4amp transformer fuse blowing

[TPG]

Hey all, im a facilities mechanic and im pretty familiar with hvac equip, i work on it pretty regularly so i have some experience.but i have a 4 amp transformer fuse that keeps popping on my American standard split system heat pump. first thing i checked was the low voltage wire that connected from the air handler to the outdoor unit and it had bad insulation in spots so i thought i had it! but after cutting back the 18/6 and re-stripping and connecting im still having the same issue. so im fishing for some tips to send me in the right direction, tomorrow i will be checking continuity and ohms on all my low voltage coils to make sure none are bad/shorted, and then checking continuity on the tstat wires. but any additional info or advice from some more experienced guys would really help me out.

[HayZee518]

do yourself a favor and pick up a 4 amp resettable circuit breaker. Siemens manufacturers these things. Look in electronic supply houses. As far as to find what is blowing the fuses [ breaker] look for some utilization device. Probably a relay coil because a thermostat is just a switch. a fan contactor coil might be shorted out.

[TPG]

Thx Hay, for the heads up. ill be spending a few hours on it today around 3-6pm PST. ill have some new info posted after that

[HayZee518]

what I find sometimes is the fan or hvac compressor contactor. they are made up of laminated iron punchings, rivetted together and have a copper shunting winding on one or two pole pieces. if this winding opens it causes a high current in the holding coil even if it operates at low voltage. the function of the coil is to short out eddy currents in the laminations. check out the reversing coil. We had a poster about two months ago that had similar problems. what they found was the wire that comes off one operating coil of one side of the reversing valve was shorted to ground and only manifested itself when the valve was operated.

[HayZee518]

please check out the HVAC section. Look for 6 postings by "sylvester36" somewhere around july 7-8 of 2010. His problems were about a 240/24 volt control transformer burning up. Maybe you could glean something from this posting.

[TPG]

Thanks a ton Hay. The time and information is much appreciated, i will make sure to take a look at those other postings.

[TPG]

So i found the problem circuit. after trying a few other steps i unhooked the 18/6 where it pigtailed to the outdoor unit.then ran the indoor unit and stat. Everything ran fine for about 10min. since the breaker was tripping very consistently after about 30-60s before this, it indicated to me that the problem circuit is on the outdoor unit 24v. so then i re hooked hot and common 24v to the outdoor unit and it ran good for 10m again. then i hooked up the white to orange24v and ran it again, still no problems. finally i hooked up the yellow and it tripped after about 40s every time again. once i had that wire isolated i traced it out both physically and on the wiring diagram. and it connects directly from the pigtail to a 6 wire plug on the PCB (printed control board).so i checked the back of the board and saw no suspect markings like burns or discoloration. Then i went to the diagram to try to find out what that wire is routed to through the plug and i cant pin it down. its model 6c0036a400a1 and attached is a scan of the schematic. so im kind of stuck atm on whats shorting me out causing the 4amp transformer to pop even though i know what circuit it is any info or advice is always very appreciated.

[HayZee518]

Is a triac used to turn on the blower? if the gate is always on, then there'll be a high current on the triac, enough to pop the fuse.

[TPG]

not sure what triac is to be honest, so i don't know. can you clarify a bit ?

[HayZee518]

A triac is a solid state semi conductor that looks like a flat transistor with three leads coming out of it. it fastenes to a metal heat sink by a bolt and nut. a high voltage is fed into one side of the triac. a triggering voltage is fed to the center terminal which causes the triac to fire or conduct. when this happens the other side of the triac switches ON and the motor starts. removing the gating voltage shuts off the transistor and the motor stops.

[TPG]

No it definitely does not have one of those. ty for asking tho. im actually just letting the 24v circuit run no loads are on Ive unhooked the fans and comp power since no line side voltage is needed to test what im looking for... like i said the 4 amp breaker trips only when the yellow wire is connected from indoor to outdoor i can have the R,W,B, all hooked up and let power circulate with no issues. soon as that Y gets connected it trips. im currently trying to check all my coils to ground and my tstat wire to ground. Question: if im checking the 18/6 24v wires where they connect to the outdoor unit for continuity to ground screw, should the common wire have connection to ground ? i know no others should but i was told by someone else if 24v common is tested to ground it will show a connection and its not a problem.

[HayZee518]

can you send me a larger diagram of the unit? I tried to expand the attachment but it gets all blurry. send it to my email address which should be in my profile page

[fgrir]

I had the exact problem on my carrier heat pump about a month ago. called my cousin that used to be a hvac tech and he said those unit's are known for the wiring in the bottom of the outside unit's to short against the copper lines. i popped the top fan and found several spots where the yellow wire had melted to the lines.taped it up and secured them, problem solved.

[TPG]

Got it! at one point i had checked continuity and resistance on all the 24v coils, as well as testing ea coil to ground. oddly enough i saw no readings that would indicate a bad coil or short on any of them. But once i took amperage reads on all the coils, the compressor contactor coil was reading 4.7 amps !! so i replaced the contactor and all is well again ....for now

[HayZee518]

check out that yellow wire that other user referred to.