Aerial hydraulics, stabilizers and interlocks

Pressure on demand, held by valves

A hydraulic pump, usually PTO-driven, supplies flow. A relief valve caps system pressure. Control valves direct that flow to a cylinder or motor, and holding or counterbalance valves keep a loaded cylinder from dropping when the control is centred.

So a ladder that creeps down under load is usually a leaking holding valve or cylinder seal, not a weak pump - the pump is not doing anything while the control is centred.

Interlocks are a safety chain, not a fault

A typical chain: PTO engaged, parking brake set, stabilizers deployed and pinned, and often a torque-box or short-jack sensor. Break any link and the aerial refuses to move.

That refusal is usually electrical - a switch, a proximity sensor, a connector - not hydraulic. Check the chain before touching anything hydraulic, and never bypass a link to test it.

Slow versus dead

Dead on one function while others work points at that function's valve, its solenoid or its interlock. Dead on everything points upstream: PTO, pump, main relief, or an interlock gating the whole system.

Slow on everything usually means low fluid, a suction restriction, a worn pump, or fluid too cold to flow. Slow on one function points at that circuit's relief or a bypassing cylinder seal.

How to test it 🔊 Read aloud

1. Confirm the interlock chain firstPTO, parking brake, stabilizers down and pinned. Most no-operate calls end right here.
2. Check fluid and temperatureLevel, condition and temperature. Cold thick fluid mimics a failing pump.
3. Compare functionsOne dead function versus all dead tells you whether the fault is local or upstream.
4. Watch for creepLoad it, centre the control and watch. Creep means a holding valve or cylinder seal, not the pump.
5. Read pressure at the test portCompare against the placard before condemning the pump or the relief valve.

Numbers to work to

Common mistakes

• Bypassing an interlock to prove it - the interlock exists because of a fatality somewhere.
• Condemning a hydraulic pump for creep, which happens while the pump is idle.
• Testing on cold fluid and calling the whole system slow.
• Overlooking short-jack or torque-box sensors, which silently gate everything.