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Discussion Starter · #1 ·
Hey folks, I have a old Westinghouse roof mount HVAC unit and it looks like the pilot valve is not opening. It uses a Robertshaw 780-745 control unit and a 7200IPER valve. I have verified 24 volts AC through the limit switch to the TH terminal. I can hear the spark operating as appropriate and can hear the main valve open when 24 V is applies to the MV terminal. I can not hear anything when 24 V is applied to the PV terminal. (But I am not sure whether it I should be able to hear it open) When I test the voltage at the PV terminal during ignition I get wildly pulsing voltages. I am thinking the readings are distorted by the proximity of the sparking as the voltage fluctuation is in time with the sparking. So my thoughts are twofold...the pilot valve may not be opening when energized and hence defective (if I am supposed to be able to hear it open) or the control unit is defective (because of the fluctuating voltages)

Any thoughts or help?

Thanks in advance

Bob
 

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Check the incoming voltage to the module.

I would think the pilot valve should be getting sufficient voltage.

You may not hear the valve open; maybe disconnect and spark wire from the module and sniff for gas around the burner. if the unit is induced draft as most rooftops newer than 25-30+ years will be, you may not smell it; can sniff the exhaust but the gas smell would be very faint.

Overall with proper input voltage to the valve and erratic output to pv, i would be condemning the module.
 

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Discussion Starter · #3 ·
User, thanks for the response. I do have 25.6 VAC to the module at the TH terminal. I think I will take another voltage meter over and see if it shows the fluctuating voltage during spark as well. My meter is an inexpensive digital one and I suppose it could be affected by the proximity to the sparking wire.
I can't get close enough to hear or smell gas at the pilot light. I am thinking about disconnecting the gas line to the pilot valve and seeing if I can hear/smell gas at the valve. Anyone want to weigh in?

Bob
 

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Discussion Starter · #4 ·
Ok, checked it out some more...
I disconnected the line from the valve to the pilot light and put 24VAC to the pilot terminal on the valve. No gas flow detected by pressure or smell. Tells me the pilt valve solenoid is not functioning. But I still have what appears to be a malfunctioning control module. 0 VAC when the unit is not attempting to ignite...that makes sense. But wildly fluctuating voltages from the PV terminal of the module when trying to ignite...timed to the sparking of the igniter. Also, when applying 24 VAC to the gas valve pilot terminal with the wire between the PV terminal on the control valve and the gas valve pilot terminal connected (in other words, a normal connection) I can actually hear what appears to be a sparking from the igniter. That seems to indicate a "short" of sorts between the spark terminal and the PV terminal on the control module. So my theory at this point...
Control module malfunctioning, sending a high voltage pulsing to the pilot valve solenoid in the gas valve, thus frying the pilot solenoid. Does this make sense? I would appreciate some feedback

Thanks again,

Bob
 

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Disconnecting the pilot line is dangerous btw; the pilot assembly has an orifice, restricts the flow of gas. run it without the orifice and it may pour lots of gas out if the regulator allows.
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I wouldn't go on that theory. The spark is in the thousands of volts.

When it sparks, may be hard to see in the light, but if you suspect sparking elsewhere check the spark wire insulation for damage.

When it's sparking i wonder if you could put a alligator clip wire from the module's th to the pv, disconnecting pv from the usual connection on the module. U would have to wait for it to spark before making the connection, let it contact metal and u'll blow the transformer.
 

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Discussion Starter · #6 ·
Yea...didn't actually disconnect it. Just loosened it at the valve and checked for any leaks. No gas flow to the pilot even when directly applying 24 VAC from the TH terminal. So I know that the pilot solenoid is not working correctly, yes? I will go back up on the roof after dark and make sure that the sparking I hear is actually at the pilot, not from a short in the wiring to it. Thanks for the idea!

Not sure what you are meaning when you say you wouldn't go with my theory...I am reading voltage pulses in excess of 100 volts off the PV terminal but the pulses are so rapid that the readings are all over the board and change so fast I can't really keep track of them. can you think of any reason I would be getting those kind of readings off the PV terminal of the control module? And if there really are hundred + VAC spikes, wouldn't that fry the pilot solenoid?

Inquiring minds want to know...

Bob
 
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