I am troubleshooting an LP-fired furnace (Dayton 3E384) with electronic spark ignition. The unit uses a Honeywell S8600M Control Module and a Honeywell VR8104A Gas Valve. When the thermostat calls for heat, the control module initiates continuous sparking at the pilot and the pilot valve opens. But the flame is unstable: flickering, and repeatedly extinguishing and re-igniting. During this time, I can occasionally hear some faint clicking from the gas control valve. This continues until the flame sensor finally gets hot enough to signal the control module to open the main valve. As soon as that happens, the main burners light-off, the control module stops the sparking, and the pilot becomes a steady normal flame.
I suspect that, while the control module is generating the spark, the pilot valve is not getting enough power to open fully and that the valve is rapidly pulsing open and closed as the spark generator draws current to produce each spark. As soon as the control module stops the spark generator, then there is sufficient power to hold the pilot valve fully open and the pilot flame becomes normal. But I am unsure if the problem is in the control module or the gas valve. The system is rated for nominal 24VAC and I am measuring greater than 24VAC at the control module's power input terminals and also at the gas pilot valve terminals (PV & MV/PV). However, I am making those measurements with a Fluke digital multimeter, and although I set the meter to use manual ranging, it's not able to detect/display high-frequency voltage transients (i.e., rapid drops in power each time a spark is generated). I have also measured the resistance between the PV & MV/PV terminals, and the MV & MV/PV terminals, and both readings were essentially identical (i.e., both the PV and MV solenoid coils have the same resistance).
To eliminate the possibility of problems with the wiring, high-limit switch, etc., I connected jumper wires directly from the power supply transformer to the PV terminals on the gas valve (bypassing everything, including the control module). The symptoms are identical: intermittent & irregular flame while the control module is generating spark; normal flame after the MV opens and the spark generator ceases.
So either the gas pilot valve solenoid is "weak" (and thus too sensitive to fluctuations in the power), or the control module is defective (and is drawing too much amperage during spark generation, thus robbing the gas control valve of enough power to stay fully open). I suppose a third possibility is that the transformer is "weak" and not able to keep up with the current demands. But I think that is unlikely.
Any assistance in narrowing down this problem would be greatly appreciated.