutral is set by the machine linkage. The M46 is a servo pump with manual, electric or hydraulic displacement controls, which is why it has neutral adjustments the smaller frames do not.

Does it need a rebuild, or just a valve?

Run the factory checks before you pull anything. A lot of "dead" Series 40 drives are a check valve, a charge relief or a control problem.

Symptom	What Danfoss has you check first
Drives one direction only	Swap the two check/high pressure relief valves. If the problem changes direction, that valve or its cartridge setting is bad. On an M46, also check control input, displacement limiters and both servo supply and drain paths.
Will not drive either way, or no power	Oil level, inlet filter, charge pressure with and without load, charge relief valve, charge pump, then system pressure. On an M46 also look at the bypass valve and servo pressure.
Running hot	Oil level, heat exchanger and its bypass, charge pressure, the check/relief valves (a partly open valve makes heat), filters and machine load.
Neutral hard to find (M46)	Control input, then swashplate neutral and control neutral, set in that order (see below).

The M25 relief cartridges carry a three digit code stamped on them that is the setting in bar, and on the M25 pump you have to pull the cartridge out of its plug to read it. For wear inside the rotating group, the case drain flow test is the best check you can do without a teardown, and the whole-loop gauge procedure is in our hydrostatic transmission troubleshooting guide.

What can you fix without opening a Series 40?

These are the factory minor repairs. Replace every O-ring you disturb and lube it lightly with clean petroleum jelly.

• Shaft seal (M25): pull the unit, remove the retaining ring (push in on the shaft against the block spring if it fights you), and the seal carrier will move out about 6 mm (1/4 in). Tap the shaft end with a soft mallet to free the carrier. Only remove the shaft with the mounting flange up, or the cylinder block can shift. Press the old seal out, press the new one in, wrap the spline or key in thin plastic, and reinstall.
• Shaft seal (M35 and M44): same idea, but Sundstrand-Sauer warns not to pull the shaft out once the seal cover is off, because the slipper hold-down pins can drop out of place and you are into a major repair.
• Shaft seal and shaft (M46): the charge pump has to come off first. Stand the pump shaft up, remove the outer snap ring, seal and inner snap ring, and lift the shaft assembly out. Danfoss says do not rotate or move the pump after the shaft is out, or internal parts can dislodge and the shaft will not go back without a full teardown. Use an installation sleeve over the splines when you press in the new seal.
• Trunnion seals (M25, M35, M44): do not alter the trunnion shims, and on tapered roller trunnions put the shims and spacer back in their original order. SB-2000-026 replaced trunnion seal 5000465 with 5000604, which has a longer dust lip, and Danfoss recommends fitting the new seal whenever these units are serviced.
• Check, relief and charge relief valves: the relief cartridges are factory set and are replaced as a unit, never taken apart. Under SB-1996-014 the M25, M35 and M44 check and SCR assemblies cannot be disassembled at all. On shim-type charge reliefs, do not change the shims or swap parts with another valve.
• Charge pump (M35 PV): the gerotor spacer pin position sets the charge pump rotation: the hole farthest from the inlet port is clockwise, the closest is counterclockwise. The steel check ball goes on the inlet side. If either gerotor part is bad, replace both.
• Charge pump (M46): the gerotor and cover are replaced as a set if either shows wear, scratches or pitting. Replace the O-ring, load ring and pad seal every time it comes apart.

Minor repair fastener	Frame	Torque
Check/high pressure relief valve plugs	M25 PV and PT, M35	41-68 N·m (30-50 lbf·ft)
Charge relief valve plug	M25 PV and PT, M35	54-135 N·m (40-100 lbf·ft)
Trunnion seal cover and plain cover screws	M25 PV and PT	15-20 N·m (11-15 lbf·ft)
Trunnion seal carrier and plain cover screws	M35	24-30 N·m (18-22 lbf·ft)
Easy-Ride valve plug	M25 PV	20-47 N·m (15-35 lbf·ft)
Auxiliary pad cover or pump screws	M25 PV and PT	27-34 N·m (20-25 lbf·ft)
Charge pump cover or pad adapter screws; tandem section screws	M35	47-61 N·m (35-45 lbf·ft)
Charge relief plug	M46	108 N·m (80 lbf·ft)
SCR (check/relief) valve plugs	M46	176 N·m (130 lbf·ft)
Bypass valve	M46	20 N·m (15 lbf·ft)
Auxiliary pad screws / cover plate screws	M46	43 N·m (32 lbf·ft) / 30 N·m (22 lbf·ft)
Servo piston cover screws	M46	15 N·m (11 lbf·ft)
Front cover screws, T50 / T55 Torx	M46	58 N·m (43 lbf·ft) / 91 N·m (67 lbf·ft)
Servo neutral lock nut; M3 and M4/M5 gauge plugs	M46	37 N·m (27 lbf·ft)
Displacement limiter lock nut	M46	19 N·m (14 lbf·ft)
Case drain plug L1/L2	M46	115 N·m (85 lbf·ft)

Sources: BLN-10056 section 50; BLN-9991 pages 11-20; 11026743 pages 18-44 including the fastener and plug charts on page 44.

