I posted before about my boiler issues (short cycles, failed ignitions). the failed ignitions only happen when the system has run recently. if I leave it off for a while, it starts ok. I began just replacing components that were old, like thermostat/wire, rollout fuse was cracked, relay was very hot. however, after replacing the pilot module, which was actually kind of expensive ($40), I decided I should use my meter and rudimentary HVAC skills to actually find the problem of my intermittent boiler issues. Thanks to the very helpful people here, I think I've been able to narrow it down. if you guys could be so kind as to help me with a sanity check of my conclusions, that would be great.
what I tested:
since the system would try to re-ignite after the short cycle, that ruled out any of the interlocks like water temp, damper, rollout, etc. I also tested these individually by jumpering them out one at a time, just to be sure. since I replaced the relay and pilot module, I can rule out the flame rod, piezo igniter, and pilot tube as well as the relay. I can feel the circulator pump running, so I don't think that's the issue (though, I'm not sure how to do a more conclusive test). so, my primary suspicions were the gas valve and the control board. so, I tested the sequence on a good startup by placing my meter between PV and MV. this allowed me to see 0v before startup, then 25V when power was applied to PV, then back to 0v when power was connected to MV, at which time the burner came on. I also test each with respect to the PV/MV "return" just to be sure, but having only one multimeter it was hard to test timing when measuring with respect to PV/MV return. it seemed like whenever the valve had 24V, that it supplied gas just fine. but at the end of a short cycle, both PV and MV would be 0V while the system tried to reignite (clicking piezeo spark is all that came on). this seems to point me to the control module, and not the gas valve.
are there any other tests I should do before ordering a new control board? I was thinking about directly connecting (for 1 second) 24V directly to the gas valve to see if it would turn on, while holding a grill lighter to the pilot/burner area.
FYI, I have some electrical debug skills, and have installed and repaired a couple of heat-pump systems. though, this is my first gas boiler repair.
also, I believe this is the circuit diagram for my Weil-Mclain CG boiler:https://f01.justanswer.com/JACUSTOMERowpc7yhg/2013-01-11_234321_2013-01-09_10.12.39.jpghttps://www.doityourself.com/forum/attachments/boilers-home-heating-steam-hot-water-systems/58141d1446098528-weil-mclain-cgx-c-wire-options-low-water-cutoff-disconnected-weilmclaincgxwiring.jpg
edit: solved: control module