Warm enough Friday to want A/C;  lowered thermostat.
Mahlian and I both fiddled thermostat Tuesday.
No response to setting fan to on this morning.
blown 3A fuse for 24VAC controls.

Cannot find my automotive fuse kit;
ordered another kit with ten 3A fuses.

• Soldered two 12V 20W halogen bulbs in series
  for testing 24V short circuits.
• jumpered fuse contacts with 24V short tester;
  overrode cover interlock switch
  - no light (low current), and furnace blower ran until cooling engaged..
  - then short tester lit, indicating a short to A/C unit
  - control wire to A/C unit is OK
  - contactor failed coil measures 1.5 Ohms, essentially shorted
    comparable replacement has 11 Ohm DC, 250 mA (AC) rating
  
  $10 replacement from Amazon

Carrier AC Unit 24ACC460A300 mfg date Jun 2017


Coil mfg May 2017


Furnace 58STA135-22 installed 19 Jul 2007


Blower capacitor replaced 3 Apr 2014

24V HVAC circuits

Greasy black rectangle at top center is cover interlock;
right of that is 24VAC transformer
- black/white wires are 120VAC to transformer
- blue/red wires are 24VAC from transformer to circuit board, below fuse (removed)
- diagnostic red LED near transformer blue wire
- 24V test lamp plugged to hot side of 3A fuse connectors, just below:
- black barrier terminal strip for (24VAC) thermostat and A/C signal connections
Top-to-bottom
G - fan   (green)
Com 24V (blue)
W - heat  (white)
Y - A/C compressor relay and thermostat (yellow)
R - 24V condensate overflow switch in series with thermostat (red)

Heat:   thermostat connects R to W
Fan:  thermostat connects R to G
Cool:  thermostat connects R to G and Y

furnace diagram