310T Practical Skills and Training 3 — Questions and Answers
Question 1: A technician is using an inductive clamp ammeter to measure parasitic drain on a truck. Where should the clamp be placed for an accurate reading?
- Around the positive battery cable only
- Around both battery cables together
- Around only the negative battery cable (Correct answer)
- Around any individual circuit fuse leg
Correct answer: Around only the negative battery cable
Placing the clamp around the negative cable measures total current draw from the battery with all other loads disconnected, giving an accurate parasitic drain reading.
Question 2: While diagnosing a no-start condition on a diesel engine, the technician finds cranking RPM is adequate but no injector pulses are detected. The NEXT logical step is to check:
- Fuel pressure at the injection pump
- Crankshaft and camshaft position sensor signals (Correct answer)
- Glow plug resistance values
- Compression across all cylinders
Correct answer: Crankshaft and camshaft position sensor signals
The ECM requires valid crankshaft and camshaft position sensor signals to calculate injection timing and enable injector pulses.
Question 3: A multiplexed body controller on a coach is not responding to inputs. After confirming 12V power and ground are present, what should the technician check next?
- CAN bus termination resistance (should be 60 ohms at the DLC) (Correct answer)
- Fuse for the passenger lighting circuit
- Relay output voltages on the controller
- Sensor supply voltages from the ECM
Correct answer: CAN bus termination resistance (should be 60 ohms at the DLC)
A CAN bus with missing or open termination resistors will disrupt communication; the combined resistance at the DLC should read approximately 60 ohms for a properly terminated two-wire CAN.
Question 4: When using a diesel exhaust fluid (DEF) quality sensor scan tool reading, a value below the acceptable specific gravity range indicates:
- DEF is over-concentrated and will crystallize
- DEF is diluted with water and SCR efficiency will drop (Correct answer)
- The sensor is faulty and must be replaced
- The tank is near empty and must be filled
Correct answer: DEF is diluted with water and SCR efficiency will drop
Low specific gravity means the urea concentration is below the required 32.5%, reducing SCR catalyst efficiency and potentially triggering derate conditions.
Question 5: A technician performs a cylinder contribution test on a diesel engine. Cylinder 4 shows no RPM drop when cut out. This MOST likely indicates:
- Cylinder 4 injector is stuck open
- Cylinder 4 is not contributing to engine output (Correct answer)
- The fuel system has excessive air entrainment
- The EGR valve is stuck open
Correct answer: Cylinder 4 is not contributing to engine output
No RPM change when a cylinder is disabled means that cylinder was already not contributing power, pointing to a misfiring cylinder due to a bad injector, low compression, or valve issue.
Question 6: When replacing a diesel particulate filter (DPF) on a Class 8 truck, the technician must also:
- Replace the turbocharger compressor wheel
- Reset the DPF soot and ash load data in the ECM (Correct answer)
- Replace the EGR cooler simultaneously
- Perform a manual regen before installing the new DPF
Correct answer: Reset the DPF soot and ash load data in the ECM
After DPF replacement, the ECM must have its soot and ash accumulation counters reset so it can accurately track the new filter's status and schedule regeneration correctly.
Question 7: A coach alternator output test shows voltage is within spec at idle but drops significantly at high electrical load. This MOST likely indicates:
- A faulty voltage regulator allowing overcharge
- High resistance in the alternator output circuit or worn brushes (Correct answer)
- A sulfated battery accepting excessive charge current
- An open diode in the rectifier bridge causing AC ripple
Correct answer: High resistance in the alternator output circuit or worn brushes
High resistance in the charging circuit or worn brushes increases voltage drop under load, preventing the alternator from delivering adequate voltage when electrical demand is high.
310T Practical Skills and Training 3
A technician is using an inductive clamp ammeter to measure parasitic drain on a truck.
Where should the clamp be placed for an accurate reading?