Okay, you've done some good troubleshooting steps already. Since the pulsating only happens when the AC is on, and you've ruled out the dimmer and loose connections, here's a breakdown of possibilities and what to check

Likely Culprits (with AC interaction)
Voltage Drop Under Load

The AC unit draws a significant amount of power. This can cause a voltage drop, especially if your wiring isn't sufficient for the increased load. LEDs are much more sensitive to voltage fluctuations than incandescent bulbs.

How to Test



Multimeter at the fixture

Using a multimeter, measure the voltage at the LED fixture while the AC is running. Ideally, you want to see around 120V (in North America). If it drops significantly below that (say, 110V or lower), you've confirmed a voltage drop.

Multimeter at the panel

Check the voltage at the breaker for the lights and for the AC unit when the AC is running. If the voltage drops significantly at the panel for either breaker, the issue could be with the main service coming into your house.

Possible Solutions



Upgrade Wiring

If the wiring to the lights or the AC is undersized, consider upgrading it to a thicker gauge. This is a job for a qualified electrician.

Dedicated Circuit

The AC unit might be overloading the circuit it's on. Having it on its own dedicated circuit can help.

Move Lights to a Different Circuit

Try moving the affected lights to a different circuit on your panel. If the pulsating stops, it indicates the original circuit is overloaded. This is a temporary fix; you still need to address the underlying issue.

Voltage Stabilizer

In some extreme cases, a voltage stabilizer/regulator can be installed to provide a more consistent voltage to the lights. This is a more expensive solution.

Neutral Wire Issues

A loose or corroded neutral wire can cause voltage fluctuations on various circuits, especially when a heavy load like an AC unit is running. This is dangerous and needs immediate attention.

Why it matters

A bad neutral return path can cause voltage from one circuit to "bleed" into another.

How to check

Visually inspect the neutral connections in your electrical panel. Look for signs of corrosion, burning, or loose wires. Carefully tighten the neutral connections (make sure the breaker is off first!).

Important Warning

If you are uncomfortable working in your electrical panel, do not attempt this. Call a qualified electrician immediately. Neutral problems can be very dangerous and cause fires.

Grounding Issues

Similar to a bad neutral, a faulty ground can also lead to voltage fluctuations.

Check grounding connections

Make sure the ground wires are securely connected in the panel and at the light fixture.

Electrical Noise/Harmonics

The AC motor could be generating electrical noise or harmonics that are interfering with the LED drivers. LED drivers are sometimes more sensitive to this than older lighting technologies.

This is harder to diagnose. Try different LED Bulbs

As you mentioned, this is a good next step. Some LEDs are better at filtering out electrical noise than others. Look for LEDs specifically designed for compatibility with dimmers and appliances.

Line Filter

An electrician might be able to install a line filter to reduce the noise on the circuit.

Less Likely, But Possible



AC Unit Problems

While less likely since you said it only happens when the AC is on, a failing capacitor in the AC unit could be causing voltage fluctuations. This is more likely to cause the AC unit itself to have problems (starting difficulties, etc.).

Have the AC unit inspected

A qualified HVAC technician can check the capacitors and other components of your AC unit.

Utility Company Issues

Rare, but sometimes the problem originates with the power company's equipment.

Contact the utility company

If you suspect this, call your electric utility and report the issue. They may be able to monitor the voltage coming into your house.

Important Safety Reminders



Turn off the power

Whenever working with electrical wiring, always turn off the power at the circuit breaker.

Work safely

If you're not comfortable working with electricity, hire a qualified electrician. Electrical work can be dangerous.

Test after any changes

After making any wiring changes, use a multimeter to ensure that the voltage is correct and that there are no shorts.

Don't overload circuits

Make sure that you're not overloading any circuits by plugging too many appliances into them.

Troubleshooting Steps (in order of ease/likelihood)


Change the LED Bulbs

Try different brands/types of LED bulbs. Look for bulbs specifically designed for dimmer compatibility and appliance load.2.

Measure Voltage at the Fixture

Measure the voltage at the affected light fixture while the AC is running*. See if it's significantly lower than 120V.3.

Measure Voltage at the Panel

Measure the voltage at the breakers for the lights and the AC.4.

Inspect Neutral and Ground Connections

Visually inspect the neutral and ground connections in the panel.5.

Try a Different Circuit

Temporarily move the lights to a different circuit to see if the problem goes away.6.

Call an Electrician: If you're uncomfortable with any of these steps, or if you suspect a serious wiring issue, call a qualified electrician. They can diagnose the problem and make the necessary repairs.Good luck! Pulsating lights can be annoying, but with systematic troubleshooting, you should be able to find the cause and fix it. Flag for review