Generator output voltage is too low or too high

Symptoms

• Lights connected to the generator are dim or unusually bright
• A multimeter reading at the outlet is well off from 120V
• Electronics behave oddly or shut off when running on generator power
• Engine speed sounds too slow or too fast compared to normal

Likely causes

• CommonEngine running at the wrong speed (RPM)Output voltage and frequency are directly tied to engine RPM; a governor issue or wear can throw this off.
• CommonOverloaded generator dragging voltage downAsking for more power than rated causes voltage to sag under load.
• PossibleFailing automatic voltage regulator (AVR)This component actively regulates output; failure can cause voltage well outside normal range.
• PossibleWorn brushes (on brushed generator heads)Worn brushes reduce the excitation needed for proper voltage regulation.

What to do

1. Reduce the connected load to well under the generator's rated capacity and see if voltage returns to normal.
2. Check the engine's idle/running speed against the manufacturer's spec (most are set for 3600 RPM for 60Hz output) using a tachometer if available.
3. Inspect and adjust the governor linkage if the engine speed is clearly off from spec.
4. Measure output voltage at an outlet with a multimeter, both unloaded and with a light load, to see how far off it is.
5. If speed and load are both fine but voltage is still wrong, suspect the AVR.
6. On brushed generator heads, check brush wear and replace if worn short.
7. Persistent wrong voltage after ruling out speed and load issues should be diagnosed by a generator repair shop, since running equipment on bad voltage can damage it.

Tools and parts

• Multimeter
• Tachometer (if available)

When to call a pro

Have a generator repair shop diagnose voltage problems that persist after checking load and engine speed, since incorrect voltage can damage connected equipment.

Safety