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How a hydraulic slide system is built

A hydraulic room is a pump that makes flow, valves that decide where the oil goes, and a cylinder that turns it into travel. Know which valve picks direction and which one picks the room, and the rest of the diagnosis follows.

The short version

The power unit

Every system in this course starts with a 12 V motor (some HWH units are 24 V) turning a fixed-displacement gear pump, bolted to a reservoir and a valve manifold. HWH calls it the pump/motor/tank assembly; Lippert calls it the power unit. The pump makes flow, not pressure. Pressure builds only when something resists, and if nothing can move it climbs until the relief valve built into the pump opens. Most HWH pumps relieve at 3500 psi.

A high-current motor solenoid switches the motor. Lippert says its motor has no serviceable parts, so a failed motor means a power unit, and HWH says the same about the whole pump/motor/tank assembly. HWH also says relief settings are not to be changed in the field.

You can tell the two pump styles apart at a glance. A bi-rotational unit has two motor solenoids and two power posts on the motor, and it reverses to pick extend or retract. A unidirectional unit has one solenoid and one post, and a directional valve on the manifold swaps the lines instead (Equalizer stamps it DV1; Lippert lists a 4-way valve).

Valves: which one does what

Think of the manifold in two layers. The first picks direction: pump rotation or the directional valve sends oil to the extend line or the retract line. The second picks the function: a valve for each jack and each room opens that function to those lines. The room moves only when both layers line up.

Function valves are normally closed solenoid cartridges. HWH opens them by putting +12 V (or +24 V) and ground across the two coil wires, and HWH valves have a manual release nut or T-handle so they can be opened by hand. Lippert and Equalizer cartridges have a manual override screw in the stem. HWH rooms run on its small valve today; from 1995 to 2002 the large valve was also used for rooms.

SystemDirection set byRoom picked by
Lippert fifth-wheel slide unitReversing pump, out the E (extend) and R (retract) portsAll rooms share the lines; an IRC block of manual valves where fitted
Lippert combined leveling and slideoutMotor reversing (bi-rotational) or 4-way valve (unidirectional)A valve coil per room: orange main, purple bed, tan kitchen, pink auxiliary
Equalizer SystemsMotor reversing, or DV1 on unidirectional pumpsV5 to V8 for up to four rooms (V1 to V4 are the jacks)
HWH room extensionsRoom manifold solenoid valves; some single-room coaches use a reversible pump with no room valvesSmall solenoid valves on the room manifold

Cylinders and how the two sides stay together

Room cylinders are double-acting, with a port at the cap end and one at the rod end. Pressure on the cap end pushes the rod out, pressure on the rod end pulls it in, and the other side needs an open path back to the tank. HWH notes that oil slipping past the piston seal makes a cylinder weak and can let it creep.

HWH room cylinders are regenerative. To extend, the valving puts the same pressure on both ends and the larger area on the cap side wins; to retract, only the rod end gets pressure. HWH builds them close to 2 to 1, piston to rod, in three sizes: 9/16 in rod with 3/4 in piston, 7/8 in rod with 1 1/4 in piston, and 1 in rod with 1 1/2 in piston.

A Lippert through-frame hydraulic room has one cylinder on the active tube, and the passive tube follows through rack-and-pinion gear packs and a cross shaft. Equalizer shaft-timed rooms tie the rails together with a timing shaft and gears on racks. HWH four-cylinder rooms use a sync cylinder with one piston per room cylinder, so each room cylinder gets the same volume of oil at the same time.

What holds the room in

There's no brake on a hydraulic room. Equalizer puts it plainly: pressure in the common retract circuit keeps the jacks stowed and the rooms closed, and closed valves trap that oil. Lippert's combined system watches its retract circuit with a pressure switch that sounds the jacks-down alarm when retract pressure falls to about 1500 psi.

That makes creep a leak question. Air in the oil, a bypassing piston seal, a cartridge valve that doesn't seat, an override left open, a leaking hose or loose mounting bolts all show up in the makers' creep charts. Some circuits add a pilot-operated check: a check valve that opens only when the opposite line is pressurized.

Some HWH room cylinders carry an internal lock. HWH says it is really a relief valve that opens at about 1000 psi, the pressure has to be on the rod side of the circuit for it to work, and it sits at the rod end or the cap end depending on which way the cylinder moves to extend the room. A cap-end lock that fails with the room out can mean cutting the rod.

Sharing the pump, and the controls

On combined systems one pump does everything. Lippert's leveling and slideout system feeds every jack and room from one 12 VDC power unit, Equalizer runs V1 to V8 off one manifold, and HWH runs rooms and leveling from one power unit and valve assembly. HWH says to confirm the leveling works when a room won't move, because a leveling problem can stop the room.

The order of operations is not universal. Equalizer says jacks first, then rooms, and rooms run only with the ignition off. HWH's operator manual says not to run the room unless the coach is at ride height with the jacks retracted, and HWH rooms need the park brake set. Many Lippert fifth wheels run every room from one in/out switch, and on a shared circuit the room that needs the least pressure moves first.

Equalizer notes the jacks down light can come on while a room moves, and retraction can pause while the pump builds pressure. Both are normal on a shared pump. HWH says to hold the room switch until the room stops, then 3 or 4 seconds more, and warns against holding it more than 10 seconds.

On the job

  1. Find the power unit and read its label: maker, model, serial number and date code.
  2. Count motor solenoids and power posts: two means bi-rotational, one plus a directional valve means unidirectional.
  3. Trace each room's hoses to its valve or IRC knob and label them if the coach builder didn't.
  4. Note how the room syncs: passive tube and gear packs, timing shaft, or sync cylinder.
  5. Check that coach's interlocks: park brake, ignition, and whether jacks must be up or down.
  6. Pull the maker's manual for that exact unit before you open any override.

Numbers

Shop tip

Put a paint-pen label on every room valve and IRC knob the first time you trace it. The next tech, and you in six months, skip twenty minutes of hose chasing.

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