Low voltage
Generator Producing Low Voltage
Low voltage means your generator is producing power, just less than your building needs. That is a different fault from no power at all, and from voltage that keeps moving. The cause is usually low engine speed, the AVR, weak excitation, an overload, a loose or burnt connection, or a winding or sensing fault, and we measure to find which one.
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- We measure before naming a part
- AVR tested separately from the alternator
- AVR adjusted only against a meter
- Onsite across Gauteng
A low but real voltage
Why is my generator voltage too low?
Something in the output path is not holding the voltage on the data plate. We take readings before we name a part, because dim lights can come from low voltage, low frequency or a bad connection on one phase, and each needs a different repair.
Engine speed comes first. If the set is not running at the speed that gives the right frequency, the voltage usually drops with it. Replacing the AVR while the engine is slow treats the result and misses the cause.
Please do not turn the AVR adjusters to bring the voltage up. The regulator is live while the set runs, and once the real fault is fixed the voltage can end up too high. If an adjustment is needed, we make it against a meter as part of the repair.
Measured, not guessed
The readings we take
Voltage: 230 V phase to neutral
South Africa’s standard low-voltage supply is 230 V between phase and neutral (NRS 048-2), and your appliances are built for it. We measure the generator’s output off load and on load, on every phase of a three-phase set, against that figure and the data plate.
Frequency at the same time: 50 Hz
The national grid runs at a nominal 50 Hz (South African Grid Code). We record the frequency alongside the voltage, because a slow engine pulls both down together.
Fault guide
Generator producing low voltage: what it can mean and what to do
Low voltage means the set is producing power, just less than it should. We check engine speed, the AVR, the connections and the alternator to find out why. This guide separates what you can safely notice from what a qualified technician tests. One symptom never proves that a particular part has failed.
Symptom, likely cause and repair
Find the line closest to what you see. The cause is the most likely one, not a diagnosis: testing confirms it before any part is replaced.
| What you see | Likely cause | Usual repair |
|---|---|---|
| Voltage low at every load | An AVR that is set too low or failing | AVR repairs |
| Voltage drops when a big load connects | Overload, or engine speed falling under load | Load and sizing check |
| Low voltage and low frequency together | Engine speed is low: fuel delivery or governor | Fuel system repairs |
| One phase lower than the others | Unbalanced load or a winding fault | Alternator repairs |
01
What you may notice
- Lights dim and appliances struggle on generator power
- The controller shows a voltage below the normal reading
- Voltage looks fine off load and drops when equipment connects
- One phase reads low on a three-phase set
02
Possible causes
Any of these can produce the same symptom. Testing decides which one applies.
- Engine speed below normal, which pulls frequency and voltage down together
- An AVR fault or a sensing problem
- Weak excitation or an alternator winding fault
- Overload for the size of the set
- A loose, corroded or burnt connection, often on one phase
03
What you can safely check
Look and note only, from outside the canopy. Do not open panels, covers, fuel lines or the cooling system.
- What voltage and frequency (Hz) does the controller show, off load and with load connected?
- Is the problem constant, or only when large appliances start?
- On three-phase sets, is one phase different from the others on the display?
- Have you added anything to the generator supply recently?
04
Stop using the generator and call if you notice
- Motors humming without starting, or getting hot
- A burning smell from the alternator, panel or wiring
- Voltage that falls steadily while the set runs
If it is safe to reach, stop the generator with its normal stop control or emergency stop, keep people away, and call a technician.
05
What a qualified technician typically tests
These are technician checks, not steps to try yourself. Several involve live electrical measurement, fuel or pressurised systems.
- Voltage and frequency off load and on load, recorded together
- Engine speed and governor response
- AVR output and sensing
- Alternator excitation and windings
- Connections on each phase, with the supply made safe
06
Common repair categories
The repair follows the test result. These are the services this symptom most often leads to.
07
What to send when you book
- Your suburb and how we reach the generator
- Make, model and kVA from the data plate (a photo is ideal)
- Fuel type: diesel, petrol or gas
- The voltage and Hz you saw, and whether it was off load or on load
- What was connected when the voltage was low
08
Related faults and services
Not sure which symptom fits? Try the generator symptom finder.
Safety before you check anything
- Exhaust contains carbon monoxide, which you cannot see or smell. Run a portable generator outdoors only, away from doors, windows and vents, and never run any generator in a house, garage or other enclosed space.
