Blower Fan Keeps Restarting


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Old 12-31-08, 08:50 PM
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Blower Fan Keeps Restarting

Hi, I'm having an issue with a Carrier 58GSC080-DB furnace. My furnace works fine for lighting the burners and kicking in the fan about a minute later. However, as the furnace enters it's cool down period (after the burners shut off), it begins to make a surging sound, as if the fan motor is stoping/starting repeatedly several times. Then once the blower does shut off, it immediately starts back up again (or tries for about 2 seconds), and then shuts off again, only to try to start 1 more time. Once it finally gives up, its fine until the t-stat kicks the process off again.

I've tested the voltage on both sides of the limit switch, and I'm reading about 25.5 volts on both sides (it varies slightly while the system is running). I have two limit switches, and both appear to be closed and completing the circuit. Also, the burner never fails to light. I've opened the circuit board cover, and the relays visually appear to be OK (nothing burnt or blackened). They click as I apply power and remove it from the furnace.

Also, if I manually turn the fan on at the thermostat, it starts instantly, as well as shuts off immediately with the switch, just as it should. If the fan starts on its own in the 'auto' mode, the motor sounds like it's struggling to get started. I thought the motor was going bad. But if I turn it on manually, it just starts right up, no growning.

Any thoughts or ideas willl be greatly welcomed, as I'm baffled at the moment. The fact that I can manually start and stop the fan without issue tells me that my relay is OK, but then the limit switches seem OK, at least when the furnace is cool.

Please help!

Thanks!
 
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Old 12-31-08, 09:34 PM
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This furnace has a fan control integrated with the circuit board. The fan is turned on and off by a timer circuit on the board.

I'd be a little suspicious that the relay is failing to switch the power on to the fan properly, and that the surging might be due to variation in the voltage applied to the motor by that defect.

So I'd measure the voltage going to the fan motor at the circuit board, usually located in the blower compartment in front of the fan.

You'd need a multimeter that measures AC voltage for that.

The voltage should be 120 VAC or so, and stay at that voltage until the fan switches off. If that's not what's happening, the circuit board probably needs to be replaced.
 
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Old 01-01-09, 10:15 AM
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Thanks, I'll check the motor voltage and see what's going on with that. I'll get back and let you know what I find.
 
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Old 01-01-09, 11:13 AM
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Another thing to check would be the relay on the circuit board. The circuit board should be in the fan compartment and covered with a sheet metal cover you can take off by removing two 1/4" sheet metal screws.

Once you have the cover off, see if you can observe any sparking or erratic operation of the relay that matches up with the erratic operation of the fan motor. There are probably two fat relays on the circuit board-- you should be able to figure out which one is operating the fan motor.

Another thing you could do is to lightly tap the relay with the handle of a knife or fork to see if that changes the operation of the fan motor.

These are ways you might test or confirm the theory that it's the vircuit board is defective and needs replacement.
 
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Old 01-03-09, 08:49 PM
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Interesting Results

Well, I crawled back up into the attic and tested the voltage going to the motor while it was running, and got what I found to be interesting results.

I tested from the common lead to the low lead (as marked on the circuit board, and got 118 volts. This reading was very solid and non-fluctuating. I then tested from common to the high lead and got 74 volts. I wasn't expecting this, but there it was. I then let the burners shut off and continued to monitor the voltages. Everything remained exactly the same up until it shut off. Then it tried to restart 2 times. Both times the voltages were as before while the relay was providing power to the motor. The circuit board is definitely sending 2 more start signals to the motor after it shuts off. I didn't get the surging this time, which probably explains the rock solid voltage readings. My guess is that the surging that I sometimes hear during cool down is the motor getting power/not getting power/getting power again. The only question is why.

As before, the relays themselves seem to be doing exactly what they are supposed to do. When the circuit board sends power to the relay, it in turn sends power to the motor. Solidly, and without hesitation. The growning that I'm hearing is definitly coming from the motor when it starts in low speed during the heat automatic cycle. When I switch the fan on manually at the tstat, it comes on in high speed mode and sounds normal. So the low speed on the motor is probably not long for this world, but that's a different issue for a later date.

The other interesting reading that I got was while I had the fan running manually from the tstat, I measured from common to the high lead and got 118 volts as expected. But from common to the low speed lead I got 172 volts! That's a pretty good trick for a 110 volt furnace that only has a single 110/120 volt line coming in! My guess is that somehow the motor is generating electricity on the low lead while being powered on the high lead. Maybe this is completely normal, I've just never observed it before, but then I can't say that I've ever tested the voltage on both leads of a 2 speed motor before while it was running!

