Good hydraulic power unit troubleshooting starts with a stopwatch and a flashlight, not a parts catalog. We get calls from shops all over Iowa that already ordered a replacement pump before anyone checked the fluid level or timed a full lift cycle, and better than half the time the actual fault was a tired cylinder seal, a stuck lowering valve, or 100 feet of undersized extension cord feeding a motor that needs real amperage. We’re Auto Lift Services out of Ames, and between installs and service calls our crew tears into power units on every major brand — Rotary, Challenger, BendPak, Atlas, and a long list of orphaned units nobody supports anymore. This walkthrough is the same order of operations we use in the field, and it will save you money.
Tell us your lift brand, model, and serial number and we’ll match the correct pump, motor, valve, seal kit, or complete power unit assembly. If the tag is gone, send photos and we’ll identify it for you.
Start By Describing the Symptom Precisely
“The lift won’t go up” covers about eight different failures, and every one of them has a different fix. Before we drive out, we ask the caller to narrow it down: does the motor spin at all? Does it spin and make noise but the carriages don’t move? Do they move slowly, or partway up and then stall? Does the lift rise fine but drift down overnight? Does it rise fine empty and refuse with a three-quarter-ton pickup on it? Each of those answers points at a different subsystem, and getting the description right cuts diagnostic time in half.
Write the symptom down with numbers attached. Time a full rise from floor to full height with a stopwatch and compare it to what the lift did when it was new — most two-post units run somewhere in the 45-to-60-second range, and a unit that used to hit 50 seconds and now takes 90 is telling you something specific about internal bypass. Note whether the noise changed, whether the reservoir is hot to the touch after two cycles, and whether the problem showed up gradually over months or all at once on a Tuesday morning. Gradual degradation almost always means wear — pump gears, cylinder seals, contaminated fluid. A sudden hard failure usually means electrical: a capacitor, a contactor, a breaker, a pinched wire. That single distinction is the most useful shortcut in hydraulic power unit troubleshooting, and it costs you nothing but a minute of thought.
Electrical Checks Come Before Anything Hydraulic
If the motor doesn’t turn, stop thinking about hydraulics entirely. Check voltage at the motor terminals with the up button pressed, not at the wall outlet — a reading that looks fine at rest and collapses under load points at wiring, a weak breaker, or a bad connection somewhere in between. We’ve found loose lugs inside disconnect boxes, corroded wire nuts up in the ceiling, and neutral-shifted 220V circuits that a shop had been fighting for two years while blaming the pump.
Single-phase 220V units usually have a start capacitor, and a failed cap gives you the classic symptom: an angry hum, no rotation, and a breaker that trips in a few seconds. Those are cheap and easy to swap once you’ve confirmed it. Three-phase units bring a different set of problems, mainly rotation direction and lost legs — a three-phase motor running on two legs will growl, overheat, and do almost no work. Also check the up button and any limit switch in the circuit; on lifts with overhead shutoff bars, a bent or waterlogged switch will kill power at random heights and mimic a failing pump perfectly. Extension cords deserve a special mention because they cause so many false diagnoses. A 100-foot 14-gauge cord feeding a 220V power unit drops enough voltage that the motor never makes rated torque, and the lift crawls. Hardwire it or use heavy cord. Half our electrical hydraulic power unit troubleshooting calls end with a wiring fix, not a part.
Fluid Level, Fluid Type, and Contamination
The reservoir is the cheapest thing to check and the most commonly ignored. Low fluid gives you a lift that rises partway and stops, usually near full height where the cylinders demand the most volume, along with a whining pump and foam in the tank. Check the level with the lift all the way down — checking it up gives a false low reading and tempts people to overfill, which then pushes oil out the breather on the way down and makes a mess someone blames on a seal.
Use what the manufacturer specifies. Most lift power units want AW32 or AW46 hydraulic oil, and some manufacturers approve ATF as an alternative. What we see in the field instead is motor oil, chainsaw bar oil, and mystery buckets from the back of the shop, all of which are too thick or too dirty to work correctly, especially in an unheated Iowa pole barn in February where a lift may sit at 20 degrees overnight. Thick cold oil starves the pump inlet and produces exactly the slow-rise symptom people assume means a worn pump. Pull the fill cap and look at the oil. Milky means water intrusion, common on units stored where condensation forms. Dark and gritty means the pump has been shedding metal, and a suction screen full of debris means you’ll be back doing this again unless you flush the whole system. Fluid diagnosis belongs in the first five minutes of any hydraulic power unit troubleshooting job.
