Compressor causing Fuse to pop

Hello all. Please excuse my ignorance. I searched and found some useful things, but none exactly what I needed.

Basically, when I call for the AC to be turned on (via thermostat, or jumping wires in the furnace) it caused the 3 amp fuse on my furnace's circuit board to pop. I can run the blower and heat without issue.

Can you please suggest what I can check?

I'm going to do a continuity check on the two wires that request the compressor to turn on (coming from the thermostat and common, to the compressor contactor). [Would I do this via an Ohm check on the multimeter. Disconnect both ends (from furnace and compressor), tie one end together, and test ohms on another?)

Thanks, Please let me know if something is unclear. I have a hard time describing this as I'm just learning.

 

Pull the wires "with the power off" to the contactor coil and tape them. Turn the power back on and turn the stat to call for cooling. If the fuse don't blow the replace the contactor. If it don't then you need to do some tracing.

 

continuity checked ok from two wires running to the compressor (with the power off) while still connected to the contactor.

mattison, so are you asking me to turn off the power to the condenser, then connect the two wires running to it from the furnace (the wires used by the thermostat)?

 

Since everything works fine until you call for cooling, kill all power and disconnect the a/c contactor control voltage and after turning the power back on call for cooling.

If the fuse does not blow then replace the contactor. If it does blow then it's probaly not the contactor, and you have to look at the thermostat wires leading to the outdoor unit.

 

ok, thanks for clarifying. I think i get it. So disconnect the control voltage from the compressor side, basically remove the two wires from the contactor. Cap the end of each wire, restore power, than try to run a/c from thermostat again to see if fuse pops.

 

You got it. The 2 small wires on the sides of the contactor. To kill the 24vac you have to also kill the breaker to the airhandler.

 

Thanks, I'll kill power to both the compressor, and the furnace. And just to be sure, I don't cap both wires together right? I cap them off separately?

 

Seperately is correct. If you put them together and power it up you will for sure blow a fuse or even the transformer.

 

thanks again. I'll give that a shot when I go home. Are contactors available at Home Depot / Lowes? Do I just need a 24v model, or are there specific models for specific compressor brands?

 

HD and Lowe's do not sell contactors. Try Grainger's, Johnstone, or a local HVAC supply house. Contactors are essentially the same except for contact ratings (usually 30 or 40 amps), coil voltage (24, 120 or 240 -- most residential applications are 24) and numbers of poles (1 or 2). If you need one, get one that matches your current one on those specs (although you can get a higher contact rating than you have currently).

 

sounds good to me. now if only they had weekend hours! thanks for all the help thus far. I'm learning quite a bit.

 

Take the contactor with you is the best bet. To get the right new one

 

but then I have to remember which wires go where

 

Draw it out, mark the wires how they go

 

Or take a picture with a digital camera. Drawing it out should be simple though. Residential don't have that much on the contactor.

 

Ok, I removed the 24v wires from the contactor and tried the AC from the thermostat as requested. The fuse did NOT blow this time.

I then pressed the button on the contactor and the compressor turned on without further issue.

So it looks like the contactor is the likely culprit as suggested. Thanks all!

I'll get a replacement and install it this week sometime.

 

well. looks like i spoke too soon. I replaced the contactor, fuse still blows on the furnace's board when I call for AC. When I press the button on the contactor, the compressor starts up, and the copper pipe running to the furnace is cold to the touch.

Any other ideas?

 

Defrost Board?

Make, Model of unit?

 

Well, the furnace is a Kenmoore, and the compressor has no brand name stickers anywhere I can see. Inside the "guts" of the unit is a fan, and underneath the fan is a large canister looking thing. That canister says "Bristol", the fan says "A.O Smith". Could be a Kenmoore as well for all I know.

How would I know if I have a faulty defrost board?

 

anyone?

could i test the voltage going to the contactor with a multimeter to see if its reading 24v? If it's not reading right around 24, is that an idicator that the transformer isn't putting out enough juice? (even though the control board has power, and I can call the blower and heat?)

Someone mentioned to me that it could b ea safety preventing the contactor from pulling in. This could be a low pressure switch (meaning low on freon?) , a water flow switch (no water leaking out the bottom, when i manually engage contactor), condensate restriction (does that mean a blockage of the condensation pipe leading away from the furnace?) circuit board bad (doesn't seem like it to me), Or loose connection in the wiring (i see no arcing, signs of burnt wire, or loose connections).

How could I test any of this? I don't want to randomly replace parts until I get it to work.

 

You can check for 24vac at the contactor coil.

Also follow the low voltage wires and see if they go through any safeties. They should be located on the copper refrigerant lines.

It is possible that your unit has an oberflow pan with a saftey in it. Check there to see if there is.

 

low voltage wires meaning the 24v wires running to the contactor coil? or is there another set?

 

Look back at post #16 and #17.

In #16 we took the wires off the contactor and called for cooling, fuse did not blow. No complete circuit.

Changed the contactor and in post #17 the fuse blew.

