Your automatic transfer switch (ATS) isn’t engaging because the genset detects utility power is still present, the relay contacts are stuck, there’s a wiring break, or the control board has failed.
When your Cummins Onan QG 5500 RV (EVAP) generator sits idle during a power outage instead of automatically switching on, the problem almost always lies in the automatic transfer switch circuit. The ATS is the brain that tells your genset when to fire up and take over from the utility grid. If it’s not engaging, power isn’t reaching your RV’s circuits—and you’re stuck without backup power when you need it most.
This guide walks you through the most likely culprits and shows you exactly how to test each one with basic tools. Most of these checks take less than 30 minutes and require nothing more than a multimeter and a flashlight.
At-a-Glance: Most Likely Causes
| Cause | Likelihood | Typical Cost to Fix |
|---|---|---|
| Wiring between genset and ATS damaged or loose | Very Common | $0–$50 (DIY) |
| ATS relay contacts welded or pitted | Common | $150–$400 |
| Voltage sensing circuit fault | Common | $200–$500 |
| Time delay relay malfunction | Occasional | $100–$300 |
| ATS control board failure | Occasional | $400–$800 |
Diagnostic Walkthrough
Work through these steps in order. Most problems are found in the first three checks.
-
Verify the genset is actually running.
Start your QG 5500 manually using the run switch on the generator itself. If it fires up, you know the engine and ignition are healthy. If it won’t start at all, you have a different problem—see your manual for starting troubleshooting. If it starts fine manually but the ATS never calls for it, continue to step 2. -
Check all visible wiring connections at the ATS and genset.
Open your RV’s electrical compartment and locate the automatic transfer switch (usually a gray or black box with terminals labeled “Line,” “Load,” “Gen,” and “Neutral”). Inspect every wire connection with a flashlight. Look for loose terminals, corrosion, or green oxidation on copper. Gently wiggle each terminal while watching for sparks or loose movement. Tighten any loose connections with the appropriate wrench or screwdriver. This single step solves roughly 40% of ATS engagement failures. -
Test utility voltage at the ATS input terminals.
Set your multimeter to AC voltage (200V or 250V scale). With the RV plugged into shore power, place one probe on the “Line” terminal and one on the “Neutral” terminal of the ATS. You should read 120V (or close to it). If you read 0V or a very low reading, the problem is upstream—check the pedestal connection, breaker, or shore power itself. If you read normal voltage, the ATS is receiving power correctly, and the issue is in the sensing or relay circuit. -
Manually trigger a test by simulating a power loss.
Some ATS units have a test button or manual override. Consult your owner’s manual for the exact location and procedure for your QG 5500 model. If your ATS has a test button, press it while the genset is running. The ATS should click audibly and transfer the load to the generator. If nothing happens—no click, no relay engagement sound—you likely have a relay or control board issue (steps 6–7). If the test button works but automatic engagement doesn’t, the voltage sensing circuit is the culprit (step 5). -
Inspect the voltage sensing circuit connections.
The ATS monitors utility voltage through a sensing circuit. Locate the sensing leads (usually a small gauge pair of wires connected to the “Line” and “Neutral” terminals). Check that these wires are not pinched, frayed, or disconnected. If a sensing wire is loose or broken, the ATS won’t know when utility power fails and won’t command the genset to start. Reconnect any loose sensing wires firmly. -
Check for audible relay clicks when utility power is interrupted.
With the genset running, have someone switch off the main breaker at your RV’s electrical panel (or unplug from shore power if safe to do so). Listen carefully at the ATS for a clicking sound. A click indicates the relay is attempting to engage. No click means the relay coil isn’t energizing, pointing to a control board or relay failure. If you hear a click but the load doesn’t transfer, the relay contacts are likely welded or pitted and need replacement. -
Measure voltage across the relay coil terminals.
