Lenox G12 / RobertShaw SP735L won't start

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Old 02-20-11, 05:22 AM
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Lenox G12 / RobertShaw SP735L won't start

I have a Lenox G12 furnace with a Robertshaw SP735L ignitor that won't start. I've had this problem for about 2 months, we always think we've fixed it but apparently never have (including a tech who charged $250 and decided it was the thermostat, which it definitely wasn't). So what happens now is when it calls for heat I get a couple clicks and then it shuts down. It sounds as if some safety mechanism is activated. I can't be sure, but it seems like I have to turn off the gas valve for a couple minutes then turn it on and it'll try again and fail again. I get 24V to the SP735L, but other than that seems not to be 24 on any other terminal.

Thoughts? Thanks!
 
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Old 02-20-11, 12:05 PM
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check for 24v between PV and MV/PV. check both at the module and at the gas valve. do this check when it first starts calling for heat. Are you getting any spark? what terminals have you check for voltage between?
 
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Old 02-20-11, 12:52 PM
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here is an in-action video:

YouTube - Robertshaw problem

I don't totally know what causes it to "try again" so I'm still trying to get PV and MV/PV measurements while it's trying to do something. The video attempt came much closer to normal than any other in the last couple days, only difference was I had left it off with the valve off for about an hour.
 
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Old 02-20-11, 01:05 PM
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Ok, a few more tests and pokes and prods and it actually sparked and lit for about 1 minute. MV/PV and PV had 24V while it was starting then I stopped measuring. While it was running, before the fan came on, I heard a few more clicks (I assume trying to fire the spark plug) and then the whole thing shut off.
 
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Old 02-20-11, 01:19 PM
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Measure the voltage between chassis ground and whatever point you want to measure. You are looking for 24 VAC.


When you have 24 VAC at the 24V terminal, the ignition sequence should begin:

1. The spark should turn on. You should get 24 VAC at PV and the gas for the pilot burner should turn on. The pilot light should light and stay lit as long as 24 VAC at 24V stays on.

2. Next the main burners should light off the pilot light. The main burners should stay lit as long as you have 24 VAC at the 24V terminal.


If that's not happening, how far is the ignition sequence getting?


You need to be able to observe what is happening at the pilot light.
 
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Old 02-20-11, 01:23 PM
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Well normally not very far. Normally I think the spark tries once and then the whole thing shuts down. I had suspected this might be because it thought the valve wasn't opening? Anyhow, the last time when it did fire up, it stayed lit full blast for 30 seconds or so, then clicked a couple times and shut off. Where can I see the pilot?
 
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Old 02-20-11, 02:16 PM
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Follow the pilot tubing and the three wire harness to the pilot burner.

You need to observe carefully whate is happening. Guessing and supposing is not allowed.
 
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Old 02-20-11, 03:37 PM
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Fair enough. There's an overlay problem which is that sometimes we just can't get the thing to even try to fire. TH has 24VAC (27.5 actually) but PV and MV just never get anything. When it does fire, PV comes on, MV comes on, it stays on for a variable amount of time generally under 30 seconds, and then it all shuts down. When we make it past the MV phase, it will all light just fine.
 
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Old 02-20-11, 03:50 PM
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We're trying to diagnose the valve as well. If I connect PV to TH (24V) and/or MV to TH, shouldn't that gas valve open? We hear no gas and cannot light the burners with a manual flame, which says to me either it's more complicated than that, or that the valve is stuck or otherwise not working. The confusion with that diagnosis is I don't understand how the controller would know that so quickly and shut down.
 
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Old 02-20-11, 05:23 PM
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YOU SHOULD NOT BE APPLYING POWER TO TERMINALS NOT KNOWING HOW THE SYSTEM WORKS! There is no room for guessing. You are dealing with EXPLOSIVE GAS. Powering the gas valve without other safeties in place presents unknown hazards which can harm you. your home, or your FAMILY!!! If you are going to be guessing at things it is best to call a PROFESSIONAL.

You may have 2 problems. 1. The gas valve may be getting slow to open. These old valves were known to have this problem. 2. The ignition control may be failing as well.

HAVE YOUR FURNACE SERVICED AND CHECKED ANNUALLY. YOUR FURNACE IS KNOWN FOR FAILED HEAT EXCHANGERS!!!!!!!
 
 

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