He said the "Furnace" has the board with the fuse.

earnold25, how many thermostat wires go to the outdoor unit? Is it only 2, or more than 2 with only 2 connected? Does the wire exit a crawl space? Is it suseptible (SP) to getting hit with the weed eater?

Glad I stubled on this site, I just though of a signature.

Back to the problem at hand. If the contactor is disconnected a call for the compressor does not blow the fuse, because the circuit is not complete.

But with the contactor connected with a call for the compressor (with new contactor) the fuse blows. Somewhere in the path of voltage from the transformer in the furnace, through the board to the stat, back to the board, then to the contactor is a dead short.

I would look at the common wire from the compressor contactor back to the board as "Y" was energized but not connected to the contactor, fuse did not blow. Thus, common on the way back to the board had no voltage until it was re-connected to the contactor.

What do you think?

My wife hates my "new" hobby while she watches House Hunters.

The Sig? "Long Distance Troubleshooting is Hard, This May Take Awhile"

 

Good thinking on the common wire. It was throwing me for a loop for a minute when the new contactor didn't work.

The signature line is great.

 

only two wires from the thermostat to the compressor. one which is wire nutted to the yellow thermostat wire (red to compressor) and one connected to the common (white). There are no extra wires being unused.

Also, I noted no safeties along that 24v two wire to the compressor. Obviously there is a condensation drain pipe leading from the furnace, but I see no pan or any way to get inside where that pipe comes from without taking the world apart.

...and I kinda see what your saying about the common. How can I check that. Unhook it segment by segment and do continuity checks?

 

ok, here's some pics. I found a third wire on the two wire that's unused (green). Funny thing is that its not on the furnace side (must be pulled far back into the sheath or something) Did continuity checks on all three to see if I had a short, but no.

furnace side thermostat and two wires (red two wire was connected to yellow), white two wire was connected to blue (common)
http://download-v5.streamload.com/962b4363-02e5-4fd4-b7cd-1c287dc1cbd9/earnold25/Hosted/furnace%20side%20stat%20two%20wire.jpg

two wire at conatactor (there a third green wire that's unused. must be pulled back into the sheath at the furnace side)
http://amd.streamload.com/earnold25/Hosted/two%20wire%20compressor%20side.jpg

wires at thermostat
http://amd.streamload.com/earnold25/Hosted/thermostat.jpg

furnace control board
http://amd.streamload.com/earnold25/Hosted/furnace%20control%20board.jpg

 

I'll bet the compressor tstat wire at the furnace is what we call 18/3 (18 gauge wire with 3 strands in one sheath) and somewhere between there and the condenser it switches to 18/2. That needs to be investigated.

I also see that the transformer in the furnace is grounded to the casing, and I see a ground wire in the condenser pic.

Did you read the red and white compressor wires to ground? And to each other?

Assuming the red wire is connected red to red at both ends. Might try connecting red back to yellow at the furnace and the condenser. Then connect the other side of the contactor to ground. This eliminates the common wire all together.

 

backward, 18/3 at condenser and 18/2 at furnace.

no one is perfect.

 

sorry, i did check the 3rd wire in the "two wire" (green) to the other two wires for continuity, and there were no shorts detected. honestly, I'd like to replace it as well, but if its not shorting, doesn't seem to be the issue at this point.

I don't follow this however. Could you explain a little further?

"Assuming the red wire is connected red to red at both ends. Might try connecting red back to yellow at the furnace and the condenser. Then connect the other side of the contactor to ground. This eliminates the common wire all together."

 

First you checked the wires for continuity to each other and found no shorts. Did you check each wire to ground? Continuity there is a short.

If you have ever taken the cover off of your breaker panel (load center) you may have noticed that all of the 120 volt breakers have a black wire and all the white and green wires go to the same buss board.

That said, the electric company sends you power which passes through your lights, fridge, and a/c to the ground which makes a complete curcuit.

We already know that the "hot" wire to the compressor contactor is not shorted to ground, because you had the wires off of the contactor and called for cooling, no pop.

When you put the wires back on the contactor the path of voltage/current was this. 120 volts on the primary side of contactor turns into 24 volts on secondary side. Common side of of secondary side is grounded.

So when the stat calls for the compressor, the 24 volts at the stat is switched to send 24 volt to "Y". This travles from "R" to "Y" to the furnace, then into the two wire out to the contactor. Through the contactor coil to the common side of the contactor back to the furnace and ground. I can say ground because the common side of the transformer secondary (24 volts) is grounded.

So, from what I can see from your pics, the transformer is grounded. If the furnace is grounded and the out door unit is grounded then you can ground one side of the contactor and eliminate the need for the white wire (common from the contactor).

If you connect the wires back togather at the furnace but not at the outdoor unit ant find the 24 volt hot wire, then turn off power and connect the hot wire to the contactor and the other side of the contactor to ground you'll have a complete circuit. Does the fuse blow?

Also, am I correct it that the "Y" (compressor) does not go throught the board in the furnace?

Guys, did I explain that correctly?