ASE Practice Test #2 3 Flashcards

A vehicle's air conditioning system is blowing warm air. The compressor clutch is NOT engaging. The refrigerant charge is confirmed low. Why does a low charge prevent compressor engagement?

Answer: The low-pressure cutout switch (or pressure transducer) detects below-minimum system pressure and disables the compressor clutch as a protection measure to prevent compressor damage from running without adequate lubrication

Refrigerant carries compressor lubricating oil throughout the system. If the refrigerant charge is critically low, there is insufficient oil circulation to lubricate the compressor. The low-pressure cutout switch (or low-pressure transducer signal to the ECM) detects suction pressure below a minimum threshold and prevents the compressor clutch from engaging. This protects the compressor from running dry and seizing. The system must be leak-tested, repaired, and properly recharged to restore operation.

What is the function of a torque converter clutch (TCC) in an automatic transmission?

Answer: To mechanically lock the torque converter turbine to the pump at cruise speeds, eliminating fluid slippage and improving fuel economy

A torque converter uses fluid coupling, which allows efficient power transfer but involves some slippage. At highway cruise, this slip wastes fuel. The TCC (torque converter clutch) is a friction clutch inside the converter that mechanically locks the turbine to the converter housing when engaged. With the TCC locked, there is no slip, engine RPM drops slightly, and fuel economy improves significantly. The ECM controls TCC engagement based on speed, throttle, temperature, and other factors.

An oscilloscope waveform of a crankshaft position sensor shows a consistent missing tooth pattern (the waveform drops to zero for one cycle at a specific interval). What does this indicate?

Answer: This is the normal reluctor ring pattern — the missing tooth is the engineered reference gap that tells the ECM the crankshaft position reference point for TDC synchronization

Crankshaft position sensors read a reluctor ring (trigger wheel) with teeth evenly spaced around the circumference. Most systems have one deliberately missing tooth (or gap) on the ring. The CKP sensor produces a waveform that corresponds to each tooth passing. When the missing tooth passes, the waveform shows a larger gap. The ECM uses this reference gap to precisely identify crankshaft position relative to TDC. This is the normal, intended design — not a fault. A broken tooth would create an additional unexpected gap.

During an oil pressure test, a technician finds oil pressure is normal at idle but drops excessively during acceleration. What does this indicate?

Answer: Worn main or connecting rod bearings — excessive bearing clearance allows oil to bypass the bearings rapidly under higher load, dropping pressure when demand increases

Oil pressure at idle is relatively low-demand — even worn bearings can maintain some pressure at idle. As engine speed and load increase, the oil pump must supply more volume against the increased demand from all bearings. With worn bearings (excess clearance), the oil escapes too easily between the journal and bearing, and the pump cannot maintain pressure under high flow demands. This pattern — normal pressure at idle dropping under load — is a classic worn bearing symptom requiring engine rebuild or replacement.

A technician uses an infrared thermometer to check brake rotor temperature after a road test and finds one front rotor significantly hotter than the other. What does this indicate?

Answer: A dragging brake caliper on the hot side — a seized piston or caliper slide pin is not fully retracting, keeping the pad in partial contact with the rotor and generating heat even when brakes are not applied

After a road test under normal driving (not heavy braking), all four brake rotors should be at approximately equal temperature. A significantly hotter rotor indicates that brake is dragging — remaining in partial contact with the pad even when the brake pedal is not applied. The most common cause is a seized caliper piston that does not retract fully, or a corroded/binding caliper slide pin that prevents the caliper from releasing. Dragging brakes waste fuel, overheat pads and rotors, and accelerate wear.

What does a P0171 DTC (System Too Lean, Bank 1) combined with a P0174 DTC (System Too Lean, Bank 2) on a V6 or V8 engine suggest?

Answer: A global lean condition affecting both banks — likely a MAF sensor reading too low, a vacuum leak affecting both banks, or low fuel pressure

When lean codes appear on BOTH banks simultaneously, the cause is upstream of the fuel delivery split — it affects all cylinders. A MAF sensor reading less air than is actually entering (causing the ECM to under-fuel) affects all cylinders. A large vacuum leak (intake manifold gasket, throttle body gasket, large broken vacuum hose) introduces unmetered air that dilutes the mixture for all cylinders. Low fuel pressure from a failing pump or clogged filter affects all injectors. Single-bank lean codes point to bank-specific causes (O2 sensor, injectors, bank-specific vacuum leak).