Sundstrand 15 Series Hydrostatic Transmission Rebuild Guide: Seals, Charge Pump, Valves and Start-Up

Rebuilding a Sundstrand 15 Series hydrostatic transmission starts with the troubleshooting charts in Sauer-Sundstrand service manual BLN-9646, then the minor repairs it covers: the gerotor charge pump, the input, output and trunnion lip seals, the charge relief, implement relief and acceleration valves, and the charge check valves on U-style units. After the rebuild, charge pressure should read 70-150 psi on a 1000 psi gauge, and it must hold at full speed.

The catch is what the factory left out. BLN-9646 lists parts by item, description and quantity only, gives no torque values and no wear limits, and every troubleshooting chart ends at "replace transmission (pump and motor)" once the outside checks pass. Sundstrand treated the rotating group as a replace-the-unit item. This page covers everything the factory did publish, so you know what you can fix on the bench and where a rebuild quote takes over.

Which 15 Series unit do you have?

The 15 Series is a 0.913 in³/rev (15 cc/rev) family with slippered pistons and a trunnion swashplate under direct mechanical control. It came in five forms:

Configuration	What it is	Weight (BLN-10006)
Split system (15 PV + 15 MF)	Variable pump and fixed motor mounted separately and connected by hoses	PV 32 lb, MF 15 lb
In-line	Pump and motor in one housing, output shaft in line with the input. Case drains through the output shaft bearing into the axle or gearbox.	37 lb
U-style (15 U)	Pump and motor in one housing in the U configuration, PTO capability, case drains into the axle or gearbox	33 lb
Tandem pump (15 PT)	Two variable pumps on a common shaft, each with its own charge pump, check valves, charge relief and implement relief	64 lb
Variable motor (15 MV)	Listed in BLN-10006 with a maximum displacement of 0.913 in³/rev	Not listed

The model number on the tag decides the build. BLN-9646 says it plainly: consult the model number of the unit to determine the specific configuration. That covers charge pump displacement (.19, .30 or .33 in³/rev on single units, .33 or .46 on tandems), implement relief setting, mounting flange and shafts. Our Sundstrand and Sauer-Danfoss identification guide walks through reading the tag, and the Sundstrand document reference index lists every 15 Series document we have.

What are the 15 Series operating limits?

Item	BLN-9646 service manual	BLN-10006 technical information
Input speed	Up to 4000 rpm	4200 rpm maximum, 4000 continuous, 1000 minimum (pump)
System pressure	3000 psi maximum operating	4500 psi (310 bar) maximum, 2500 psi (175 bar) continuous
Charge relief setting	70-150 psi	Not stated
Implement relief setting	550-700 or 700-800 psi (tandem: 700-800 or 1000-1100 psi)	Not stated
Case pressure	15 psi maximum	10 psi (0.7 bar) continuous, 25 psi (1.7 bar) at cold start
Inlet vacuum	5 in Hg maximum	5 in Hg normal; replace the filter when it passes 10 in Hg
Inlet filtration	25 micron nominal, no bypass	20 micron nominal, no bypass
Temperature	200°F maximum continuous	180°F (82°C) continuous, 220°F (104°C) intermittent, at the case drain
Viscosity	At least 47 SUS at 210°F; no more than 6000 SUS at cold start	70 SUS optimum, 55 SUS minimum continuous, 45 SUS minimum intermittent

BLN-10006 ties its pressure limits to a five-year life at 200 hours a year. Where the two documents disagree, work to the more conservative of the two figures. The oil viscosity guide and fluid cleanliness guide cover the fluid side. BLN-9646 lists anti-wear hydraulic oil, Type F automatic transmission fluid and agricultural hydraulic transmission fluid as fluids that have worked.

How do you troubleshoot a 15 Series before tearing it down?

BLN-9646 gives four flowcharts. Take readings with a 0-5000 psi system gauge and a 0-1000 psi charge gauge, both snubbed, with input speed at or near maximum rpm. The gauge ports are 1/8 pipe thread.

Symptom	Check in this order
Jerky when starting	Control linkage, acceleration valves, charge check valves (U-style only)
Works in one direction only	Control linkage, charge check valves (U-style only), acceleration valves
Running hot	Oil level, heat exchanger, bypass valve, acceleration valve, charge pressure. Low charge: inlet filter, charge relief, implement relief, charge pump. Charge good: system pressure; if high, reduce the machine load.
Loss of power or will not drive either way	Oil level, control linkage, bypass valve, charge pressure, system pressure. Low charge: inlet filter, charge relief, implement relief, charge pump. Low system pressure: acceleration valves, then charge check valves.

If all of that checks out and the unit still will not pull, the charts say replace the pump and motor. That is the point where a rebuild costs out against a replacement. A case drain flow test and our hydrostatic transmission troubleshooting guide help you make that call with numbers.

How do you rebuild the 15 Series charge pump?

1. Mark the charge pump housing to the adjacent housing with a scribe line or punch marks so it goes back in the same position.
2. Clean the shaft extension and remove sharp edges, burrs and abrasive residue so the seal is not cut coming off.
3. Remove the hex head screws (four on split and in-line units, two on the U-style) and slide the housing assembly off the shaft while holding the gerotor cartridge in place. Remove the drive pin.
4. Take the shaft seal and bearing out of the housing only if they need replacing.
5. Inspect the gerotor faces for heavy scratching or wear patterns. The cartridge is a matched pair: replace both parts or neither, and never mix parts between cartridges. Always replace the drive pin. Check the bearing, O-ring and seal.
6. Oil both sides of the cartridge and the housing face, fit the drive pin in the shaft, and slide the cartridge on.
7. Wrap the shaft extension in plastic and oil it, put the O-ring in the housing, slide the housing on, line up your marks and torque the screws.

