Bryant 398AAW furnace: random pilot gas valve activity


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Old 11-09-15, 06:31 PM
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Bryant 398AAW furnace: random pilot gas valve activity

I have a mid 80's Bryant furnace that has the three wire pilot. Gas valve model 646AX-1. When heat is called for the induction blower runs, I prove air pressure but the pilot gas does not turn on. I don't hear the pilot gas valve pop. Sometimes when the furnace has not been used (cold), I will get a normal sequence and hear the pilot gas valve pop. When the pilot lights everything else functions normally.

I need to know how to test gas valve or what else could be causeing this.

Any help is greatly appreciated!!
 
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Old 11-09-15, 08:45 PM
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Welcome to the forums.

How do you prove air pressure ? Do you have a meter connected to the switch ?

I would lean more towards a control board problem as opposed to a bad valve.
You should hear a relay on the control board click and then the pilot should light.

We need a full model number of that furnace. It will be found on an ID tag near the burner area.
 
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Old 11-10-15, 04:35 AM
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Yes, I have meter on air pressure switch.

This morning I let it go longer and pilot gas valve popped after a longer period of time. and gas went to pilot. No spark. When gas valve pops quickly I have always seen spark.

What leads on gas valve do I need to check for voltage to see if signal is coming for pilot gas? The valve pictured here is a spare I have. Mine is still hooked up.

I will get full model number at lunchtime when I go home.


Thanks!!
 
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Old 11-10-15, 09:22 AM
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Bryant 398AAW036090D is the model number
 
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Old 11-10-15, 09:29 AM
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You checked the amount of vacuum but you need to put a volt meter on the switch to see if it's actually closing.

Utilizing this service manual.....docs/1009/Public/00/58SX-18SI.pdf

I find this for your gas valve.

Name:  valve.JPG
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Terminals 1 and 3 are main gas valve.
Terminals 2 and 5 are the pilot valve.
 
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Old 11-10-15, 02:05 PM
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Thanks Pete.

I am getting 2.7 volts on 2 and 5 after I prove air pressure. No pilot gas. Sometimes I will still hear it poo and I will get pilot gas. Usually nothing.

Bad gas valve?
 
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Old 11-10-15, 02:07 PM
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I'm not certain what you should measure. I would expect to see +/- 24vac.

I've located what looks like several replacement valves and they all spec 24vac.
It would appear to be either a defective relay on the control board or a dirty connection in the wiring. Check for the voltage right at the connector on the board.
 
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Old 11-10-15, 04:40 PM
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Ok, I tested at board. I get 24 volts.

What color line brings low voltage to the gas valve? I am guessing it's somewhere after the pressure check.
 
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Old 11-10-15, 04:51 PM
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I left you the link for the manual. If it's not in there you'll need to physically follow the wiring from the valve back to the board.
 
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Old 11-10-15, 05:31 PM
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I don't know how to read those real well. A white wire comes from board directly to valve. A brown wire comes from a little board behind the air pressure switch. I thinks it's one of those but I am not sure how to verify.
 
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Old 11-10-15, 05:43 PM
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I did jumper to the pilot gas valve to confirm the valve was functioning properly. It opens with 24 volts, so the valve is working.
 
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Old 11-10-15, 07:06 PM
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Have you checked the spark generator for power in power out. Check for 24 volts going in with thermostat calling for heat. Power out is the high voltage wire. Remove the wire and with an insolated screwdriver touch the tip and then pull away about a quarter inch away to see if there is a spark.
 
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Old 11-10-15, 09:14 PM
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The manual I left you the link to is different from your furnace. Your furnace uses a dual port pressure switch and has been the cause of many problems.

Check this video out and see if it helps you.
you tube/watch?v=r7TmMjRkruY

I'm looking for the correct service manual now.
 
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Old 11-11-15, 06:31 AM
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I have not checked spark generator yet. I want to solve the pilot gas valve issue first. Should the spark generator start before pilot gas valve opens? I assumed the spark starts after pilot gas valve opens.
 
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Old 11-11-15, 06:48 AM
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I noticed that SeattlePioneer has made a lot of comments on these types of furnaces. Are there other members that are experts on my system?
 
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Old 11-11-15, 08:46 AM
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The spark generator and pilot valve are energized at the same time.
 
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Old 11-12-15, 04:52 AM
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Schematics

when does pilot gas valve receive 24 volts?
 
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Old 11-12-15, 07:20 AM
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Post another picture of the right hand side of the diagram. This part of the diagram is called a ladder diagram and is easier to interpret furnace sequence compared to the schematic in the left side of the diagram you provided
 
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Old 11-12-15, 07:46 AM
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Here is a picture of the other side of the diagram.

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Last edited by PJmax; 11-12-15 at 11:26 AM. Reason: reoriented pic
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Old 11-12-15, 09:23 AM
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I am wondering what the 11C-Link, Fusible (Over temp) is for. I have noticed intermittent activity. Sometimes when the furnace is cold it fires more often. Is the 11C-Link some sort of temperature shut down? Plus those jumper wires seam quite old and brittle. Three wire pilot was replaced one year ago.
 
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Old 11-12-15, 11:28 AM
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I combined your threads. Please keep the topic in this thread.

The fusible link is open or closed. If your furnace is operating.... the link is good.

You were checking previously and you found low voltage at the gas valve and normal voltage at the board. Did you persue that ???
 
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Old 11-12-15, 12:03 PM
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Yes. I get full, uninterrupted low voltage (24v) at terminal 4 on gas valve coming from the brown wire. There is a yellow wire on terminal 4 which heads to the pilot assembly. In line there is a fusible link. The fusible link is not blown, but it may have a bad connection. Yellow wire continues to pilot assembly. A green wire returns from pilot assembly to gas valve at terminal 5 to open pilot gas. I get intermittent voltage at terminal 5 (pilot gas valve) and subsequently at spark box. There is a loose connection, or the pilot assembly is bad. I am testing continuity today after work.

Does anyone know the temp rating for the fusible link? That link is very old, should it be replaced?
 
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Old 11-12-15, 04:28 PM
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Pilot assembly had intermittent continuity on green and yellow. I had a spare, so I replaced the pilot assembly and it seams to be ok. I will let it cycle a few times to verify. There is still a very random bit of intermittent signal by the fusible link. Does anyone know the temp rating of that?
 
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Old 11-12-15, 06:06 PM
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While I was hunting around I came across this troubleshooting guide. I believe it covers your unit. Has extensive voltage check points listed.

hvac partners/docs/1009/Public/06/TSP-398A-1.pdf

The 11C fusible link is for sale and is discussed all over the internet but no one talks about the rating. If I'm not mistaken it's rated to melt open at 117f.
 
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Old 11-12-15, 09:03 PM
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I've never seen a fusible link that was intermittent, either closed or open. Might be the connection.
 
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Old 11-13-15, 04:56 AM
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Furnace ran fine all night. Thanks for the advice everyone!!

I think the fusible link is the connection terminals. The link itself is fine.
 
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Old 11-13-15, 05:00 AM
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Thanks PJ, that document is perfect for my model. Thanks for your time finding it for me. If you're ever in Michigan on me.
 
 

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