A vacuum pump that won’t start is usually down to a power or electrical fault, a tripped thermal overload, a seized pump (thick cold oil or internal corrosion), or a capacitor or wiring problem. Work through the simple checks before assuming the worst.
1. Power and supply
Check the obvious first: fuse, breaker, isolator, plug and that the correct voltage/phase is present. Three-phase pumps won’t run (or run backwards) on a lost phase.
2. Tripped thermal overload
Many pumps have a thermal protector that trips on overheating. Let the pump cool and reset it – but find out why it overheated.
3. Seized pump
Cold, thick oil or internal corrosion (from standing, or water in the oil) can seize a rotary vane pump. Try turning the pump by hand (isolated); if it’s seized, it needs a workshop strip-down.
4. Capacitor or wiring fault
A single-phase motor that hums but won’t turn often has a failed start capacitor. A quick check: set a multimeter to capacitance mode and test the capacitor against its rated µF value printed on the case – a reading of zero, or wildly off the rating, confirms it has failed. Capacitors are a low-cost, straightforward swap – replace like-for-like. Electrical faults beyond this (wiring, windings, control circuits) should be checked by a competent person.
Pump won’t start?
If it’s seized or electrical, our workshop can diagnose and repair it – tell us the model and what happens when you switch on. Email [email protected] or request a quote.
Frequently asked questions
Why won’t my vacuum pump turn on?
Usually a power or electrical fault, a tripped thermal overload, a seized pump from thick cold oil or corrosion, or a failed motor start capacitor. Check the supply and thermal reset first.
My vacuum pump hums but won’t turn – why?
A single-phase motor that hums without turning typically has a failed start capacitor, or the pump is mechanically seized. Have it checked by a competent person.
Can a seized vacuum pump be repaired?
Often, yes. A pump seized by thick oil, corrosion or worn parts can usually be freed, rebuilt with new wear parts and returned to service in a workshop.
Written by the Girovac technical team. Girovac Ltd has supplied and serviced industrial and laboratory vacuum equipment from its North Walsham workshop since 1983. Last updated: July 2026.