H7 Transit Bus — HVAC — practice test

High-yield H7 topics

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

• Rooftop package systems and capacity control: Thermo King (X426/X430 open-drive recips, Intelligaire controls) and Carrier 05G/05K style units. Know cylinder unloaders and hot-gas bypass - a bus A/C does not cycle a clutch the way a car does, it unloads banks to match load. An unloader stuck unloaded reads as low suction plus weak cooling and gets misdiagnosed as a tired compressor every time.
• Charging by weight, not by sight glass. Transit systems carry a big charge (roughly 15-30 lb of R-134a depending on the unit) with a receiver holding reserve, so the glass can run clear while the system is undercharged and bubble at high ambient with a correct charge. Recover, evacuate to a deep vacuum (500 microns, hold it), weigh in the exact spec charge. Expect questions on why the sight glass lies on a TXV/receiver system.
• TXV diagnosis by superheat and subcooling with a live air split. Bulb placement, clamp contact, insulation, and the external equalizer line all get tested. Know the pattern differences: restricted drier or liquid line = frost at the restriction, low suction, low-ish head; TXV flooding = low superheat and a wet suction line; TXV starved = low suction, high superheat, warm return air.
• Noncondensables and condenser airflow as the real high-head causes. Compare static (equalized) pressure to the PT chart at the actual ambient soak temperature - reads high means air in the system, and the fix is recover and evacuate, not bleed off refrigerant. Also know hydraulic-motor and electric condenser fan drives on the roof stack, and the coil sandwich (condenser stacked with CAC and radiator) that kills capacity when it is packed with brake dust and leaves.
• The heat side: engine coolant loops to roof heater cores and the driver defroster, auxiliary/booster coolant pumps, water valve control, and reheat/dehumidify mode. Reheat is the mode techs never see in a car - the unit cools to drop moisture, then reheats with coolant to hold cabin setpoint on a 55 degree wet day. A stuck water valve or a dead booster pump shows up as no heat at the rear while the front blows warm.
• Hybrid and battery-electric HVAC: high-voltage electric scroll compressors, orange cable, service disconnect and lockout, and POE oil - never PAG. PAG is hygroscopic and conductive enough to trip isolation faults and destroy an HV compressor. Know the insulation-resistance check and that a shorted compressor sets an isolation/leakage fault, not a refrigerant code.

Where techs lose points on H7

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

• "Bubbles in the sight glass = add refrigerant." This is the most reliable wrong answer on H7. On a receiver/TXV system, bubbles show up with high head, low subcooling, a restricted drier, or a hot condenser with no airflow. If Tech A says add refrigerant and Tech B says check subcooling and condenser airflow, Tech B is right. Charge by weight, always.
• High head pressure read automatically as overcharge. Check equalized static pressure against the PT chart at soak temperature first - static above chart means noncondensables, and the answer is recover and evacuate, not bleed off a few pounds. Venting refrigerant is never a correct answer on this test, regardless of charge size.
• "Compressor is noisy and weak, replace it." On a bus the noise is usually the jackshaft, drive gearbox, an idler, or a dry clutch bearing, and the weak cooling is usually an unloader stuck unloaded or a suction-side restriction. The look-alike answers offer compressor replacement before anyone has checked unloader operation or the suction screen.
• Evaporator icing blamed on the freeze switch, and HV compressors oiled like a car. The freeze switch only reports - real icing causes are plugged return-air filters, a slow or dead blower, a blocked coil, or low refrigerant flow, so bypassing the switch is the trap and restoring airflow is the fix. Same logic on lubricant: "it is a 134a system so PAG is fine" is wrong on any high-voltage electric compressor. It must be POE.

A study plan that works for H7

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

1. Days 1-4: Pressure/temperature relationships, cold. PT chart for R-134a, superheat and subcooling math, static pressure vs ambient. Then build a table of the classic gauge patterns (low charge, overcharge, air in system, restricted drier, TXV starved, TXV flooding, weak compressor or unloader) with the return-air split for each. If you cannot fill that table from memory, nothing else you study will stick.
2. Days 5-8: Live the roof unit. Pull the service manual for whatever your fleet runs (Thermo King T-series/Intelligaire or Carrier AC353) and read the capacity-control section, the sensor list, and the fault-code table. Then walk a real bus: find the receiver, the drier, both TXVs, the equalizer lines, the freeze and low-pressure switches, the water valve, the booster pump, the fresh-air damper. Trace them, do not just point at them.
3. Days 9-11: Heat side and controls. Coolant flow from engine to roof cores and defroster, reheat/dehumidify logic, damper actuators, blower speed control, and how the control head decides cool vs reheat vs vent. Then EPA 609: recovery, evacuation to 500 microns, J2788/J2843 equipment, leak detection, charge-by-weight procedure. These two blocks are roughly 14 of the scored questions and they are the cheapest points on the test.
4. Days 12-14: Practice questions only, at exam pace, and audit every miss. Write down WHY the wrong answer looked right - that is the actual study product. Hit hybrid and BEV HVAC specifically (POE vs PAG, HV disconnect, isolation resistance) since a question or two will land there and most techs have never touched it.

Sample H7 questions

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