- A standby generator set to automatic can start at any moment, when the mains fails or on its exercise timer. Keep the canopy closed and stay clear of fans, belts and the exhaust.
- Generator output, the changeover and the control panel can be live, and some parts hold stored electrical energy after shutdown. Leave panels, covers and wiring closed for us to open.
- The engine, exhaust and coolant stay hot after running. Never open a radiator or coolant cap on a hot engine.
- Do not refuel a running or hot generator. If you smell gas at a gas-fuelled set, do not operate switches; leave the area and contact your gas supplier.
Carbon monoxide guidance: US Consumer Product Safety Commission: Generators and engine-driven tools (carbon monoxide).
Send the details on WhatsApp
This opens WhatsApp with a short message already typed: brand, model, capacity, symptoms and suburb. Fill in what you know and send it. Blank lines are fine.
- The data-plate photo is added inside WhatsApp after the chat opens: tap the attachment or camera icon and choose the photo. A link cannot attach the photo for you.
- Take the photo only when the generator is switched off and has cooled down. Do not reach over or into a running or hot generator, and keep away from the exhaust.
- A standby generator set to automatic can start by itself when the mains supply fails. If you are not sure how it is set, photograph the plate from a distance or send a photo of the whole set instead.
Typical repair costs for this fault
Published ranges, last reviewed October 2026. The callout is R400 – R700 in business hours and R700 – R1,400 after hours, and it is waived if you accept the quote. Your written quote after diagnosis is the price that applies.
- Voltage fluctuation diagnosis and repair
- R1,500 – R5,000
- AVR replacement
- R2,200 – R9,000
Step 9: book a diagnosis visit.
Related Pages
Regulators, alternators and related voltage faults
Common Questions
Questions about low generator voltage
What counts as low generator voltage?
Output that is present but below what your set should deliver, which is normally 230 V between phase and neutral in South Africa. We confirm it with a meter rather than judging by how dim the lights look.
Is low voltage the same as no power?
No. With no power, there is no usable output at all. Low voltage means a reading exists but it is too low.
Is low voltage always the AVR?
No. Engine speed, the load, the connections, the excitation and the windings can all cause it. The AVR is one possibility, and we test it alongside the others.
Can I turn the AVR screw to raise the voltage?
No. We make that adjustment during the diagnosis, against a meter. Turning it yourself can push the voltage too high once the real fault is fixed.
The voltage is fine with nothing on and low when we connect. Why?
The set cannot hold its voltage under load. We then test for overload and check the fuel supply, the frequency and the alternator.
Can one loose connection cause low voltage?
Yes, often on one phase. We find the joint and remake it properly rather than covering it up with an AVR adjustment.
Do you repair the alternator if the voltage is low?
Only when our tests show the alternator has failed. Many low-voltage jobs end at the AVR, the engine speed or a connection.
Still unsure what the fault is? Call or WhatsApp 082 057 4605 for onsite diagnosis.
Service Areas
Onsite visits across Gauteng
We come to your generator anywhere in Gauteng, 24/7. Choose your region for local suburbs, call-out details and the faults we see most there.
What to send before we attend
These details help us plan the visit and bring the right parts. Send what you can on WhatsApp; if you can’t find the model number, we still book the visit.
- Location
- The suburb or site, so the visit can be booked in the right area.
- Make and model
- The generator brand and model, if they are known.
- kVA
- The size printed on the data plate.
- Fuel type
- Diesel, petrol, LPG, natural gas, or dual-fuel.
- Controller model
- The module name on the panel, when it is visible.
- Fault or alarm
- What the set does, and any text on the screen.
- Photos
- The generator, the data plate, and the controller or alarm. Send these on WhatsApp.
Book a Visit
Send us the voltage you saw
If your controller shows a number, send it and say whether it was off load or with equipment connected. Please leave the AVR alone until we arrive.
- Call
- 082 057 4605
- Message us on WhatsApp
What happens next
- We reply with questions about the generator, or to book a visit.
- A technician comes to you. For a breakdown we arrive within 1 hour.
- The fault is diagnosed on site and you get a written quote. No work starts until you accept it.
Charlie, lead technician: Degree in Mechanical Engineering, Degree in Electrical Engineering, Diesel Plant Fitter. Generator specialists since 2000.