Why Ford Pro Power Onboard won't start RV air conditioner

I've seen this before, but now I have measurements to confirm my theory of failure

Hello Mike, my name is Tom,

I have an air conditioner on my slide-in camper with a Soft Starter installed, trying to run it off of the 2,000-watt Pro Power Onboard inverter in my 2024 Ford Super Duty Truck. The air conditioner fan runs, but the compressor won’t kick in. However the inverter doesn’t trip off.

This setup works fine plugged into shore power so I figured the Soft Starter must be properly installed. However, when I tried plugging in a generator bonding plug,the inverter instantly trips off due to overload. Any idea what’s happening?

Hey Tom,

I have a theory. Can you measure the Hot/Neutral, Hot/Ground and Neutral/Ground voltages on the Pro Power inverter outlet and send them to me?

Hey Mike,

Here you go. Thanks for taking a look.

Hot to Neutral is 114.4 volts

Hot to Ground is 70.2 volts

Ground to Neutral is 48.03 volts

Everyone

I think I know what’s happening. The Pro Power inverter doesn’t have a floating neutral. It’s something known as balanced power. Note that the neutral to ground measures 48 volts, while the hot to ground is 70 volts. Now, those 2 voltages are 180 degrees out of phase so they’re additive. You can see that 70 volts + 48 volts = 118 volts.

But the neutral and ground outputs are not floating like on an inverter generator and they’re not bonded like a large contractor generator. Essentially there’s a 48-volt inverter circuit and a 70-volt inverter circuit that are connected out of phase with each other. And their common tie point is connected to the ground conductor. That’s what causes the inverter to trip in overload mode when you plug in a generator Neutral/Ground bonding plug. The bonding plug shorts out the 48-volt neutral inverter circuit.

Why won’t the compressor run?

Many modern air conditioners and refrigerators have an open-ground detection circuit that won’t allow the compressor to operate if the ground and neutral wires aren’t bonded and within a few volts of each other. This is a safety mechanism that allows refrigerators to be plugged into a non-GFCI outlet in your kitchen. And apparently your RV rooftop air conditioner has the same open-ground safety circuit.

I don’t know of any simple workaround for this problem except to install an air conditioner without this open-ground detection circuit. But I’ll do a little more studying on your problem, so please stand by.

Let’s play safe out there… Mike