The manual gives no torque value for these screws, so we do not either. Check the current manual for your unit.

How do you replace 15 Series shaft and trunnion seals?

All the seals are lip seals, and they can be replaced without splitting the transmission. Changing the input or output seal does mean taking the transmission out of the machine. BLN-9646 counts one charge pump seal, two trunnion seals, one input shaft seal and one output shaft seal on a typical unit.

1. Pry the old seal out carefully without distorting the housing or marking the bore or shaft. It is not reusable.
2. For trunnion seals, remove the retaining rings (15) and washers (14) first, and replace any washer that is bent or distorted.
3. Polish the shaft extension, wrap it in thin plastic and lubricate it with hydraulic oil.
4. Slide the seal over the shaft and press it into the bore.

Two factory notes on sealing the OD. BLN-9646 allows a sealant on the seal OD (it names Loctite High Performance Sealant #5923) to help where the bore is marked. The later service bulletin SB-1992-010, which covers press-fit seals on both the 15 and 18 Series, calls for cleaning the bore with Loctite Primer T and a thin coat of Loctite RC680 on the seal OD, kept off the lip and out of the bearings. Some motor shaft seals carry a sleeve that locates a drive gear; that sleeve must stand 0.010-0.030 in beyond the seal face.

How do you check the 15 Series relief, acceleration and check valves?

• Charge relief valve: remove the plug, then slide out the spring and poppet. Do not change the shims or swap parts with another valve. Inspect the poppet and its seat, clean the bore, replace what is damaged.
• Implement relief valve (optional): same procedure. On units built without an implement circuit, the charge relief parts sit in the implement relief bore and the original charge relief port is plugged, so know which bore you are in before you start.
• Acceleration valves (not on U-style): remove the plugs on each side and slide out the two valve assemblies and the spring. Inspect valves and bores and clean out debris.
• Charge check valves (U-style): remove the cartridges complete with seals. Press the poppet and confirm a slight spring load. Fill the valve with solvent and let it sit 15 minutes; there should be no leakage.
• Bypass valve: on PV, tandem and in-line units it is a screw-type valve, fully open at half a turn of the stem. U-style units use buttons on the charge check valves instead. Either way it must be fully closed for normal operation.

Which seal kit does a 15 Series take?

BLN-9646 does not give kit or seal part numbers. The kit numbers below come from a Sauer overhaul kit list we have on file, not from the service manual, so match them to your model number before ordering. We do not list 15 Series kits on the store yet; ask and we reply with price, availability and lead time.

Unit type	Overhaul kit (Sauer kit list)
15 PV variable pump	9510354-0001
15 MF fixed motor	9510354-0003
15 MV variable motor	Not listed (shown as N/A)
15 tandem pump	9510353
15 in-line transmission	9510356-0001
15 U-style transmission	9510172
15 U-style, four lip seals only	9510171-0002

One detail from Danfoss kit instruction 11019500: 15 Series pump end caps were made with two different port O-ring grooves, a trepanned groove and a standard groove. The pump seal kit carries O-rings for both, and you use the pair that fits your end cap. Tandem pumps take double the quantities. Kit includes but is not limited to the items the instruction shows. The Sundstrand seal kit guide lists the kits we stock for other series.

How do you start up a rebuilt 15 Series?

1. Inspect the transmission for shipping damage and make sure the reservoir, hoses, fittings and heat exchanger are clean.
2. Fill the reservoir with fluid passed through a 25 micron nominal filter. Never reuse fluid.
3. Fill the inlet line from the reservoir to the pump (by hand if gravity will not do it), check the fittings are tight and the line is unrestricted, and fill the pump and motor housings with clean fluid.
4. Put the control in neutral and disconnect the control linkage for the first start-up.
5. Remove the charge pressure port plug and turn the input shaft slowly, by hand if you can, until fluid comes out.
6. Fit a 1000 psi gauge in the charge port on a short hose with a snubber or needle valve.
7. Start and run at the lowest possible rpm until charge pressure is up. If there is an implement circuit, cycle its valve slowly to fill it.
8. Bring the speed up to full rpm. If charge pressure does not hold (it may rise but should not fall), shut down and find out why.
9. Run at full input and output speed in both directions, watching charge pressure.
10. Run at least 15 minutes, shut down, replace the inlet filter, remove the gauge, plug the port and check the reservoir level.

After that, BLN-9646 calls for a fluid change at least every 2000 hours with a sealed reservoir or 500 hours with an open one, an inlet filter change at least every 500 hours sealed or 100 hours open, a breather service every 500 hours, a new filter with every fluid change, and a daily look at the reservoir. Cloudy or milky oil means water; a rancid smell means it has been running too hot. The general pump start-up procedure covers the rest of the system.

Should you rebuild a 15 Series yourself?

Seals, the charge pump and the valves are straightforward bench work with the manual. Past that, there is no factory procedure, no wear limit and no torque chart to work to, so the rotating group comes down to parts availability and experience. Mixed housings after 2006, a reused drive pin, or a gerotor with one new ring are the usual ways a home rebuild fails early. If the transmission has lost power with good charge pressure, have it torn down and quoted before you buy parts.

Frequently asked questions

Related reading

• Sundstrand and Sauer-Danfoss pump identification
• Sundstrand parts catalogs, Series 15 through 90
• Sundstrand document reference index
• Hydrostatic transmission troubleshooting
• Hydraulic pump case drain flow test
• Sundstrand and Sauer-Danfoss hub
• Sundstrand piston pumps
• Hydraulic motor repair and rebuild
• All guides and resources