Okay, this is a tricky situation, and potentially dangerous. Let's break down what you're describing and think through the possibilities.

Safety First



HIGH VOLTAGE IS DANGEROUS! This is not a drill. 310V, even 240V, can kill you. Work on this only with the power OFF, verified with a meter. Lockout/Tagout

Properly lock out and tag out the disconnect before doing any work. Make sure no one can accidentally re-energize the circuit while you're working.

If you're uncomfortable or unsure, call a qualified electrician. Seriously. It's better to be safe than sorry.

Analyzing the Readings



310V Phase-to-Phase

This is significantly higher than the nominal 240V you'd expect. This is a big red flag. Overvoltage can damage equipment and cause fires.

240V to Ground/Neutral on One Line, 120V on the Other

This imbalance is also a problem. In a properly wired 240V circuit, you should see approximately 120V from each leg to neutral/ground. The 240V to ground on one leg indicates something is going wrong with your voltage divider which may be in the Controller box.

Possible Causes and Troubleshooting Steps (with the power OFF, of course)


Neutral Problem

This is the most likely suspect.

Check the Neutral Connection

Carefully inspect the neutral connection at the disconnect, the controller, and the well pump. A loose, corroded, or broken neutral connection can cause voltage imbalances and overvoltage. It's also possible the neutral is open somewhere along the circuit.

Neutral/Ground Bond

Make sure the neutral is properly bonded to ground ONLY at the service entrance (main panel). Multiple neutral-ground bonds can create circulating currents and voltage issues. This is a common mistake.2.

Controller Issue (Capacitor Bank, Relays)



Capacitor Bank

You're right to suspect this. A failing capacitor can definitely cause voltage fluctuations and imbalances. If you can safely and carefully disconnect the capacitor bank (with the power OFF!), do so and re-measure the voltages.

Relays

Relays can also cause issues if they are failing or sticking.

Controller Wiring

Double-check all wiring connections within the controller. Make sure everything is clean, tight, and properly terminated. Look for signs of overheating, discoloration, or damage.3.

Wiring Errors



Incorrect Wiring

Verify the wiring is correct at the disconnect, the controller, and the well pump. It's easy to make a mistake, especially if you're not familiar with the specific controller.

Short Circuits

Check for any potential short circuits between conductors or to ground.

Loose Connections

As mentioned before, loose connections are a common cause of voltage problems.4.

Utility Problem



High Utility Voltage

It's possible the utility company is providing voltage higher than the nominal 240V. Call them and report the overvoltage issue. They will be able to check their equipment.5.

Voltage Meter Error

While unlikely, confirm your voltage meter is accurate on a known voltage source.6.

Transformer issue

If you have a step-up or step-down transformer on your property, make sure the transformer is not damaged or failing.

Troubleshooting Procedure (After confirming safety, power OFF)


Visual Inspection

Carefully inspect all wiring, connections, and components for any signs of damage, overheating, or loose connections.2.

Continuity Testing (Power OFF!)

Use a multimeter to check the continuity of all conductors (hot, neutral, ground) from the disconnect to the well pump. Look for any open circuits.3.

Isolation

If possible, isolate the controller from the rest of the circuit (disconnect the wires going to the well pump) and measure the voltages at the controller output. This will help determine if the problem is within the controller or further downstream.4.

Component Testing (Power OFF!)

If you're comfortable doing so, test the components within the controller (capacitors, relays) using a multimeter.5.

If all else fails, consult an electrician.Important Notes



Pictures Will Help

When you provide pictures, please try to include clear shots of the wiring diagrams (if any), the connections, and any labels on the components.

Controller Model

Knowing the model number of the controller would be helpful, as we might be able to find a schematic or troubleshooting guide online.*

Well Pump HP and Voltage: What is the well pump rated at? This can help determine if the voltage is not adequate.The fact that you have 310V phase to phase is very concerning, and you definitely need to find the source of that overvoltage. Be methodical, be safe, and don't hesitate to call a professional if you're not comfortable. Flag for review