On Friday, November 29, 2019 at 8:16:08 PM UTC-5, wrote:
On Fri, 29 Nov 2019 18:56:50 -0500, Ralph Mowery
wrote:

In article ,
says...

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.



The control circuit is 24 V AC. The block diagram shows a relay contact
or two in series with the motor contactor for the compressor.

I had forgotten I had the diagram from last year when it acted up. Just
cutting it off and back on cleared the problem, so I did not go into it
last year. I had so much going on the day it quit on me this year, my
mind was on other things.

The LED has 3 or 4 flash rates to indicate a couple of problems. It
flashes normal. It may be as simple as a bad connectiion of dirt on
one of the relay contacts.

The thing quits so infrequent it is difficult to trace when other things
are going on.

Often in simple circuits (and more complicated ones) a product will have
several known faults that hapen 90 % or more of the time. I just though
someone may have ran into it before.

You are right, mass production means mass failures if there is a flaw
in the process. In my biz, every machine usually only had a few things
that went wrong. If you could identify the flaw and come up with a fix
in the process, there was money to be had.

The people who work for me fall into couple of groups on troubleshooting.

One is in the camp of: It's a brand XXX model 123 so it's always a faulty output relay. They're right a lot of the time and that reinforces the approach. When it's wrong they're lost.

Another is: Gather all the data, check everything, avoid a diagnosis until the last moment. These guys are slow but get it right. I've maybe got one like that.

The most common though is to make the diagnosis too soon and then exclude all data that doesn't confirm it. I try to keep these guys away from expensive equipment but there's no way to fix their approach. It's not that they ignore conflicting data, they can't actually see it.