Furnace: low voltage at the gas valve? RUUD UGDE-06ELAES

[dshanks]

I have a RUUD UGDE-06ELAES "Deluxe High Efficiency" furnace that suddenly stopped working while we slept last night.  It was installed in 1991 or 1992.

In my attempt to diagnose the problem, I've discovered that the gas valve gets 17-19VAC instead of 24VAC when the relay closes at the end of the ignitor cycle. 

Here's what I'm seeing:

1. Set thermostat high enough to call for heat
2. Blower goes on
3. 30 seconds or so after blower on, ignitor lights up
4. 20 seconds or so after ignitor lights, I hear a click and see approximately 17-19 VAC across the gas valve relay.
5. 5 seconds or so after I see voltage at the gas valve, the ignitor goes off and the voltage at the gas valve drops to zero.

I also measured the voltage across Valve and Common on the Robertshaw Model HS780 34PL-398A control board.  Surprisingly, the control board is putting out 17-19VAC.  Not 24VAC.  Not 0VAC. So my question is, what does this mean?  What could cause the control board to put out low voltage?

I should probably also mention that this furnace was repaired three or four years ago.  At that time, a repairman replaced the ignitor and I think he may have replaced the Robertshaw control board too.  But I'm not sure the control board was replaced.  It just looks "newer" than the rest of the parts in the furnace.

I checked the output voltage at the transformer, and it's 26.9 VAC.  The input voltage to the Robertshaw control board, measured across TH & COM is also 26.9VAC.  But the output to the gas valve is 17-19VAC.  This suggests to me that there's something wrong with one of the circuit boards.

How do I tell which of the circuit boards needs to be replaced?

Edit - Conclusion: I decided to gamble that replacing the Robertshaw board would solve the problem.  After all, 26.9V in and 17-19V out probably means something's wrong, right? 

Anyway, I replaced the board and the furnace works again.  I couldn't have done it without the information I found in this forum.  Thanks, Samurai Appliance Repair Man!

[applianceman18007260692]

Check all your connections for corrosion and make sure they are tight. Clean the sensor and retry