Your Generac RTS transfer switch is receiving a signal to switch but the mechanical contacts are either stuck or the control board isn’t commanding the final transfer—a fixable problem that usually doesn’t require a technician if caught early.
What’s Happening?
When your Generac RTS transfer switch hums but fails to actually transfer power from utility to generator (or vice versa), the control board is likely energizing the relay coil, but the physical contacts aren’t moving or aren’t moving far enough to make a solid connection. This is different from the switch being completely silent—the hum tells you the control board is trying to do its job, but something mechanical or electrical is blocking completion.
The good news: this symptom is usually traceable to one of five specific failure modes, and most can be diagnosed with basic tools and a multimeter.
At-a-Glance: Most Likely Causes
| Cause | Likelihood | Typical Cost to Fix |
|---|---|---|
| Relay contacts stuck or corroded | Very Common | $ |
| Mechanical interlock binding or misaligned | Common | $–$$ |
| Generator voltage too low to trigger transfer | Common | $–$$$ |
| Time delay relay stuck or not timing out | Occasional | $$ |
| Control board relay driver weak or failing | Occasional | $$$ |
Diagnostic Walkthrough
Work through these steps in order. Stop when you identify the problem.
- Listen and observe during a transfer attempt. When the utility power drops (or you manually trigger a test transfer), note exactly when the hum starts, how long it lasts, and whether it stops abruptly or fades. Does the hum happen on every attempt? Does it occur when switching back to utility? Document this—it narrows the failure mode significantly.
- Check generator voltage at the transfer switch input terminals. Use a multimeter set to AC voltage. With the generator running under load (or simulated load), measure the voltage at the generator input terminals of the RTS switch. It should read between 220–240V for a 200A unit or 110–120V for a 100A unit. If it reads below 200V (200A) or below 100V (100A), your generator is not producing sufficient voltage to trigger the transfer logic. Low voltage is a common culprit and often points to a weak generator AVR (automatic voltage regulator) or an overloaded generator.
- Verify utility voltage is present and stable. Measure the utility input terminals. Voltage should be steady at 220–240V (200A) or 110–120V (100A). If utility voltage is fluctuating wildly or dropping below acceptable levels, the control board may be confused about which source to use. Unstable utility power can prevent the switch from committing to a transfer.
- Perform a manual transfer test with the generator running. If your RTS has a manual override or test mode (consult your manual for the exact procedure), engage it while the generator is running at full speed. Listen for the hum and watch for any visible movement of the switch lever or contacts. If the lever moves slightly but not fully, or if you hear a grinding sound, the mechanical interlock is likely binding. If there’s no movement at all, the relay coil may not be energizing or the contacts are severely stuck.
- Inspect the relay coil for signs of burn or corrosion. Open the RTS enclosure (turn off both utility and generator first). Locate the main relay coil—it’s a cylindrical component with wire leads. Look for discoloration, burn marks, or a burnt smell. If the coil appears scorched or smells like burnt insulation, it has likely failed and needs replacement. A weak coil may energize (causing the hum) but lack the force to fully actuate the contacts.
- Check for mechanical obstruction or corrosion on the contact arms. With power off, gently try to move the main contact arm by hand (if accessible and safe). It should move freely with light pressure. If it’s stiff, stuck, or makes grinding sounds, corrosion or debris is binding the mechanism. Look for white or green oxidation on the contact surfaces. Light corrosion can sometimes be cleaned with a dry cloth or fine contact cleaner; heavy corrosion usually requires contact replacement.
- Measure the time delay relay output with a multimeter. If your RTS has a visible time delay relay (a small cylindrical or cube-shaped component on the control board), set your multimeter to continuity or resistance mode and test the relay terminals. The relay should switch states (change from open to closed or vice versa) when power is applied and removed. If the relay is stuck in one state, it will prevent the transfer from completing. A stuck time delay relay usually requires replacement.
- Test the control board relay driver with a load test. This is the most advanced step and may require a technician. If all previous checks pass but the switch still hums without transferring, the relay driver circuit on the control board may be weak. You can perform a rough test by measuring the voltage across the relay coil terminals while a transfer is being attempted. If voltage is present but very low (below 18V for a 24V coil), the driver is likely failing. The control board will need replacement or professional repair.
Parts You May Need
- Main relay coil (RTS-specific; verify part number from your manual)
- Contact kit or contact assembly (for corroded or pitted contacts)
- Time delay relay module
- Control board (if relay driver is confirmed failed)
- Contact cleaner or electrical cleaner spray
- Multimeter (digital, with AC voltage and continuity modes)
When to Call a Pro
Stop troubleshooting and contact a licensed electrician or Generac service technician if:
- The relay coil shows burn marks or smells burnt. Coil replacement requires safe electrical disconnect and proper testing of the control board.
- The contact arm is stuck and won’t move even with gentle hand pressure. Forcing it risks damaging the mechanism further.
- Generator voltage is consistently low (below 200V on a 200A unit). This points to generator failure, which requires specialized equipment to diagnose.
- You measure voltage across the relay coil during a transfer attempt but the contacts don’t move. The relay driver on the control board is likely failing and the board needs replacement.
- You’ve completed all diagnostic steps and the switch still hums without transferring. At this point, the control board or a combination of internal failures requires professional replacement or repair.
Frequently Asked Questions
Why does the switch hum but not transfer?
The hum is the relay coil energizing—the control board is sending power to the coil. However, the physical contacts are either stuck in place due to corrosion, mechanically blocked by a binding interlock, or not receiving enough force from a weakened coil. The control board is trying, but the mechanical or electrical path is broken.
Can I clean the contacts myself?
Light surface corrosion on contacts can sometimes be cleaned with a dry cloth or electrical contact cleaner spray, but only if the switch is fully powered down and isolated from both utility and generator. Heavy corrosion, pitting, or arcing damage requires contact replacement. If you’re not comfortable working inside an electrical enclosure, have a technician do it.
What if the generator voltage is low—does that cause humming?
Yes. If generator voltage is below the threshold needed to trigger the transfer logic (typically around 200V for a 200A RTS), the control board may repeatedly attempt to energize the relay coil without committing to a full transfer. This can sound like a stuttering or continuous hum. Check your generator’s AVR and load balance first.
How long should a transfer take?
A normal transfer from utility to generator (or back) should complete within 1–3 seconds once the control board detects the need to switch. If the switch is humming for longer than a few seconds without completing the transfer, a mechanical or electrical fault is present.
Disclaimer
This article provides general troubleshooting information for homeowners and small contractors. Always consult your Generac RTS Transfer Switch owner’s manual and installation guide for model-specific procedures, safety warnings, and electrical specifications. Transfer switches handle high-voltage circuits; if you are not confident working with electrical equipment, contact a licensed electrician. Improper diagnosis or repair can result in electrical shock, equipment damage, or loss of backup power during an outage.
Source: Information adapted from official manufacturer documentation (reference). Always consult your generator owner’s manual for model-specific procedures.
Leave a Reply