TE3 Truck Equipment — Auxiliary Power — practice test

High-yield TE3 topics

The material that shows up year after year. If you're short on time, start here.

• Pump math you will actually be asked to do: GPM = (RPM x pump CID) / 231, and HP = (GPM x PSI) / 1714. From there, PTO ratio selection is just arithmetic - if the pump needs 1000 rpm and the truck makes power at 1200 engine rpm, you need roughly an 83 percent ratio. Also know cylinder force = PSI x piston area, and that the rod-side area is bore area minus rod area, which is why a hoist retracts faster and weaker than it extends.
• PTO mounting and gear lash. Know split-shaft vs transmission side-mount (6-bolt and 8-bolt SAE apertures) vs rear-mount vs front crankshaft-driven, and clutch-shift vs hot-shift. Backlash between the PTO drive gear and the countershaft gear runs about 0.006 to 0.012 in, set with gaskets, and gasket thickness moves lash about 1:1. Too tight whines and cooks the gear; too loose knocks. Also know the interlocks: neutral, park brake, and roughly 60 psi minimum for air shift, plus a cab indicator light.
• Reservoir and suction-side design. Mobile reservoirs run about 2 to 3 times pump GPM (not the 3x-plus industrial rule), with 10 percent air space, a baffle between the return and the suction, and both lines below fluid level. Pump inlet vacuum should stay under about 5 in Hg. Suction strainer is typically 100 mesh (about 150 micron); the return filter does the fine work at about 10 micron. Cavitation = restricted or cold-thick oil flashing to vapor, high-pitched whine, pitted pump. Aeration = air pulled in through a suction leak, milky oil, spongy controls.
• Open-center vs closed-center valving matched to the pump. A fixed gear pump needs an open-center directional valve so neutral flow goes back to tank. Closed-center valving only belongs on a pressure-compensated or load-sense piston pump. Know why: hang a closed-center valve on a gear pump and the pump dead-heads over the relief, all that horsepower turns straight into heat, and the reservoir cooks past 180 F in minutes.
• Auxiliary air off the truck's supply. You do not tap the primary or secondary brake reservoirs directly. Air for a body, tarp, or air-shift PTO comes off the supply/wet tank through a pressure protection valve that closes around 70 to 80 psi so an auxiliary leak can never drain brake air below the FMVSS 121 floor. Know the wet tank as the moisture drop, the air dryer's place upstream, and that auxiliary tanks still need drains.
• Electrical power generation numbers. Frequency = (RPM x poles) / 120, so a 4-pole head needs 1800 rpm and a 2-pole needs 3600 rpm to hold 60 Hz - that is why a hydraulic-driven or engine-driven gen set lives or dies on engine speed control. On inverters: DC amps = AC watts / (DC volts x efficiency), so a 2000 W inverter at 12.6 V is pulling roughly 185 to 190 A. That drives 4/0 cable, an inverter mounted within a few feet of the battery bank, a fuse within about 18 in of the positive post, and 3 percent max voltage drop. Pure sine for electronics and medical loads; modified sine will buzz or kill them.

Where techs lose points on TE3

Good technicians miss these — not from lack of skill, but because the question is built to catch them.

• The slow cylinder. The stem says the hoist raises slowly, and the tempting answer is to raise the relief setting. Wrong. Flow controls speed, pressure controls force. Slow means you lost flow - engine rpm, PTO ratio, worn pump, restricted suction, bypassing cylinder seal, or a flow control set wrong. A relief setting only shows up when the load will not lift at all or stalls part way. ASE loves this pair and will always leave both answers on the page.
• Milky oil vs whining pump. Both are suction-side and both give you spongy controls, so techs blur them together. Cavitation is fluid flashing to vapor from restriction, cold thick oil, or a plugged strainer, and it whines and pits the pump. Aeration is air getting in through a loose suction fitting, low fluid, or a return line that pipes above the fluid level and foams the tank. Same neighborhood, different repair, and the distractor list will offer you the other one's fix.
• Sealant on the wrong fitting. JIC 37-degree flare and O-ring boss seal mechanically - the flare seat and the O-ring do the work. Tape or pipe dope on either one is a wrong answer and a contamination source. Only NPTF tapered threads get sealant. Related look-alike: torquing an ORB fitting by turns of the wrench instead of seating the O-ring against the boss face and locking the jam nut.
• Sizing an inverter or generator by watts and calling it done. A 2000 W inverter is not a 2000 W problem, it is a 190-amp DC problem, and a 160-amp alternator makes maybe 40 to 60 amps at curb idle. The correct answer usually involves high idle, a dedicated bank, or a bigger charging source - not just a bigger inverter. Same trap with motor loads: an inverter or gen set has to survive a 3x to 7x starting surge, so the continuous rating is not the number that decides it.

A study plan that works for TE3

Roughly two weeks of real preparation, in the order that actually builds on itself.

1. Days 1-4: Math and the pump/valve match. Drill GPM = (RPM x CID) / 231, HP = (GPM x PSI) / 1714, force = PSI x area, Hz = (RPM x poles) / 120, and inverter DC amps until you do them without a calculator. Then read one good pump-manufacturer catalog section (Muncie, Chelsea, or Parker will do) on open-center vs closed-center and fixed vs variable displacement. This block alone is worth more points than anything else you can do in four days.
2. Days 5-8: PTO and driveline install. Mounting styles, shift types, torque ratings intermittent vs continuous, gear lash and gasket shimming, interlock wiring, driveline angles and phasing, pump support. Pull an actual Chelsea or Muncie install manual and read the specs pages, not just the pictures. If you have a wet kit in the shop, go put your hands on the aperture and count the bolts.
3. Days 9-11: Reservoirs, plumbing, filtration, heat, and air. Sizing rules, inlet vacuum, strainer vs filter micron, hose velocity ranges by line type, JIC vs ORB vs NPTF sealing, bend radius and live length. Then the air side: supply tank, dryer, pressure protection valve, FMVSS 121. Air is a small slice of the test but it is easy points and you can own it in a day.
4. Days 12-14: Electrical generation plus mixed practice. Alternator idle output vs rated output, isolators, low-voltage disconnect, battery bank type, inverter and gen set sizing, bonding and GFCI. Then run mixed practice sets in timed blocks and force yourself to say out loud whether each question is a flow problem or a pressure problem before you pick. Review only the ones you missed.

Sample TE3 questions

Straight from the bank — answers highlighted, with the explanation underneath.