Sauer-Danfoss (Sundstrand) Series 90 Pump Rebuild Guide: Teardown, Seal Kits, Torque and Start-Up

To rebuild a Sauer-Danfoss (Sundstrand) Series 90 pump, pull it as one unit with any auxiliary pump, do the outside work first (shaft seal, multi-function valves, charge relief, charge pump and displacement control), then take off the end cap to reach the valve plate and rotating group, replace every seal with the overhaul kit for your frame size and date code, and torque everything to the Danfoss chart. Start-up is done with a charge pressure gauge in port M3, and the pump is not ready until charge pressure, measured above case pressure, is inside the range for the code on the nameplate.

One thing to know up front: Danfoss's Series 90 service manual (520L0818) covers installation, adjustment and minor repair only. Removing the end cap or side cover is what Danfoss calls a major repair, and it voids the factory warranty unless an authorized service center does it. So the factory gives you full procedures and torques for everything bolted to the outside, and service bulletins, not a step-by-step, for the rotating group.

What do you need before you start a Series 90 rebuild?

The nameplate. It carries the model code (for example 90R055 MA 1 NN 80 L 3 C6 D 03 GCA 42 42 24), the model number and the serial number. Danfoss says the model code completely defines the unit and must be used to order parts. The serial number is where the date lives: in A-01-26-23456, the letter is the plant (A is Ames, Iowa), 01 is the year, 26 is the week, and the last five digits are the build sequence.

That date matters more than most people expect. Danfoss parts lists often give the same item number two or more part numbers, split by date code. Their own example is the end cap gasket, item G30: a pump dated 87-02 through 89-16 takes 8000151, and one dated 89-17 or later takes 8000243. Same model code, different gasket. If you order by model code alone, you are guessing. Our Sundstrand and Sauer-Danfoss identification guide explains the date code rule, and the Series 90 model code decoder breaks down every field of the code.

Tools: common mechanic's tools, an arbor press for the shaft seal, snap ring pliers, a set of internal hex wrenches, a torque wrench, and calibrated gauges with snubbers: 1000 bar (10,000 psi) for system pressure at M1 and M2, 50 bar (1000 psi) for charge pressure at M3 and servo pressure at M4 and M5, and 10 bar (100 psi) for case pressure at L1 or L2.

How do you know the pump needs a rebuild and not just an adjustment?

Run the Danfoss troubleshooting checks first. A lot of "bad pumps" are a bad charge relief, a leaking multi-function valve or a worn motor.

Symptom	Check first (per Danfoss)
Works in one direction only	Swap the two multi-function valves. If the problem changes direction, the valve is bad. If charge pressure decays in one direction, look at the motor loop flushing valve.
Will not drive either way	Oil level, control input and orifices, bypass valves closed, charge pressure in neutral and in stroke, charge relief, inlet filter, then the charge pump.
Running hot	Oil level, heat exchanger, charge pressure, inlet vacuum, relief settings, and motor case flow with loop flushing defeated. If the motor's case flow is excessive, Danfoss says replace the motor.
Noise or vibration	Oil level, air in the oil, inlet vacuum, loose couplings and shaft alignment.
Sluggish response	Oil level, multi-function valve settings, inlet vacuum, engine speed, charge and control pressures.
Neutral hard to find	Control input, control linkage and orifices, and a leaking multi-function valve.

The full symptom-by-symptom version, with gauge readings for the whole loop, is in our hydrostatic transmission troubleshooting guide. For wear inside the rotating group, the case drain flow test is the best single check you can do without a teardown.

What order do you take a Series 90 pump apart?

1. Clean it, tag it, pull it. Clean the outside with the engine off, tag and cap every line, plug every port, and remove the pump together with its auxiliary pump if it has one.
2. Look at the oil. Discoloration, foam, sludge or metal means all filters get replaced and the system gets drained and flushed before the rebuilt pump goes back in.
3. Shaft seal and shaft. Stand the pump shaft up. Do not lay it on its side or knock it while the shaft is out, or the cylinder block can shift and the shaft will be hard to refit. Remove the three or four retainer screws (L70): 10 mm hex on the 042, 5 mm internal hex on 055-100, 6 mm internal hex on 130-250. Spring force usually pushes the seal carrier (L30) out about 5 mm. Remove its O-ring (L50), press the lip seal (L40) out of the carrier, and lift out the shaft (L100) with its roller bearing (L10).
4. Multi-function valves. Remove both cartridges (P2A1 and P2B1) with a 1-1/4 in hex on 030-100 frames or 1-5/8 in on 130-250. Danfoss sells these as complete units, with O-rings available separately. Pre-1988 valves have different internals.
5. Charge relief valve. On the screw-adjustable type, mark the plug (K90), lock nut (K10) and housing so you can return close to the original setting, then remove the plug, O-ring (K50), spring (K70) and poppet (K80). On early shim-adjustable valves, do not mix up the shims.
6. Charge pump. Remove the six retainer screws (H80), the retainer (H70) and the cover (J15), note which way the gerotor (H05) sits, then take out the coupling (H50), drive key (H60), gerotor, alignment pin (H40) and inner port plate (H31). On early 75 frame pumps with twin ports, clamp the end cap to the housing first so the gasket is not damaged.
7. Displacement control. Remove the six 5 mm control screws (M90), lift off the control with its orifice check valve (T40) and spring (T30), and discard the gasket (M11). If you pull the orifice plugs, do them one at a time and note where each came from; they are not interchangeable.
8. End cap, valve plate and rotating group. This is the major repair stage that 520L0818 does not cover. In our shop we mark the end cap to the housing, keep the valve plate with its end cap, and keep the parts of each rotating group together. The parts manuals list some valve plates by rotation (CW and CCW), so note which way the pump turns before anything comes apart.

What do you inspect inside a Series 90 pump?

Here is what Danfoss actually tells you to look at, and the service bulletins that change how the rotating group is rebuilt.

• Shaft and bearing: check for rust, wear or contamination, and spin the bearing by hand feeling for roughness. The seal carrier, new seal and O-ring are checked for nicks.
• Charge relief: inspect the poppet and its seat in the end cap for damage or debris.
• Charge pump: inspect all parts for abnormal wear. Danfoss is firm that charge pump parts from different production periods must not be mixed; install it as a complete assembly.
• Integral filter head: if the head or lock nut sealing faces are nicked or scratched, replace them.
• Valve plate: from serial N-95-48-xxxxx Danfoss changed the standard valve plate on 42 through 130 cc pumps (SB-1995-026). An old-style plate may be replaced with a new-style one, but never fit an old-style plate to a pump that had the new style.
• Slipper hold-down: in 1988 the hold-down went from spring loaded to a fixed clearance design (SB-1988-016). If old-style hold-down parts need replacing, rebuild with the new-style bearing, spacer and washer together. The hold-down screws carry locking compound and must always be replaced once removed.

Frame	Old standard valve plate	New standard valve plate
42 cc	500337	758383
55 cc	8000071	8000431
75 cc	8100145	501332
100 cc	8200041	501327
130 cc	700203	758425

Source: SB-1995-026, printed in the 55 cc parts manual 520L0889. The special hardware code in the model code changed from NNN to GBA with this change.

The charge pump also changed. On 55 through 130 cc pumps the outer port plate became part of the charge pump cover in 1993 (week 93-10 on 55 and 75 cc, 93-15 on 100 cc, 93-34 on 130 cc). The new complete assembly fits pumps from 89-17 on, but the piece parts do not interchange, the old hardware is obsolete, and for pumps built before 89-17 Danfoss says to consult them (BLN-10107). To pick a replacement charge pump kit you need the frame size, the charge pump displacement, whether there is an auxiliary pad, and the rotation.

A block or valve plate face that is scored, a slipper that is loose or worn through, or a swashplate with slipper tracks you can catch a fingernail on is a parts decision, not a seal kit job. Danfoss publishes no figures for these, so the call is made by eye and by experience, which is a good reason to have the rotating group looked at before you order.

Which overhaul seal kit does a Series 90 pump take?

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

Frame	Overhaul seal kit (parts manual)	From date code	Notes
042	11095646	87-42	520L0838, page 86
055	11095647	87-42	520L0889, page 114. We carry 80020S30.
075	Parts manual not on file	-	We carry 81020S30 for the 75 cc variable pump.
100	11095649	87-42	520L0893, page 182. SB-2007-018C lists overhaul kit 82020S30 replaced by 11035669 from serial A-07-28-xxxxx, non-interchangeable.
130	11095651	89-41	520L0895, page 170. A 2007 redesign lists overhaul kit 83020S30 replaced by 11035671 from serial N-07-10-xxxxx.
180	11095663	94-20	520L0897, page 85
250	11095664	95-28	520L0898, page 56

The 100 and 130 cc rows show exactly why the serial number matters: the same frame has more than one correct kit depending on when it was built. Send us the full model code and serial number and we will match the kit before it ships. Kits we carry are in the Sundstrand seal kit collection, and the Sundstrand seal kit guide covers the other series.

How do you reassemble a Series 90 pump and what are the torque values?

Use new O-rings and gaskets throughout, lightly lubricated with clean petroleum jelly, and keep the pump shaft-down on the bench while torquing covers, controls and valves.

• Shaft seal: press the new seal into the carrier, wrap the spline or key with plastic film or use a sleeve so the lip is not cut, lubricate the O-ring and seal, then torque the retainer screws in a pattern and recheck.
• Multi-function valves: reinstall the bypass actuator with the pressure limiter at 40 N·m (30 lbf·ft). If the limiter came out of the actuator, it has to be reset to the model code setting. Do not overtorque the cartridges.
• Charge pump: the alignment pin position and the gerotor outer ring orientation set the rotation, so check the model code if you are unsure. Lubricate the gerotor ID, OD and faces, and hold the drive key in with petroleum jelly.
• Control: new gasket, orifice check valve and spring in place, linkage pin into the swashplate link. Clean orifice plugs go back at 3 N·m (2.2 lbf·ft).

Fastener	Frame size	Torque
Shaft seal retainer screws (L70)	030-042 / 055-100 / 130-250	12 N·m (9 lbf·ft) / 16 N·m (12 lbf·ft) / 32 N·m (24 lbf·ft)
Multi-function valve cartridges (P2A1, P2B1)	030-100 / 130-250	89 N·m (66 lbf·ft) / 210 N·m (155 lbf·ft)
Bypass actuator with pressure limiter (in cartridge)	All	40 N·m (30 lbf·ft)
Charge relief lock nut, screw adjustable (K90/K10)	All	52 N·m (38 lbf·ft)
Charge relief plug, shim adjustable	All	68 N·m (50 lbf·ft)
Charge pump retaining screws (H80)	030-100 / 130-250	16 N·m (11.85 lbf·ft) / 32 N·m (24 lbf·ft)
Control and cover plate mounting screws (M90)	All	16 N·m (12 lbf·ft)
End cap screws (G90/G97)	030-042 / 055-100 / 130 / 180-250	13.5 N·m (10 lbf·ft) / 32.4 N·m (24 lbf·ft) / 38 N·m (28 lbf·ft) / 110 N·m (81 lbf·ft)
Auxiliary pad / end cap screws (J80)	030 / 042 / 055 / 075-130 / 180-250	58 N·m (43 lbf·ft) / 75 N·m (55 lbf·ft) / 122 N·m (90 lbf·ft) / 298 N·m (220 lbf·ft) / 580 N·m (429 lbf·ft)
Servo cylinder screws (F111/F211)	030-055 / 075-250	13.5 N·m (10 lbf·ft) / 32.5 N·m (24 lbf·ft)
Integral filter head lock nut (N20)	030-042 / 055-130 / 180-250	70 N·m (52 lbf·ft) / 122 N·m (90 lbf·ft) / 156 N·m (115 lbf·ft)
Speed sensor lock nut (B83)	All	13.5 N·m (10 lbf·ft)
Gauge port plugs, 9/16-18 (B70, B72, B82, B83, G45-G50)	All	40 N·m (30 lbf·ft)
O-ring plug, 1-1/16-12 (B80)	All	115 N·m (85 lbf·ft)

Source: Danfoss 520L0818, pages 49, 51, 54, 59-61 and the fastener and plug charts on pages 69-70. The side cover screw (E60) values in that chart are printed with metric and inch figures that do not agree, so we left them out; check the current manual for your unit. Plugs and fittings that go into aluminum housings take the lower value given for an internal hex plug of the same size.

