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Practical Questions

Electric-driven Air Compressor Troubleshooting

Suppose this electric-driven air compressor refuses to start when the switch is in the “Auto” position, but starts up immediately when the switch is placed in the “Hand” position.

Electric-driven Air Compressor

Electrical Air Compressor

The first test performed by a technician is to measure AC voltage between test points A and F with the switch in the “Auto” position. There, the meter registers 117 volts AC. You are then called in to help:

Electric-driven Air Compressor Troubleshooting

Identify the likelihood of each specified fault for this circuit. Consider each fault one at a time (i.e. no coincidental faults), determining whether or not each fault could independently account for all measurements and symptoms in this circuit.

Identify the below mention faults are possible or impossible?

  • PSHH failed open
  • PSH failed open
  • PSL failed open
  • “Hand” switch position failed open
  • “Auto” switch position failed open
  • OL contact failed open
  • Auxiliary “M” contact failed
  • Contactor “M” coil failed open

Also, comment on whether or not the initial test between points A and F was a useful one (i.e. did it provide any new information to help diagnose the problem?).


The initial test between points A and F was useless. We already knew from the symptom of the compressor running in “Hand” but not in “Auto” that the fault must be an open, and it must lie between the Hand/Off/Auto switch and test point A somewhere.

An open fault anywhere between points A and F would of course drop the full control voltage, so the measurement of 117 volts AC should come as no surprise.

  • PSHH failed open – Impossible
  • PSH failed open – Possible
  • PSL failed open – Possible
  • “Hand” switch position failed open – Impossible
  • “Auto” switch position failed open – Possible
  • OL contact failed open – Impossible
  • Auxiliary “M” contact failed – Impossible
  • Contactor “M” coil failed open – Impossible

Share your answers with us through the below comments section.

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Credits: Tony R. Kuphaldt

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Electric-driven Air Compressor Troubleshooting

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