Q:

Old Carrier Furnace, 3-Wire Pilot Assembly, Electrode Sparks Continually While it Runs

Our 29 year-old Carrier furnace relies on a 3-wire pilot with an intermittent spark igniter. It sparks probably around 30 times before the gas valve opens and the furnace fires up. That's a lot of sparking, and surely not good in and of itself, but the real problem is, it just keeps on sparking continually throughout the cycle, up until exactly the moment when the blower stops. 
I would estimate it sparks twice per second throughout the cycle duration, roughly twice as frequent as the sparking that happens prior to the blower coming on. 
Please note this furnace does not have a flame sensing rod. This is the bimetal type. Any suggestions on troubleshooting are appreciated.

A:

I experience this exact same problem (sparking throughout the entire cycle) when the 646AX gas valve assembly failed and had to be replaced. No direct replacement was available but the stated equivalent replacement was the EF32CW.
I soon found out that the EF32CW doesn't actually have true separate PICK and HOLD coils -- it just ties the two connections together to energize a single PICK/HOLD coil. Unfortunately that means that the sparking that is supposed to happen only during the PICK phase now occurs throughout the entire HOLD phase.
My kludgy fix was to disconnect the fake PICK input and route the HOLD to the PICK. So the PICK signal only energizes the spark generator while the HOLD opens the PICK/HOLD valve and keeps it open. Works fine...