How do you start up a rebuilt Series 90 pump and check charge pressure?

This is Danfoss's start-up procedure for a new or reinstalled pump, condensed:

1. Make sure the oil, reservoir, hoses, valves and heat exchanger are clean, fit new filter elements, and check the inlet fittings for air leaks.
2. Install the pump with a 50 bar (1000 psi) gauge in charge port M3.
3. Fill the housing with filtered oil through the upper case drain port, opening the case plug on top of the control to let air out.
4. Fill the reservoir through a 10 micron filler filter and fill the inlet line.
5. Disconnect the pump control input signal.
6. With the engine disabled, crank for several seconds, wait 30 seconds, crank again, and top up the reservoir.
7. When charge pressure reaches 3.5 bar (50 psi), start the engine and run at low idle for at least 30 seconds. Check for leaks and listen for cavitation. Do not run to full pressure until the air is out and the oil has been filtered.
8. With proper charge pressure established, raise to normal operating speed to purge the remaining air.
9. Shut down, reconnect the control signal, restart, and confirm the pump stays in neutral. Then cycle forward and reverse for at least five minutes to bleed air and flush the loop.
10. Top up the reservoir and remove the gauge.

To check charge pressure properly, read M3 with the pump in neutral and subtract case pressure (tee a 10 bar gauge into L1 or L2, or use M4 or M5). Danfoss lists these ranges at 1500 rpm and 50°C (120°F) reservoir temperature:

Charge setting in model code	Measured charge pressure above case
20	18.1-21.7 bar (262-315 psi)
24	22.0-26.9 bar (319-390 psi)
28	25.8-30.7 bar (374-445 psi)

Charge pressure rises above the setting at higher input speeds and drops in forward and reverse, so test in neutral. On screw-adjustable valves, clockwise raises the setting at roughly 3.9 bar (50 psi) per turn; lock the nut at 52 N·m. Early shim-adjustable valves are changed with shims behind the spring.

Other Danfoss operating limits worth checking on a rebuilt system: charge pressure minimum 18 bar (261 psi) and maximum 34 bar (493 psi) in the 2016 technical information 520L0603 (the service manual 520L0818 lists 10 bar (150 psi) and 35 bar (508 psi)); case pressure rated 3.0 bar (40 psi), maximum 5.0 bar (75 psi); charge inlet rated 0.7 bar absolute (9 in Hg vacuum), with 0.2 bar absolute (24 in Hg) allowed only at cold start; cleanliness ISO 4406 22/18/13 or better. Danfoss calls for a fluid change after the first 500 hours. Our fluid cleanliness and flushing guide and oil viscosity guide cover the fluid side.

Should you rebuild a Series 90 yourself?

Shaft seals, multi-function valves, the charge relief, the charge pump and controls are all bench jobs with a press, good gauges and the torque chart above. The rotating group is where a home rebuild usually goes sideways: a valve plate from the wrong era, hold-down screws reused, or a scored block put back because it "looked OK." If the case drain flow is high or there was metal in the filter, have the pump torn down and quoted before you buy parts.

Frequently asked questions

Related reading

• Sauer-Danfoss Series 90 model code decoder
• Sundstrand and Sauer-Danfoss pump identification
• Hydrostatic transmission troubleshooting
• Sundstrand parts catalogs, Series 15 through 90
• Series 90 pump parts
• Sundstrand and Sauer-Danfoss hub
• Sundstrand Series 20 and 90 seal kit guide
• Hydraulic pump start-up procedure
• Hydraulic pump case drain flow test
• All guides and resources