Why is my AC unit measuring 120 V at the contactor?

I am not an electrician. This is the first spring in my new house, and the AC unit outside is not turning on. The contactor pulls in, but nothing else fires up. I measured the voltage across the contactor with a multimeter, but it is only showing 119 V, not the ~240 I expected. Why would that be? Would a dual breaker with one bad side cut the voltage in half, maybe?

240 VAC residential service is almost always split single phase; if you measure across both “hot” feeds you’ll get ~240VAC, if you measure from one hot feed to neutral you’ll get ~120VAC.

In the parts of the world where you get 120V RMS as a single phase… Which answers the OP, as he is in Wyoming USA.

One side of the double circuit breaker for a 240 volt unit may have tripped - leaving just one side on. Fully turn off the double breaker, then turn it back on.

Note there may be a couple of breakers for this - one at the main panel and one outside near the unit.

Do you know that the A/C was working recently and has it been working OK for some time?

If so and you’re out there measuring voltages you’re enough of an electrician to try replacing the starting capacitor(s) on the outdoor unit. I did this two weeks ago, same no-start symptom; 3rd time in 8 years; $25-$35 versus a service call…

If you are truly measuring it across the contactor output terminals, it should either measure 240V or 0.

So, either you are measuring one leg to neutral, or you are looking at the wrong contactor (like, maybe the fan control).

The compressor contactor will be a big, beefy one, rated for 30-50A.