Hydraulic Power Unit Troubleshooting for Slow Rise and Drift Down
Slow rise under load with normal speed empty is the signature of internal bypass. Fluid is going somewhere other than the cylinder — past worn pump gears, past a relief valve that’s opening early, or past cylinder piston seals. To split pump problems from cylinder problems, we cap the pressure line and watch whether the unit can build and hold pressure on its own. A pump that builds full pressure against a plug but can’t lift a loaded truck is fine; the leak is downstream in the cylinders or a fitting.
Drift down is a different animal. Raise the lift, let it settle onto the mechanical locks, then take it off the locks and mark the carriage position with tape. If it creeps down with the pump off, fluid is escaping past the lowering valve or the check valve in the power unit, or past a cylinder seal. A quick discriminator: watch for oil actually returning to the reservoir. If the tank level rises as the lift falls, the valve is the culprit. If it doesn’t, the leak is internal to a cylinder and you’re looking at a seal kit or a rebuild. Debris on a lowering valve seat is common and sometimes clears with a careful clean rather than replacement. We cover more of this sequence in our writeups on lift hydraulic power unit troubleshooting and power unit bypassing.
Air in the System and Uneven Carriages
Air is the reason a lift jerks, chatters, and feels spongy at the top of its travel. It gets in through a low reservoir, a loose suction fitting, a cracked line, or any hose you opened during a repair and never bled. On two-post lifts, the standard bleed is to run the lift to full height and back down three or four times, then crack the bleeder on each cylinder with the lift raised slightly and let the foam out until clean oil flows. Top off afterward — bleeding always costs you fluid.
Uneven carriages get misread as a hydraulic fault constantly. On most two-post lifts the two sides are equalized by cables and sheaves, not by hydraulics, so if one side sits an inch or two low the answer is cable adjustment and sheave inspection, not a new pump. Check the cables for broken strands and check the sheaves for flat spots or seized bearings while you’re in there. On four-post lifts it’s the same principle with four cables and more sheaves. Where hydraulics genuinely do cause unevenness is on lifts with two cylinders plumbed in series or with flow dividers, and there a trapped air pocket or a plugged divider will hold one side back. Sorting mechanical from hydraulic here saves real money, and it’s why we treat cable inspection as part of hydraulic power unit troubleshooting rather than a separate job.
When the Power Unit Really Is the Problem
Sometimes the answer is the unit. A pump making a hard metallic grinding noise, a motor that trips a good breaker on every cycle after you’ve verified voltage and the capacitor, a housing weeping oil from a cracked casting, or a unit that simply won’t build rated pressure against a capped line — those are replacements. On older orphan lifts, the original power unit may not exist anymore, and a universal replacement matched on horsepower, voltage, reservoir capacity, flow rate, port configuration, and mounting bolt pattern is the right move.
Some of those calls end with a broader conversation. If a lift is 25 years old, the cylinders are seeping, the cables are frayed, and the arm restraints don’t hold, sinking money into a new power unit is a poor investment compared with replacing the lift. We’ll tell you that honestly, even though we sell power units, because we’d rather have the shop that trusts us in five years than one more part sale today. When replacement is right, we match on the specs above and confirm hose and fitting compatibility before anything ships. Send us the lift brand, model, and serial and we’ll do that matching for you — the parts lookup above is the fastest route, or call and describe what the unit is doing. Good hydraulic power unit troubleshooting sometimes ends in a new pump, but it should never start there.
What Keeps a Power Unit Alive
Nearly every failed unit we pull has one of three histories: it ran low on oil, it ran on the wrong oil, or it ran on dirty oil. Fix those three and a quality power unit will outlast several sets of cables. Check the reservoir level quarterly, change the fluid every two or three years in a busy shop, keep the breather clear, and don’t let a leaking fitting go for months while topping off the tank — that leak is pulling air and dirt in every cycle.
Cold matters here in Iowa. If your lift lives in an unheated building, cycle it empty a couple of times before putting weight on it in the morning so the oil moves and warms. Keep the top of the reservoir clean so grit doesn’t fall in during a fill, and use a clean funnel, not the one that’s been sitting on a shelf collecting brake dust. Annual inspection is worth it too — we look at anchors, cables, sheaves, locks, arm restraints, hoses, and the power unit together, because those systems fail in patterns and catching one problem early usually reveals another. We handle these inspections across central Iowa and ship parts nationwide, and we’re happy to talk through a diagnosis on the phone before you spend anything. Related reading: two-post lift power unit troubleshooting. Call 800-674-9302 and describe the symptom.
Our Clients Include: