Automotive Service Technician exam: Electrical diagnosis

Key points

• Test voltage drop with the circuit loaded. Unloaded readings hide bad connections.
• Commonly accepted drop on a ground side is about 0.1 V per connection.
• Measure parasitic draw with an ammeter in series after modules go to sleep.
• An open circuit has full voltage across the break.
• Use a fused jumper, never a plain wire, when bypassing a component.

Study tip

If a question says "the circuit works but weakly", think high resistance and voltage drop.

Try these

1. Voltage drop tests must be done:

   1. With the circuit off
   2. With the circuit loaded
   3. With the battery removed
   4. Using an ohmmeter
   Show answer

   B. With the circuit loaded Current must flow.

2. Parasitic draw is measured:

   1. In parallel
   2. In series
   3. With a test light only
   4. At the alternator
   Show answer

   B. In series Ammeters go in series.

3. Across an open switch in a working circuit you would read about:

   1. 0 V
   2. Source voltage
   3. Half voltage
   4. Negative voltage
   Show answer

   B. Source voltage The whole drop is across the open.

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