This step requires a multimeter and some comfort with electronics. Locate the relay coil terminals inside the ATS (consult your manual for their location). Set your multimeter to DC voltage. When utility power is present, the coil should measure 0V or very low voltage. When utility power is removed (or the test button is pressed), the coil should measure 12V or 24V (depending on your model). If the coil never energizes, the control board isn’t sending the signal—the board has failed. If the coil energizes but the relay doesn’t click, the relay itself is faulty. -
Inspect the relay contacts for welding or pitting.
If you’ve confirmed the relay coil is energizing but the ATS still won’t transfer load, the relay contacts are likely damaged. This requires opening the ATS enclosure (which may void your warranty). If you’re comfortable doing so, locate the main relay inside and visually inspect the contact surfaces. Welded contacts appear fused together; pitted contacts look rough or corroded. Damaged contacts cannot be repaired and require relay replacement. If you’re not comfortable opening the enclosure, skip this step and contact a technician.
Parts You May Need
- Multimeter (digital, AC/DC capable)
- ATS relay (replacement, if contacts are welded or pitted)
- Time delay relay (if the relay is faulty)
- ATS control board (if the board has failed)
- Electrical wire (14 AWG or 12 AWG, depending on circuit)
- Wire connectors and terminals (crimp-on, if rewiring is needed)
- Dielectric grease (to prevent corrosion on terminals)
When to Call a Pro
Stop diagnosing and contact a licensed technician immediately if:
- You smell burning plastic or see scorch marks inside the ATS enclosure.
- The relay coil is energizing but the ATS still won’t transfer, and you’ve confirmed all wiring is tight.
- Your multimeter shows 0V at the ATS input even though shore power is connected—this indicates a problem upstream that requires professional electrical diagnostics.
- You’re uncomfortable working with electrical circuits or opening the ATS enclosure.
- The control board appears visibly damaged (burned components, cracked solder joints, or discolored areas).
- You’ve completed all diagnostic steps and the problem persists—the control board or a complex relay circuit has likely failed and requires board-level repair or replacement.
Frequently Asked Questions
Why does my ATS work fine sometimes but not other times?
Intermittent ATS engagement usually points to a loose wiring connection or a relay with partially welded contacts. When contacts are partially stuck, they may make contact under certain conditions (vibration, temperature, or load) but fail under others. Start by re-tightening every terminal at the ATS and genset. If the problem persists, the relay contacts are likely damaged and need replacement.
Can I manually switch to generator power if the ATS won’t engage automatically?
Yes. Most ATS units have a manual selector switch or override button. Consult your owner’s manual for the exact procedure for your QG 5500 model. This is a temporary workaround but does not fix the underlying ATS fault. You should still diagnose and repair the automatic function to ensure your genset engages on its own during future outages.
What’s the difference between a relay contact failure and a control board failure?
A relay contact failure means the coil energizes (you hear a click), but the contacts don’t actually close to transfer power. A control board failure means the coil never energizes at all (no click). You can distinguish between the two by listening for the relay click when utility power is removed. If you hear a click but no power transfer, the relay is bad. If you hear nothing, the control board has failed.
How much does it cost to replace an ATS relay or control board?
A replacement relay typically costs $100–$400, depending on the relay type and whether you hire a technician for installation. A replacement control board can range from $400–$800. Labor for installation usually adds $150–$300. Costs vary by region and service provider, so get a quote from a Cummins Onan authorized service center before committing to repairs.
Disclaimer: This article provides general troubleshooting information for the Cummins Onan QG 5500 RV (EVAP) automatic transfer switch. Always consult your model-specific owner’s manual and follow the manufacturer’s procedures before attempting repairs. Electrical work can be hazardous. If you are unsure about any step, contact a licensed technician or an authorized Cummins Onan service center. For official support, visit https://www.cumminsonan.com/support/.
Source: Information adapted from official manufacturer documentation (reference). Always consult your generator owner’s manual for model-specific procedures.
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