So, I'm left to my original dilema. What's causing the circuit board to try to restart the fan? Is it getting a signal from some sensor that is triggering it, or is it doing it on its own? And how do I tell for sure? If this were a $10 or $20 part, I'd just replace it and move on. Being that the circuit board will be considerably more than that, it would be nice to know for sure before I go changing it!

Any thoughts would be greatly appreciated.
Thanks!

P.S. Just for reference, my furnace is using circuit board # HH84AA011, in case that tells you any more about my unit and possible causes.
 
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Old 01-03-09, 11:32 PM
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Originally Posted by kaman
So, I'm left to my original dilema. What's causing the circuit board to try to restart the fan? Is it getting a signal from some sensor that is triggering it, or is it doing it on its own? And how do I tell for sure? If this were a $10 or $20 part, I'd just replace it and move on. Being that the circuit board will be considerably more than that, it would be nice to know for sure before I go changing it!

.


All right, you've done a good job of collecting information which can be used to diagnose the problem.

What you lack is a reasonably detailed underestanding of how the circuit board works, although your assessment that the fan relay is working OK sounds correct.

That means that something upstream of the fan relay is causing the problem. And upstream of the fan relay is a timer circuit that turns the fan on and off. There's no way to test that directly, but my judgement is that this circuit is in the process of failing completely, and has reached the point where it operates erratically. That kind of intermittent failure is fairly common in a wide variety of equipment, and is often the bane of repairman because of the uncertainty it creates and because it can be hard to diagnose a problem that isn't there.

The timer circuit should have a delay after the burner turns on before turning the fan on, and should continue to run the fan after the burner shuts off for a period of time. But there is no way it should turn on and off as you describe --- which means that the circuit board is defective, whether it's the timer, relay or whatever.

In your case, it is there. It just hasn't failed completely ---yet. But I don't doubt it will before long.

So you have a choice. Wait until it fails completely or change it now.

You get to take your pick.


Perhaps you can appreciate the diagnostic procedures I have suggested you follow here to gain information about what is happening, and comparing that with how the furnace is designed to operate. The problem is at the point where there is a discrepancy between the two.

And think positively --- there are quite a few different functions controlled by that circuit board. When you replace it, they are all new again, which means you greatly reduce the likelihood of having other circuit board related problems with the furnace for years to come.

Think of it as a bargain.
 
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Old 01-04-09, 12:03 AM
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The voltages in the other blower motor wires are very likely induced voltages from the fan speed that is operating.

Each fan speed has it's own winding, vaguely similar to the different windings on a transformer, and I'd guess that that's whats causing the voltages your measured.
 
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Old 01-04-09, 12:40 PM
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Thanks for the input

Thanks for all of your input. I completely agree, and so have consequently gone ahead and ordered a new circuit board. It should arive this coming week, and then I'll let you know if it solves my problem. Searching online, I found a wide range of prices from a number of different vendors, most for the exact same board! I can understand some variance in pricing, but can't explain a $100 difference from one to the next for the same item (the exact same make and model, all brand new!).

I'll post back once it arrives and I have the results.

Thanks for all your help!
 
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Old 01-09-09, 11:23 PM
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It Works!

Well, the circuit board arrived today. So, I carefully installed it, making sure that I got all the wires correctly migrated over (the low and Hi connectors were reversed on the new board), and ran a test cycle. Badabing, it works!!

Thank you, SeattlePioneer for all your help. Your advice was spot on, and now my furnace is humming again, and for $40 no less. It pays to shop around for your parts. Some sites were asking as much as $100 more for the exact same part!

I hope my experience can help someone else.

Good luck, all, and thanks again for all the help!
 
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Old 01-10-09, 12:33 AM
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Glad to be of help, but your own ability to observe and report what you saw was the key to being able to determine the problem.
 
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Old 01-10-09, 12:41 AM
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A further note--- oil the blower motor every five years and you'll probably never have to buy a new motor. Leave the date of each oiling posted on the furnace as a reminder.
 
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Old 01-10-09, 12:55 PM
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$40 only? Wow, that's good! At that rate, a person would not feel quite so bad if you bought it and that was not the problem. How long did it take to get? Where did you get it?
 
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Old 01-14-09, 11:29 PM
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I wound up ordering it from a vendor on Amazon. It's an aftermarket replacement board that was an exact fit for the OEM original. It had all of the exact connections, in all the right locations, and functions exactly as the original. In my case, it was an ICM 271, made here in the United States, if you can believe it! The vendor shipped it on the 2nd day after order, and it took 3 days to get here, which in my case was 3/4 of the way across the country. Not bad at all.

I am extremely pleased and would highly recommend both the vendor and the product The vendor's name was HVAC Mega Store. They didn't have much feedback, so I'm not sure how long they've been around, but hey, they came through for me.

Good Luck!
 
 

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