How do you take a Series 40 pump apart?

M25 variable pump and tandem (factory major repair)

1. Remove the outside parts first: auxiliary pad cover and coupling, Easy-Ride valves if fitted, charge relief, and both check/relief valves.
2. Mark the housing to the end cap. Remove the four housing screws (5/8 in hex) and washers.
3. Lift the housing off the end cap and valve plate without letting the valve plate drop. Pry the valve plate off carefully if it sticks; do not mark the plate or end cap faces.
4. Lay the housing on its side, slide the cylinder block out, then take the slipper guide and pistons out of the block. Hook the thrust plate off the swashplate with an O-ring pick.
5. Remove the shaft seal carrier and pull the shaft and bearing.
6. Mark both trunnion covers, remove them, and pull the journal bearings, or on tapered roller units the spacers, shims, cups and cones. Do not change the shim stack. Tilt the swashplate and lift it out.

On a tandem, the center section is the end cap for both pumps. Mark everything, remove the four screws holding each housing, and treat each section like a single pump. Only the front section has a shaft seal carrier.

M46 variable pump (Sauer-Sundstrand repair manual)

1. Remove the shaft seal, auxiliary pad, charge pump, control, filter adapter, charge relief, check/relief valves and bypass valve.
2. Set the pump front cover up and remove the seven 12-point screws (3/8 in or 7/16 in socket). The front cover comes off with the shaft and bearing.
3. Pull the swashplate journal bearings from the front cover and lift the swashplate out.
4. Lift the cylinder block kit out with large snap ring pliers, then the slipper guide, pistons, guide retainer, hold-down pins and pin retainer.
5. Lift the valve plate out, then remove both servo covers and the servo piston. Do not force the centering screw.

What should you inspect inside?

• Valve plate and cylinder block: no cracks are allowed on any surface. Running faces must be flat and free of nicks and burrs. Replace a plate or block that shows metal transfer, wear or erosion. If a scratch on the bronze valve plate face or the block running face can be felt with a fingernail or pencil lead, polish or replace the part. (M25 manual.)
• Pistons and slippers: replace all the piston assemblies if the brass slippers are scored or rounded over at the edges. Pistons and bores are not selectively fitted, so no bore orientation is needed on reassembly.
• Bearings: replace spalled or worn tapered roller cups and cones as a matched pair; never mix them.
• Needle bearing: when you press a new one in, the printed end of the cage faces the press pin. It is set to protrude 2.0-2.5 mm (0.08-0.10 in) from the end cap on the M25, and from the inside of the housing on the M46, because it pilots the valve plate. The M46 valve plate locating pin protrudes 4.19-4.70 mm (0.165-0.185 in). On an M46 fixed motor end cap the needle bearing protrudes 2.41-2.67 mm (0.095-0.105 in).

The service bulletins printed in the parts manuals change how some of these parts are rebuilt:

Bulletin	Applies to	What it means at the bench
SB-1996-010	M25 PV and PT	From serial A-95-05 the swashplate and trunnion bearings changed so no shims or spacers are needed for bearing preload. The old swashplate is serviced only as kit 4510126, and the old and new trunnion bearings do not interchange.
BLN-10151	Series 40 swashplate design change	An old-design swashplate must be replaced with the new design swashplate and its bearings, and the old shims and spacers are discarded.
SB-1996-014	M25, M35, M44 PV and PT	Check valve and SCR assemblies are now one-piece and are exchanged complete.
SB-2003-018	M35 and M44 pumps and variable motors	Shaft lip seal 9008000-0121 replaced by one-piece seal 5000689 from serial K-03-49.
SB-1999-058	Series 40 motors including M25, M35, M44 and M46	New finish on motor valve plates and new part numbers; interchangeable.
SB-2009-048	M25 fixed motor	Housing, shaft, bearing and seal changed from serial A10-09 (March 2010) and do not interchange with the old parts.
SB-2002-021	Series 40 tapered shafts	Woodruff key replaced by a straight key; replace shaft and key together.

Danfoss does not publish wear limits for the block, valve plate or slippers in any Series 40 document we have, beyond the visual rules above. That call is made by eye and by experience, which is a good reason to have the rotating group looked at before you buy parts.

Which overhaul seal kit does a Series 40 take?

