Bryant plus 90 blower wont stop running

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Old 11-11-16, 10:28 AM
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Bryant plus 90 blower wont stop running

Hello all -
I have an older Bryant i think from mid 1980's where the blower will not stop running - have to cut the incoming power to get it to stop. everything else functions properly in fact it still cycles the heat on correctly when i turn up the thermostat.

Model is 398AAW048100ABBA

I replace the high limit switch and have the same results.

The main circuit board has no lights to look at. It is part number HH84AA021.

What else can i check?
 
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Old 11-11-16, 02:07 PM
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Sounds like the fan relay is stuck. Try tapping on the two black relays. Do you have a voltmeter?

Make sure the stat is set to fan auto.
 

Last edited by skaggsje; 11-11-16 at 02:09 PM. Reason: added
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Old 11-11-16, 03:33 PM
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If there is no 24vac on the G terminal then the board is bad. There is no limit switch to operate blower. It's timer controlled from the board. It could possibly be a dirty control pot. See if your board has an adjustable control on it. It will have an arrow and say incr. That should be the blower off delay setting. Try turning it towards the decrease setting. (counter-clockwise)
 
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Old 11-11-16, 04:06 PM
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Many older hot air gas fired furnace controls had a dual function sensor. One function was to shut down the furnace when the exhaust gas temperature reached the high limit setpoint and the other was to turn off the blower when the exhaust gas temperature was decreasing and setpoint was reached after the t'stat was satisfied.
This device was located in the exhaust plenum of the furnace. As I recall, there were restrictions when adjusting the limits. They were mechanical sensors and sometimes got stuck due to friction. Due to the heat they can't be lubricated. Best approach was to operate (rotate) them manually once or twice to remove the cause of the friction.
If your high limit switch has more than 2 wires attached, it most likely is a dual function. If not , look for a second sensor in the plenum and try operating it manually.
Hope this helps.
 
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Old 11-12-16, 04:42 AM
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Here is what I found so far

First thanks to all for your help - deeply appreciated.

Thermostat is set to auto

Tapping relay has not helped

Do have a electric-electronic background and have no 24 vdc on G or anywhere i can probe with and with out the thermostat calling for heat

Turned the potentiometer on the board again no change in symptoms

Not sure what the plenum is but if it is where the high limit switch is housed the only other devices I see are the spark igniter and a solid state timer connected to it.

Just notice on the board just below where the transformed 120 vac to 28 vac on term SEC 1 AND SEC 2 there are 4 diodes - two of them are corroded to the point that they are no longer connected. Could these be the 24 vdc bridge and the reason there is no 24 vdc?
 
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Old 11-12-16, 06:55 AM
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Shouldn't be any 24 VDC, set your meter to 24 VAC and check again.
 
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Old 11-12-16, 04:21 PM
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Ok checked again
there is 24 vac at the secondary of the transformer which is the terminal SEC 1 and SAC 2. No 24 vdc anywhere.
 
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Old 11-13-16, 05:13 AM
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The plenum in your furnace is the enclosed volume above the burners up to the connection to the chimney.
Diodes normally fail due to over-current or over-voltage, not corrosion. While it is easy to replace the diodes (maintain anode to cathode orientation), I would include a fuse holder and fuse to protect the bridge in case there is a short somewhere on the 24 vdc load. Size the fuse to the current rating of the old or new diodes, whichever is smaller. Hope this helps.
 
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Old 11-13-16, 09:03 AM
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Your blower is controlled by a timed function on the control board. It doesn't have any other heat operated switches. There is not a switch in the plenum for it.

I repair the furnace boards myself. My advice to you.... replace the board. There are too many safeguards on it then can be damaged.

The Carrier board that you left the part number for should have a 3A protection fuse on it.
 
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Old 11-13-16, 02:41 PM
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Thanks again to all.
I ordered a board today and will report the outcome.
 
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Old 11-19-16, 02:08 PM
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Ok so the new board fixed the blower problem of staying on all the time but....the burners shut down after 10-15 minutes of running even though the thermostat is still calling for heat.

any help is appreciated.
thanks
 
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Old 11-19-16, 04:08 PM
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Your furnace is overheating. Possible causes are dirty air filter, blower running too slow, too many registers closed, blocked return vent and dirty A/C coil.

When you installed the new board.... did you set the right blower speed for heat ?
 
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Old 11-19-16, 05:55 PM
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Well - this one may be off the norm but here is what i found....

Earlier in my struggles I notice that there was a slow drip of water coming from under the inducer fan and also a drip coming from a clear tube connected to the bottom of the furnace...tightened the clamp and didn't think much of it at the time.

So why was it dripping? After taking apart the inducer motor and seeing a ton of water in the furnace and some actually in the bottom of the inducer fan assembly i decided to investigate the drains and found that the PVC tube/valve where both tubes connected was clogged. It was full of sediment and rust and who knows what.

So i think that the the cause of the circuit board failure was water dripping on to it in the first place as the water collected and ultimately dripped onto the board causing its failure.

All my issues circuit board failed (from slow water leak) causing fan motor to stay on....replaced the board and the furnace was shutting down early because the fan motor...all because the water couldn't drain.

THANKS AGAIN FOR ALL THE HELP!
 
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