Testing an RV Air Conditioner Capacitor

Lockout, discharge, isolate

1. Kill 120V to the roof unit (coach AC breaker and pedestal or generator as needed). Verify 0 VAC at the unit supply.
2. Photo or label every wire on the capacitor before you move anything. Dual cans often have two or three wires on C.
3. Discharge each terminal pair, then meter VDC across those terminals and confirm it reads ~0 V before you handle the spades.
4. Disconnect the leads so the capacitor is isolated. Never take a µF reading in-circuit—the meter will also see motor windings and the number will lie.

Visual fail — replace without bothering to meter

• Domed, bulged, or split top on a metal run can.
• Oily residue or a leak at the terminals or seam.
• Burnt, melted, or badly corroded spades; a terminal post that turns in the can.
• Cracked or swollen plastic start-cap case.

A can that looks bad is bad. A can that looks perfect can still be electrically weak—that is why you meter µF.

What the meter is telling you

Use a DMM with a capacitance (µF) range. That is the pass/fail test. An ohms check only catches a dead short or a wide-open can.

• Good: measured µF is within the tolerance printed on the can. If no tolerance is printed, treat a run (or FAN/HERM) section as fail if it is more than about 10% below rated µF. Start capacitors are often ±10%. Caps almost always fail low, not high.
• Weak / out of tolerance: replace. A low run cap makes the compressor hard-start, run hot, and pull high amps. A low fan section makes the condenser fan slow or stall.
• Shorted: near 0 Ω between terminals (or the µF range pegs/errors). Replace.
• Open: OL on µF and no charging action on ohms. Replace.

Separate run and start capacitors

You will see this layout when the unit (or a hard-start kit) has two distinct cans: a metal run capacitor that stays in the circuit, and a higher-µF start capacitor that is in the circuit only for a second at start. A potential relay, current relay, or PTC drops the start cap out. Test each can on its own. Do not leave a start cap in the circuit continuously.

1. Isolate the run capacitor (two terminals). Measure µF across those terminals and compare to the µF and VAC printed on the can.
2. Isolate the start capacitor the same way. Compare µF to its own rating—not to the run cap. Start µF is much higher.
3. If a start relay or PTC is in the circuit, note it. A good start cap with a stuck-open relay still will not help the compressor start. A welded/shorted start circuit can overheat the start winding.
4. Replace with the same µF. Voltage rating must be equal or higher (a 440 VAC can can replace a 370 VAC can; do not go the other way).

Combo capacitor (one can for fan run and compressor run)

This is the common RV rooftop can: one oval metal capacitor with three terminals stamped C, FAN, and HERM. It is a dual-section run capacitor, not a start capacitor. The HERM section is the compressor run capacitor (PSC motors use that run winding/cap to start and to run). The FAN section is the condenser-fan run capacitor. A hard-start kit, if fitted, is still a separate start cap.

Typical marking looks like 45/5 µF 370 V: 45 µF is HERM (compressor), 5 µF is FAN. Always read the can in front of you—do not assume those numbers.

1. Identify C, FAN, and HERM from the stamp on the can. If the stamp is gone or unreadable, use the identification tips below before you meter or land wires on a new can.
2. Measure FAN to C. That is the fan section. Compare to the smaller µF on the can.
3. Measure HERM to C. That is the compressor (HERM) section. Compare to the larger µF on the can.
4. If either section is out of spec, shorted, or open, replace the entire capacitor. You cannot service one section.
5. Match both µF values on the replacement. Do not swap FAN and HERM leads—the fan motor will not survive a 45 µF feed.

HERM to FAN is the two sections in series. You do not use that reading for pass/fail. Pass/fail is FAN→C and HERM→C against the printed ratings.

How to tell which terminal is which?

• Common — often white; two or three wires on one post.
• Herm — highest µF (e.g. 60) is HERM → Common.
• Fan — lowest µF (e.g. 5) is Fan → Common.

Pass / fail and putting it back

• Pass: no visual damage, and each isolated section measures within tolerance of the rating on the can.
• Fail: visual damage, short, open, or µF out of tolerance. Replace. Do not “try it anyway.”
• Reconnect exactly as photographed. Seat every spade fully. Dual-can C often takes two or three wires—none of them are optional.
• After the unit is buttoned up, restore 120V and clamp compressor running amps against nameplate RLA. High amps with a proven-good new cap points off the capacitor (locked rotor, mechanical, or sealed-system)—do not keep cycling it.