The overhaul seal kit is item Q210 in each Danfoss parts manual. These are the kits as listed, with the first date code each applies from:

Unit	Overhaul seal kit	From date code	Parts manual
M25 variable pump (PV)	4510151	92-07	520L0813
M25 tandem pump (PT)	4510016	91-07	520L0814
M25 fixed motor (MMF025C)	4510035	93-07	520L0812 (the kit's shaft lip seal changed to 11103135 from 10-10)
M35 and M44 variable pump (MPV035/44D)	11181820	95-45	520L0601, 520L0591
M35 and M44 tandem pump (MPT035/44D)	11181821	95-45	520L0601, 520L0591
M35 and M44 fixed motor (MMF035/44D)	11114238	95-42	520L0602
M35 and M44 variable motor (MMV035/44D)	4510148	95-42	520L0602
M46 variable pump (PV)	4510017	93-39	11066890; kit sheet BLN-10046
M46 tandem pump (PT)	4510018	93-39	Tandem parts manual 11077090; kit sheet BLN-10047
M46 variable motor (MMV046C)	4510184	84-31	11063895
M46 variable cartridge motor	4510183	93-39	11063895

The M46 fixed motor section of 11063895 does not list an overhaul seal kit, so we do not give one. The M35/M44 pump kit list in 520L0601 includes, but is not limited to, the shaft lip seal, the end cap gasket, the trunnion lip seal and a set of O-rings. We do not list Series 40 overhaul kits in our store today, so send us the full model code and serial number and we reply with price, availability and lead time. Kits we carry for other Sundstrand series are in the Sundstrand seal kit collection, and the Sundstrand seal kit guide covers the other families.

How do you reassemble it and what are the torques?

Coat every surface that moves against another with clean hydraulic oil so it is lubricated at start-up, replace every O-ring and gasket, and clean all gasket faces.

• M25 swashplate preload (tapered roller trunnions, older units): if the housing, swashplate or bearings were replaced, install the spacers with no shims, read swashplate end play with a dial indicator, and shim for 0.05-0.1 mm (0.002-0.004 in) preload. Keep the shim difference side to side at 0.20 mm (0.008 in) or less. A correct assembly takes 0.56-2.26 N·m (5-20 lbf·in) to rotate the swashplate. Units from A-95-05 on do not use shims (SB-1996-010).
• Valve plate: hold it on the end cap with petroleum jelly on the steel side, bronze face out, notch on the locating pin, and use the correct plate for the rotation. On a tandem, a clockwise unit takes a CW plate in front and a CCW plate in the rear, and the reverse for a counterclockwise unit. Motor valve plates are bi-directional.
• Closing it up: the block spring holds the end cap (or M46 front cover) away from the housing by about 3 mm (1/8 in). Never force it down. Torque evenly, then turn the shaft by hand to check the build.
• M46 servo: eliminate all axial play between the centering screw and servo piston before you lock it, or you will never find neutral. Always fit a servo piston with new bearing and seal rings, and use new grade 8 flange head screws on the servo covers.

Major repair fastener or check	Frame	Value
Housing to end cap screws (also tandem housing to center section)	M25 PV and PT	81-95 N·m (60-70 lbf·ft)
Trunnion cover screws (new screws)	M25 PV and PT	15-20 N·m (11-15 lbf·ft)
Maximum torque to turn the shaft when assembled	M25 PV, PT and fixed motor	9.5 N·m (7 lbf·ft)
End cap screws	M25 fixed motor	54 N·m (40 lbf·ft)
Front cover screws, 3/8 in / 7/16 in (12-point)	M46 PV	53-64 N·m (39-47 lbf·ft) / 81-100 N·m (60-74 lbf·ft)
Torque to turn the shaft when assembled	M46 PV	5.4-10.8 N·m (4-8 lbf·ft)
Servo centering half nut	M46 PV	24-37 N·m (18-27 lbf·ft)
Servo cover flange head screws	M46 PV	14.9-17.6 N·m (11-13 lbf·ft)
Preliminary neutral lock nut	M46 PV	17.6-24.4 N·m (13-18 lbf·ft)
End cap screws	M46 fixed motor	69-84 N·m (51-62 lbf·ft)
Torque to turn the shaft when assembled	M46 fixed motor	5.4-13.6 N·m (4-10 lbf·ft)

Sources: BLN-10056 sections 61-63; Sauer-Sundstrand M46 repair manual pages 16-19 and 24. The newer Danfoss M46 service manual 11026743 lists the front cover screws as Torx fasteners at 58 and 91 N·m (see the minor repair table), so use the figure that matches the hardware in your pump.

How do you start up a rebuilt Series 40 and check charge pressure?

