Pilot on, no main gas

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  #1  
Old 02-13-11, 11:36 AM
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Pilot on, no main gas

Hi

I read some old threads and did some testing and I think I have a bad gas valve, but I'd like confirmation from experts before I get it replaced.

I have a Robertshaw SP715A ignition control and Robertshaw 7100 DER gas valve on my hot air gas furnace. The ignitor is an intermittent pilot type. The pilot ignites OK, but the main gas valve does not turn on. The pilot stays on. For a while I was getting a momentary turn on of the main gas (barely a second) when I switched it on but now nothing. I replaced the flame sensor and still have the same problem.

From advice given in a past thread I've measured the following;
MV to PV : 9v
TH to TR : 24v
M to ground : 9v
TH to ground : 24v
P to C : 24c
M to C : 9v
Pulled the connection from M off and measured the lead to C : 24v

So from the last measurement it looks like the ignition module is OK. It's giving the right voltage to the pilot (P to C) and the pilot is coming on. It's also giving the right voltage to the control valve (with M removed measuring to C). But when the M lead is replaced the voltage drops from 24 to 9V indicating that the valve is pulling the voltage down and not switching the main gas on. I watched the voltage as I flipped the switch on and it momentarily got up to 16v before going down to 9v.

Does it seem like I got this right? Any advice would be appreciated. I've got the house at 50F with electric heaters but I'd like to get this fixed...

Thanks

Mike
 
  #2  
Old 02-13-11, 12:04 PM
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Good diagnostic job so far, but I'd disagree with a couple of your suggestions.


The probability is that the gas valve is good and the ignition module is bad.

I'd check that guess by disconnecting the M wire that powers up the gas valve and using a wire to jump 24 VAC to power up the gas valve independent of the module. If the gas valve switches on, you have a bad module.
 
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Old 02-13-11, 03:47 PM
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Thanks for your help SP, I'll try that tomorrow (not at home this evening).
 
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Old 02-14-11, 06:15 AM
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I'll be checking as you suggested this afternoon or this evening when I get home. In the mean time I've been looking for replacement parts so that I can get this sorted out in time for the long weekend.

I've found a Robertshaw SP715A module (original) but if the control valve is bad I've not found a direct replacement. It's a Robertshaw 7100 DER 71F 11A 000

I found a 71B 11A 001 which has the same form factor, also a 71N 41A 001. For other manufacturers this link which says it's a replacement..
Direct Spark or Hot Surface Ignition Gas Valve (Natural Gas or L.P.Gas) (Amana, Goodman, Robertshaw): American HVAC Parts

And this link which says it's "7100DER Exact Original control may have slight variances" and "ROBERTSHAW 71F-11A-000 Functional". But when I look at the valve there's a right angle turn for the gas (side and front) where as the one I have and the other 2 with close part numbers are direct through connections.
Patriot Supply - 720-079

Can you or anyone tell me if the 71B 11A 001 or 71N 41A 001 are viable options, should I go for one of the replacement parts or are there any other direct fit options?

Hopefully it's the ignition control but I'd like to get both options ready so that I can place the order tonight if at all possible after I test.

Thanks

Mike
 
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Old 02-14-11, 10:13 AM
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I didn't see that you cleaned the pilot. Please do that first, even if you think it is clean.
 
  #6  
Old 02-14-11, 10:51 AM
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You're right I didn't clean the pilot. Is this because of concern that the flame is not enveloping the sensor or because the current through the flame may not have a good path back to ground to supply the current sense? What current should I be getting? Can I measure it with DC amp in line (I believe it's rectified)?

I am getting 24v on the main gas valve (when I remove the lead from the controller). Doesn't this show that the pilot and flame sensor are satisfied?

I'll try cleaning the pilot tonight if the 24v jump doesn't turn the gas valve on.

Thanks

Mike
 
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Old 02-14-11, 02:47 PM
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I'm a bit confused, you have 24 v to P" & "M" on the gas valve and no gas to burners?? I guess I thought you only had voltage to "P" and nothing else. What lead are you moving?
 
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Old 02-14-11, 04:06 PM
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Hi, no not quite;

I had 9v to M but 24v to P. The pilot is lighting OK but not the main burner.
When I disconnected the lead to the M connection on the control valve the voltage (between the lead I'd removed and C) went up to 24V. So it seems as though something is pulling the voltage down from the controller so that the main valve wasn't getting the juice to open. I thought it may be the control valve itself but SP above suggested it was the controller.

I just got home and jumped 24v from the transformer to the main gas valve and it fired right up. Then it got interesting. I decided to leave it running while I was here (I have to leave in an hour or so). I knew I'd have to remove the transformer connector to secure the wire but as I pulled the wire away I touched the transformer case (ie ground) while the other end was still connected to M, ie I shorted what should have been the 24v supply to the main valve to ground for a fraction of a second.. I got a small spark of course, but the burner kept running even though my jumper cable was no longer connected! Now when I meter the voltage between C and M it's 24v rather than 9v. Wierd, it seems as though there's a problem in the controller that got reset with a connection to ground, iffy SRC?

I ordered a new controller which should be here in a few days.

Thanks!

Mike
 
 

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