I notice that some HVAC capacitors have labels printed or sprayed on them like the black "negative circle" label on top of the capacitor in the picture. What is it supposed to indicate?
Electrical – What the purpose of a label on top of a capacitor
air-conditioningelectricalhvac
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The capacitor needs replacement. While it is possible there is a circuitry problem causing the capacitor to overheat (and thus fail like this), it is also possible it was simply time for this electrolytic capacitor to fail while nothing else is wrong.
See this photo for a similar failure mode.
Based on the new information... It looks like the blue wire from the secondary side of the transformer is the R, which would make the yellow secondary wire C.
I should have read the transformer label, that clearly says BL (Blue) is common, and yellow is 24V.
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Best Answer
The black circle is not a label; it is a safety vent. If an electrolytic capacitor overheats due to high DC voltage, to a large AC component, to reverse connection or to internal failure, it will build up pressure until something gives... hiss, or sometimes bang.
To prevent B A N G, vents are designed to fail gracefully, or at least not too abruptly. The vents are marked below: a weak thermoplastic disk in the harder thermoset top of the large capacitor, and the score marks to weaken the thin aluminum in the smaller, which has already failed.
There should be clear polarity marks on the electrolytic (unless it's a non-polarized cap), often a black stripe along the negative side, plus mark(s) (+) on the positive, or embossed marks near the terminals. (Barely visible in the photo below is a plus mark below the right terminal of the big blue cap.)
BTW, from the terminals, I might guess that this is a non-polarized motor-starting capacitor, but take that surmise with some circumspection. Check the schematic or parts call-out if it needs to be replaced!