Danfoss's M46 start-up, condensed (the M25 and M35 manuals follow the same pattern):

1. Clean the reservoir, hoses, valves, fittings and heat exchanger, fit new filter elements, and check the inlet fittings for air leaks.
2. Install the pump with a 50 bar (1000 psi) gauge in the charge port M3.
3. Fill the housing with filtered oil through the upper case drain port.
4. Fill the reservoir through a 10 micron filler filter and make sure the inlet line is full.
5. Disconnect the pump control input. With the engine disabled, crank for several seconds, wait 30 seconds, crank again, and top up the reservoir.
6. When charge pressure starts to build, start the engine and idle for at least 30 seconds. Check for leaks and listen for cavitation. Do not run to maximum pressure until the air is out.
7. Bring it to normal speed, shut down, reconnect the control, restart and confirm the pump stays in neutral. Cycle forward and reverse for at least five minutes, then top up and remove the gauge.

On the M25, the manual says to run the system at full input and output speed in both directions for at least 15 minutes with the wheels off the ground, and to look for at least 7 bar (100 psi) of charge above case pressure once it is at operating speed. The M25 manual also warns never to start the prime mover unless the pump is in neutral.

Measure charge pressure in neutral and subtract case pressure. These are the factory settings:

Unit	Charge relief setting (above case)	Source
M25, M35, M44 PV and PT	14.0 bar (200 psi) standard; shims adjust within 7.5-14 bar (110-200 psi)	520L0635 Rev EJ, pages 12 and 27
M46 PV and PT	19.5 bar (285 psi) standard; shims adjust within 19.5-26.2 bar (285-380 psi)	520L0635 Rev EJ, pages 12 and 27
M25 PV and PT (older manual)	Usually 13.8 bar (200 psi)	BLN-10056
M25 "U" transmission	Usually 11.7 bar (170 psi)	BLN-10056
M35 (older manual)	7-8 bar (100-120 psi) at 1800 rpm; certain M35 pumps 14-15 bar (200-220 psi)	BLN-9991

The settings in 520L0635 are factory set at 1800 rpm in neutral. Pumps with no integral charge pump are set with an outside charge flow of 19 l/min (5 US gal/min), or 38 l/min (10 US gal/min) on tandems, and the system has to supply at least that. Whatever the setting, Danfoss says keep charge at a minimum of 6 bar (87 psi) above case, and keep case pressure at 1.7 bar (25 psi) continuous, 5.2 bar (75 psi) at cold start. Charge is changed with shims behind the spring. On the M46, adding shims raises it, and the plug goes back at 108 N·m (80 lbf·ft).

Setting neutral on an M46

1. Swashplate (mechanical) neutral first: 600 bar (10,000 psi) gauges on MA and MB, a hose bridging servo ports M4 and M5, engine at operating speed. Turn the neutral screw until one system gauge rises, then the other way until the other rises, and set it halfway. Lock the nut at 37 N·m (27 lbf·ft).
2. MDC control: 20 bar (300 psi) gauges on M4 and M5. Rotate the neutral return bracket until one servo gauge rises, then the other, set it halfway and torque the screw to 14 N·m (10 lbf·ft).
3. EDC or HDC: same split-the-difference method on the adjusting screw, lock nut at 3 N·m (27 lbf·in).
4. HC EDC: 50 bar (1000 psi) gauges on M4 and M5, total adjustment is less than 120 degrees, lock nut at 12 N·m (9 lbf·ft). Danfoss accepts a 1.5 bar (22 psi) differential or less and says zero is usually not possible.

Fluid and filters: Danfoss gives 2000 hours for a sealed reservoir and 500 hours for a breather reservoir when the machine maker gives no interval, and recommends the first M46 fluid change at 500 hours. Our fluid cleanliness and flushing guide and pump start-up procedure cover the system side.

Should you rebuild a Series 40 yourself?

Seals, valves, the charge pump and an M46 control are fair game for a good bench with a press, gauges and the torque tables above. The rotating group is where it goes wrong: a valve plate for the wrong rotation in a tandem, a shim stack changed on an older M25, a scored block put back because it "looked OK." If there was metal in the filter or the case drain flow is high, have it torn down and quoted before you buy a kit. A seal kit will not fix worn slippers.

Frequently asked questions

Related reading

• Sauer-Danfoss Series 42 pump rebuild guide
• Sundstrand and Sauer-Danfoss document reference index
• Sundstrand and Sauer-Danfoss pump identification
• Hydrostatic transmission troubleshooting
• Sundstrand piston pumps
• Sundstrand and Sauer-Danfoss hub
• Hydraulic pump case drain flow test
• Hydraulic oil viscosity guide by pump type
• All guides and resources