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Old 06-27-2010, 01:31 PM   #1
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Please help: blown fuse, slow compressor fan, and hissing noise from compressor


Hi all,

My Carrier compressor (21+ years old) started to give up the ghost friday night. The fan was turning very slowly and obviously the air coming out the vents in the house was at room temp. I've since replaced the capacitors, and when that didn't help, replaced the fan motor. Problem still persisted.

Now I've come to find out that one of the two delay fuses in the disconnect box is blown (should have checked these first!). Replaced the fuse. When I turned on the system via the thermostat, nothing happened -- no activity from the compressor (however the air return fan did turn on).

Went outside to the unit, and noticed that the contactor was not engaged. Carefully pushed in the manual test button on the contactor with an insulated screwdriver and the fan started turning at full speed. But I didn't hear any compressor sounds.

Turned off the thermostat and let the system sit for a bit, then turned the thermostat back to "cool." Again, no activity from the compressor unit. Decided to let it sit for a while, and after a few minutes, heard a hissing sound from the compressor (the fan wasn't on though). I'm assuming that this is the relief valve.

Anyway, now I'm at a loss as to what to do next. Maybe I should replace the contactor (I did, after all, only get the fan to engage once I pressed in the test button)? Of course, since the compressor didn't start when I pressed the test button, perhaps it is dead as well? Maybe the compressor is on a delay and only kicks in after the fan is on for over a minute or 2 (I only held in the test button for about 30 secs)?

Any advice is greatly appreciated!

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Old 06-27-2010, 01:36 PM   #2
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Please help: blown fuse, slow compressor fan, and hissing noise from compressor


When you turn it to cool check if you have 24 volts AC voltage across the 2 terminals on the contactor for the coil. NOT the high voltage ones and be CAREFUL where you stick your probes and fingers or you will get electrocuted!! If you are uncomfortable with this called a Pro. $$ ain't worth dying over.

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Old 06-27-2010, 01:48 PM   #3
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Please help: blown fuse, slow compressor fan, and hissing noise from compressor


If you manually engage the contactor and the compressor won't start then it's likely the compressor (or internal compressor overload protection), 220V wiring, or capacitor.
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Old 06-27-2010, 03:25 PM   #4
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Please help: blown fuse, slow compressor fan, and hissing noise from compressor


Thanks for the advice! I tested the control voltage, which I understand is normally 24V, but mine is registering about 120V (I'm using an analog multimeter, so can't get an exact measurement).

After further testing, it appears that the contactor is definitely malfunctioning. With all power off, if I test the resistance between contacts (when the contactor is "open" of course) I get a resistance of over 1000 Ohms (my meter doesn't go higher than this). So there must be a short inside this thing.

Also, I read that the resistance between the control contacts should be 10-20 ohms when the power if off (so contactor is not engaged), but my meter is also registering over 1000 ohms for these two contacts.

Guess I need to replace the contactor and then just hope that the compressor hasn't been fried.

Do you guys recommend anything else to check or replace at this point?
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Old 06-27-2010, 04:13 PM   #5
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Please help: blown fuse, slow compressor fan, and hissing noise from compressor


Shut the high voltage off for the outside unit and have the thermostat call for cool. Check the two low voltage wires again. Each wire on a meter lead and not to ground. It should read 24 volts. If you still have 120 then check between Y and C at the furnace with the low voltage wires going out disconnected.

Fan running slow and a fuse blown can be a sign of a very dangerous situation so be extra carefull.
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Old 06-27-2010, 04:40 PM   #6
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Please help: blown fuse, slow compressor fan, and hissing noise from compressor


Going to pick up a digital multimeter right now so that I can get a more accurate reading. When I tested the control voltage earlier, I also tried it with the high voltage off -- still was in the ~120V range according to my analogue meter.

I'll test the voltage between Y and C on the furnace in about an hour and report back!

Thanks again for all the advice.
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Old 06-27-2010, 05:33 PM   #7
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Please help: blown fuse, slow compressor fan, and hissing noise from compressor


Gee... I must be slow.... you guys already pretty much have this one covered.
I was going to suggest replace fuse, speed up fan and kill the snake....
Ah well. Po)

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Old 06-27-2010, 06:47 PM   #8
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Please help: blown fuse, slow compressor fan, and hissing noise from compressor


I guess my analog meter was way off -- new digital multimeter reads 25.9V at the control contacts now. Also, the new meter has the ability to read shorts. Turns out my single pole contactor is not shorted between the terminals that the contactor "contacts" -- the meter only reads a short when I manually press the contactor's test button.

When I restore the main power to the compressor unit with the thermostat off, the compressor tries to start up (not reaching a full start I would assume though because it isn't humming very loud and it's only getting 1/2 the voltage it should due to the fact that the single pole contactor isn't engaged). After this happened, I immediately cut the main power as I'm sure this situation isn't good for the compressor. Obviously the compressor shouldn't be trying to start when the contactor isn't engaged.

Anyway, time to hand this situation over to a professional. I've got an HVAC contractor coming out tomorrow... probably going to ask him to switch out the old single pole contactor with a double pole and see if that solves the problem. I'd like to try this myself, but my wife (who has been dealing with no a/c for two days now) put the kabosh on any more DIY attempts to fix this thing...
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Old 06-27-2010, 07:27 PM   #9
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Please help: blown fuse, slow compressor fan, and hissing noise from compressor


You need to back off and not touch the unit as the compressor may be internally shorted and power may be flowing to ground. If you touch it and complete the circuit to ground you may die!! Tell the tech what is happening so he does not get zapped. Shut off the power to it and have him test the 3 compressor terminals to the copper freon line and ground to see if it is shorted.
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Last edited by yuri; 06-27-2010 at 07:31 PM.
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Old 06-27-2010, 08:29 PM   #10
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Please help: blown fuse, slow compressor fan, and hissing noise from compressor


Thanks for the safety tips -- I won't be messing with it anymore. I'll be sure to tell the tech what has happened so far and to pass along your words of caution.

So I guess that if there is a short in the compressor, then that would explain the one blown fuse. And before, when the thermostat was set to cool and the contactor was engaged, the fan must have been only getting 120V instead of 240V, which would be why it was only spinning at low speed.

So I'm guessing that I'll at least need a new compressor out of this. Might make sense to go ahead and replace the entire unit, no? Looking at Carrier's website, the Performance Series seems like a good choice (probably half as loud and 3 times more efficient than what I have now!). Anybody have experience with those models?
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Old 06-27-2010, 10:08 PM   #11
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Please help: blown fuse, slow compressor fan, and hissing noise from compressor


I had one with the exact same symptoms. Problem is when the compressor shorts it also literally burns up inside and contaminates the whole system. A 21 yr old evaporator/indoor coil may not last long either. Carrier units are pretty good. Check with your local gov't and utilities if there is any rebates for higher SEER/more efficient units.

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