How to Test an RV Air Conditioner Capacitorby Mark Corgan
RV air conditioners have two capacitors: a motor run capacitor and a motor start capacitor. A motor run capacitor is used in the blower fan circuit while the motor start capacitor is used in the compressor circuit. While each capacitor serves a different purpose, each is testable in the same manner. The two types of tests are resistance testing and capacitance testing. Resistance testing provides a quick indication of a capacitor fault. Capacitance testing provides a more accurate reading comparable to the capacitor's specifications.
Turn off the main AC circuit breaker to the RV. The circuit breaker is located in the RV's electrical load center. Remove the shore power AC connection to the RV if it is connected.
Climb on to the RV roof and remove the air conditioner protective housing by unscrewing all Phillips head screws located around the base of the housing.
Locate the motor run and start capacitor enclosure. The enclosure is typically located near the top right corner of the air conditioner assembly when facing towards the front of the RV. It may also have a wiring diagram sticker on it. Remove the enclosure cover screws and cover.
Inspect the enclosure for two capacitors. The motor run capacitor is typically a silver oval-shaped canister two to three inches in length. The motor run capacitor is either black or silver, cylindrical in shape and is three to four inches in length.
Discharge each capacitor by shorting the electrical terminals located on the top of the capacitor using the flat blade screwdriver.
Remove the electrical leads from the motor run capacitor, taking note of what terminal each wire is connected to.
Measure the capacitance of the motor run capacitor by switching the multimeter to capacitance mode and place the positive lead (red) on positive or "+" terminal of the capacitor and the negative (black) lead on the negative or "-" terminal.
Compare the reading to the value printed on the side of the capacitor. If it is out of range, replace the capacitor. Repeat Steps 6 and 7 to test the motor start capacitor.
Switch the multimeter to Ohms mode. Place the positive lead (red) on the positive or "+" terminal of the capacitor and the negative (black) lead on the negative or "-" terminal.
Check to see if the resistance measurement gradually increases to near infinity. If it does not, the capacitor is leaky and requires replacement. If there is no resistance --- a zero reading --- the capacitor is shorted and requires replacement. If there is no resistance reading, the capacitor has an open circuit and requires replacement.
Repeat Steps 1 and 2 for the motor start capacitor.
- A telltale sign of a bad capacitor is bulging at the ends. This means the capacitor has overheated and requires replacement regardless of what the meter readings are.
Things You'll Need
- Insulated flat blade screwdriver
- Multimeter with capacitance setting
- Battery-powered electric drill
- #2 Phillips head bit
- When discharging the motor run and motor starting capacitors, use an insulated screwdriver or other insulated tool. Otherwise, there is the risk of serious electrical shock which can be fatal.
- Take caution when climbing or walking on the RV roof. Most RV roofs are strong enough to hold an average person's weight but some ultra-light models might not. If in doubt, contact the RV manufacturer to learn about what the maximum roof load is.
Based in northern Nevada, Mark Corgan has been writing RV-related articles since 2007. His articles have appeared on ModMyRV.com and other websites. Corgan holds an Associate of Science in electronics engineering from ITT Technical Institute. An information technologist by trade, he also has a technical writing background and writes how-to guides on blogging technologies.