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Practice questions · 310S C

Diagnoses and repairs vehicle module communications systems

2 questions from this exam area, each with the full explanation shown — not just the answer. Free, no account.

10% of the exam

1

A Ford vehicle with a two-wire J1850 PWM network has one of the two network wires shorted to ground. What will happen?

  1. The entire network will fail, and no communication is possible.Answer
  2. The network will continue to operate normally on the one good wire.
  3. The network will fail, and the resistance will read 60Ω.
  4. All modules will short to ground and be destroyed.

Why that is right

Ford J1850 PWM is a two-wire differential network that requires both bus wires to read the differential signal. Shorting either wire to ground collapses the differential voltage, so the entire network loses communication.

Why the other options are wrong

  • The network will continue to operate normally on the one good wire. — J1850 PWM is not fault-tolerant single-wire; it cannot keep operating on one good wire because it needs the differential pair to communicate.
  • The network will fail, and the resistance will read 60Ω. — A 60Ω reading describes the termination of a high-speed CAN bus (two 120Ω in parallel), which is not how J1850 PWM is diagnosed and does not describe this fault outcome.
  • All modules will short to ground and be destroyed. — A short to ground on a bus wire disables communication but does not deliver destructive current to every module, so the modules are not destroyed.

2

What is a critical safety step that should be performed prior to servicing or disconnecting components in an integrated network?

  1. Disconnect the battery ground cable.Answer
  2. Honk the horn to test the battery.
  3. Disconnect the gateway module first.
  4. Disconnect the PCM first.

Why that is right

Disconnecting the battery ground (negative) cable removes power from the modules before service, preventing voltage spikes, shorts, or programming corruption that can occur when a powered module or connector is disturbed.

Why the other options are wrong

  • Honk the horn to test the battery. — Honking the horn is not a valid safety procedure for protecting network modules before service.
  • Disconnect the gateway module first. — Disconnecting the gateway module while the system is still powered risks damage and does not remove power the way disconnecting the battery ground does.
  • Disconnect the PCM first. — Disconnecting the PCM while it is still powered can cause voltage spikes and damage; the correct step is to remove power at the battery ground first.

Exam-style practice written to the published Red Seal standard. These are not real exam questions — the official Red Seal sample questions are deliberately excluded from everything we publish.

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