24v on P terminal but only get 1v on MV terminal - Raypak p-r336a heater

[stevecespedes]

Hi Fellas,

First post on the forum, I just found the site and it has already been very helpful. I'm having a little bit of trouble getting a heater running. My LCD screen does not work very well, I can only make out what it says late in the afternoon when there is minimal glare. I ordered a replacement. In the interim, the modes are few and simple to interpret what the unit is trying to do. Modes are OFF/SPA/POOl. I can make out when it is set to POOL or SPA and it initiates the CFH, as I have the temp set high.

Symptoms - I see CFH then SPRK then it shows the temp of water (51) and set temp (80). I can visually see the pilot is lit and remains on but I get no main burner.

I can't tell if there are any codes. I pulled the pilot out to make sure the flame sensor is inline with the flame and I tested all of the safetys. I checked continuity on HL1, HL2, Roll-Out, pressure/flow and all check out ok. I looked at all the wiring and it appears to be sound - no mouse chew damage or cuts. Hours later...

I am suspecting either the PCB board or the gas valve. Using my multi-meter I verified 24v on the P terminal (red) when checked against the common (yellow), when I check the MV terminal (purple), I only get 1v. Not having that much experience my concerns are the following:

1 - It can be a bad PCB board, which is why I'm getting the 1v which explains why the main burner is not kicking on.
2 - Perhaps there is a safety which is preventing the PCB from supplying 24v.
3 - Gas valve is not initiating burn, but it would need 24v, so not convinced it is the valve. How can I rule out the valve?

Questions:
1 - While I wait for a new LCD, what else can I do?
2 - Should there be 1v on MV terminal? Is that a typical voltage when not putting out 24v or is that a fault in itself? Pointing to a bad PCB board.
3 - If it is a common voltage when the PCB board has a safety triggered, which one would you suggest I check again? More importantly, what is the best procedure to rule it out?

Equipment Details:
Unit: Raypak p-r336a heater
PCB board - 601769
Gas Valve - 003900F

 

[califman2000]

You should not not see 1 Volt on a 24 Volt gas valve ever for proper functioning. I would suggest cleaning the ignitor rod which also doubles as the flame sense (check with ohm measurement on top and bottom). this is how flame sense is detected also with a back microvolt DC current on Raypak units it seems...called flame rectification. Also, verify the pilot is in the correct position with respect to burner tubes and check resistance on your ground wire and clean if find high ohms/no continuity. (Lower DC microvolts are more sensitive to oxidation/same goes for igniter rods which become rusted over time-clean with steel wool(easiest)or brass brush(softer metal) preferably. check ground terminals at the gas valve and down near the heat tray... try again to light

if the unit has a flame sense in the heater column clean that also...the above was for older units that have the combined pilot light and ignitor with no flame sense on separate circuit

 

[stevecespedes]

Thank you for the suggestion. One question though, if the PCB should never put out 1v on the MV terminal, should I continue trying to clean and troubleshoot or does it suggest the PCB has gone bad? I don't mind testing, just want to make sure.

 

[stevecespedes]

Can someone let me know how I can bypass the flame sensor and also a method to deliver 24v to test that the gas valve will work? I'd like to run these 2 test to ensure that the issue is the PCB and not the gas valve. Any help would be appreciated.

 

[stevecespedes]

Any help on how best to rule out and isolate the gas valve as the problem would be appreciated.