Very old forced air furnace has thermocouple sitting in continuously lit
pilot light. End of tcpl "wire" is screwed into a Baso pilot safety
switch, model 850. It appears that the current from the tcpl energizes
a small solenoid inside the Baso. I could not get the Baso to stay in
the on position so this evening I replaced the tcpl, eventually twice.
The first one worked for one cycle of the furnace, then the Baso tripped
and could not be reset to on. I then installed the second tcpl, and it
will occasionially let the burner gas come on for about a second, before
the Baso would trip.

I can think of four possibilities as to what's wrong.

1. The Baso needs replacing. The only reason I haven't done it is that
it is $90 and I can't see anything in it that could fail. It appears to
be extremely simple.

2. The pilot is under driving the tcpl. It gets the last 1/2" up to
medium red.

3. I have a bad connection where the tcpl is screwed into the Baso.

4. The pilot is overheating and killing the tcpl.

I would appreciate if you would please hold the comments that any
furnace with a pilot safety switch is an inefficient dinosaur and must
be replaced. It is in an area with extemely moderate weather and our
gas bill is less than $40 monthly in the middle of the winter, including
gas water heater and stove. Thus, the break-even date on a new more
efficient furnace would be... never. And, I like equipment that you can
fix with a large hammer.

Any ideas as to what's wrong?

Henry