On Fri, 29 Nov 2019 16:50:04 -0500, Ralph Mowery
wrote:

In article ,
says...

The LED indicator will still be a way for a quick peek without having
to get your meter out. I would start with it on the white wire from
the thermostat inside the air handler and get that out of the picture.
The first rule of trouble shooting is "can you draw a circle around
the problem"? You want to get as much as you can outside that circle.
You already seemed to have eliminated the condenser since you don't
have the pick shot to the relay. If you eliminate the thermostat and
wiring into the air handler you are getting pretty close to that
control board. Do a visual inspection of all connections before you
buy anything.



One of my metrs is a Fluke T5-1000. Very handy as for checking voltages
it goes from low voltage to high voltages and auto from AC to DC. Just
slap the leads across a circuit and if any voltage from a volt or so up
to maybe 1000 they will show up.
It also does ohms and is very well protected. While at work using them,
we would go across fuses on a 240 volt circuit and if the fuse was open,
the meter took it just fine.
It will also do AC amps like the clamp on type. All this about the size
of a bannana.


My favorite for simple trouble shooting is a Fluke T2. Same size,but
has about 8 leds on it that light up on voltages of 6 volts to 600 volts
AC or DC. It will show continuenty of a circuit that has a very low
ohms in it. Just 2 wires like the led thing you are talking about, but
does much more.

I did take time today to pull the wiring diagram. If it stops on me
again, I will try to follow the 24 volt control circuit to see where it
stops. It must be in that as the relay did not have any voltage across
it that starts the compressor and outside fan. They both run if I push
the relay in with a screwdriver.

The control board is just a block with not much information on it. It
does have a trouble shooting LED, but it blinks once per second like the
paper says it should if nothing is wrong.


There must be a processor on that card. If the thermostat wires go
right to the card and the connections are good you really only have
the output to the relay. Maybe the driver transistor/Triac is bad. It
might also look at the card to see if there is a flaky land pattern or
a cold solder joint. Is this AC to the relay or DC? Typically HVAC is
AC but I wouldn't be surprised that these CPU controlled ones use DC.
If it is AC, you also have the connection from the AC supply into the
card.
The LED is really just the "heartbeat" indicating the processor is
running.