Great service, here, at askmehelpdesk/heating & ac. Been reading up on past Q&A, and have performed basic TS on my 9 yr-old Lennox condensing unit (mod# HS29-030-1P). When it failed a few days ago, the first thing I noticed was the rusty dual-run cap (C12). It showed no "bowing", but I could feel the internal structure was wobbling inside the case. The schematic on the panel (and other documents) does not show the value of C12, so I phoned Lennox. Unfortunately, their technicians couldn't be bothered to answer my simple inquiry as to the correct value of the dual-run cap. So, I phoned a local commercial electrical contractor that I found in the yellow pages. They directed me to Grainger Industrial Supply. I phoned Grainger, and got the correct value (35+5uf370VAC) for the cap. I registered at their website, logged on and ordered it. It arrived two days later. I installed it, but that was too easy. The unit still wouldn't kick ON.
The 1st time I took readings - low-voltage (26v) was present at the contactor with the unit
powered ON, and the coil activated properly. High-voltage (241.7v) was present at the
contactor (L1 across L2) with the unit powered ON or OFF. There was no low voltage present at the contactor with the unit powered OFF.
I like to be thorough, so I took readings 5-6 min later to verify.
The 2d time I took readings - again, low-voltage (27.1v) was present at the contactor with
the unit powered ON, and the coil activated properly. High-voltage (243v) was present at the contactor (L1 across L2) with the unit powered ON or OFF. Again, there was no low voltage present at the contactor with the unit powered OFF.
Readings on the dual-run cap, with the unit powered ON, were as follows:
C - gnd = 122.4v
H - gnd = 122.2v
F - gnd = 122.1v
Still, NOTHING. Just for the heck of it, I started some random measurements with the unit powered ON. As soon as I measured from H-C12 (wired to S-B1 compressor) to the T1 lead on the contactor (wired to C-B1 compressor) the unit kicked ON. Done and DONE. Must be kind of like shorting out the starter on a car in order to fire a stuck solenoid. Who knew?
Looks like my problem is solved--based largely on previous posts/replies from hvac1000. The unit has been running fine for the past few hours.
Can someone verify that I've installed the correct value (35+5uf 370VAC) dual-run
capacitor?
Hope this post will encourage/help others. Thanks to ALL for the good info; special thanks to hvac1000.

Almost sounds like you are saying, when the unit is on and 240 volt is present on load side of contactor, it puts 27 volts to compressor coil.
Or are you saying when 27 volts is present at contactor coil, it energizes and puts 240 volts out of load side of Contactor?

"Almost sounds like you are saying, when the unit is on and 240 volt is present on load side of contactor, it puts 27 volts to compressor coil."
"Or are you saying when 27 volts is present at contactor coil, it energizes and puts 240 volts out of load side of Contactor?"

The unit was only powered ON (not running). By taking a few "random measurements",
I was hoping to stumble across an explanation for the non-working unit. I realize that it's basically a backwards approach, but...
I wasn't actually trying to "read" the voltage to the compressor coil. I didn't even have access to the compressor and fan. As soon as I placed one test-lead on C12-H and the other test-lead on K1-T1, the unit instantly fired up. I was a bit shocked by the noise, so my eyes didn't really have a chance to focus on the meter. At that point, I just picked up a pen and quickly wrote down the very last thing I had done. Then, I went back and checked the schematic. As it turns out, C12-H is wired to B1-S, and K1-T1 is loaded to both the fan and the compressor. It kind of looks like my meter added tiny extra amounts of copper (load) and nicely oscillating milli-volts into a "failing" part of the circuit. Add to that a few solar-flares, a couple of cosmic-rays, and you could get a recipe for correction. For whatever reason, the unit "liked" it enough to fire up. This "failing" part of the circuit might have actually been less of a failure and more of a "design"--engineered into an IC on the SureLight ignition control board as a way for Lennox to "appear" to be mindful of safety... quote from one of their documents--"...has built in device for protection from excessive current and temperature." Sure, it does. Well, it could be. NOTE: The unit did fail on what was probably the hottest day of summer, so far, in this area.
Getting tired... meds are killer... not thinking clearly.

See this

http://0190acd.netsolhost.com/manual...ries_1_Ton.pdf