Heat Pump blows 5 amp fuse..

______________________________________________________________________ Situation: I have a Carrier Heat pump that will not start in either “cool” or “heat” modes. From the thermostat, switching the “fan” switch to the ON position doesn’t turn on the blower. Equipment: a) Air Handler- Model# FB4ANF048 b) Air Handler- Product# FB4ANF048000ACAA c) Compressor- Carrier Model# 38YCA042300 d) Thermostat- Honeywell Model# T841A1001 e) Control Board- Texas Instruments Model# HK61GA003 Steps taken: 1) I checked all breakers and found them still be in the ON position; however, just for good measure, I cycled each breaker OFF and then back ON. Still no change. 2) With a volt meter, I checked the wires at the thermostat, and didn’t receive a reading from wire posts marked [W2] and [R]. (From what I could find, there should be 24VAC at those terminals) 3) I opened Air Handler pannel to check the low-voltage transformer. I discovered a 5-amp fuse on the Control Board. This fuse had blown. I replaced it, and ensuring all settings were in the OFF position, I re-applied power to the four breakers associated with the system. 4) From the Thermostat, I moved the FAN switch to the ON position. (The SYSTEM switch was placed in the OFF position) The blower motor did come on and remained on until I again moved the FAN switch to the AUTO position. 5) I then adjusted the temp setting to a position that would cause the unit to heat the home. When I moved the SYSTEM switch to the HEAT position, I noticed a small flash from the Air handler. Checking the 5-amp fuse, I found it had again blown. 6) I moved the SYSTEM switch back to the OFF position and replaced the 5-amp fuse. I, again, repeated step 4 — with the same result. Next, I moved the SYSTEM switch to the COOL position (after adjusting the temp settings to a position that would cause the system to cool the house), and again noticed a small flash from the Air handler. Placing the SYSTEM switch to OFF again, I checked and found that the 5-amp fuse had blown once more. 7) I turned all breakers back to OFF. Moving to the compressor, I checked the relay which applies the 220VAC to the compressor with my OHM meter and found it to be good. 8) I disconnected the wires from the coil side of the (compressor) relay, replaced the previously mentioned 5-amp fuse and turned all breakers back ON. I then repeated step 4 and 5 — with the same results. I did these things to confirm that the compressor relay wasn’t causing the problem. 9) Back at the Air Handler, I found a relay on the Control Board. Thinking that this relay must be used to apply the 110VAC to the blower motor, I figured it would be working as intended. However, just to be thorough, I tested it with my OHM meter. The results were as expected — so this doesn’t seem to be the cause either. 10) A close examination of the Control Board revealed a resistor that appears to have over-heated and was discolored. Thinking that the resistor may have blown, I removed it for testing. The colored bands of this resistor indicated that it was a 1000-OHM type. Using my OHM-meter, I received a reading of .948 K-Ohm. I think that this should be within tolorence, but have replaced it with a new one anyway. 11) After reconnecting everything, I started again at step 4; same results. Moved to step 5 and also had the same results as before. So now I’m at a loss as to what I should check next. If there is anything you could provide in the way of guidance, I would be most appreciative. Replies: Mastertech “Re(1):Heat Pump blows 5 amp fuse..” , posted Sun 30 Jan 23:32 ______________________________________________________________________ Sounds like u have a short in the wires going outside or in the control wires in the outside unit. Mark the wires to the outside, then remove them from the air handler and retry test. If it doesnt blow u now its the outside unit. Hook the wires back up and go out side mark all the wires and then remove them at the outside unit(make sure to tape off or wire nut the wires. Retest if not blown then its in the outside unit wires. Remove the control wires from the contractor and ohm out the coil should be above 10ohms or it will cause a short. If pass then check one terminal of the coil to the ground should be infinite. If me i would check contactor first. Then if okay do as i say above. Michael-E “Re(2):Heat Pump blows 5 amp fuse..” , posted Tue 1 Feb 00:25 ______________________________________________________________________ Thanks for the quick response. Per your recommendations, I disconnected the wires (from the Control Board) that run to the Compressor. I repeated steps 4 and 5. This time, the fuse DID NOT blow. The system continued to run until I moved the SYSTEM switch back to the OFF position (the fan continued to run for a short while before turning off as it usually does). Next, I reconnected the wires to the Control Board, and removed them from the connector at the Compressor. Again, repeating steps 4 and 5. Again, the fuse DID NOT blow. Checking the contactor, I did not receive a reading > 10 Ohms as you mentioned, instead the reading was 0.1 Ohms. If I’m understanding this right, that would mean that the contactor is causing a direct short between terminals — which would be the cause of my problem, yes? Gatorguy “Re(2):Heat Pump blows 5 amp fuse..” , posted Mon 31 Jan 21:45: ______________________________________________________________________ The other possibility is a short to ground in the “Y” circuit, usually the yellow thermostat wire. This is the compressor contactor circuit. You can take the yellow wire (not bad to do them all) loose at the air handler and at the condenser. Set you ohm meter to around 200k ohm and check from each wire to ground. You can also check each wire to each other to make sure you don’t have a short from say yellow to common. I say the 200k ohm scale because it is very possible to have a high resistance short to ground that you will not see on a lower scale. One example is a bare wire in the dirt. You may have to get up on the bigger scale to see that type of short, especially if the ground is a little dry. The “Y” circuit is energized in both heating and cooling and is why I suggested you look there if you find the contactor coil okay per Mastertech. Another little tidbit. Place a jumper wire across the fuse holder spade terminals. Put an ammeter around the jumper wire. Proceed to do your testing. Be prepared to pull the jumper wire off very quickly if your ammeter shows more than 5 amps (where it would blow the fuse). Normally you will not see this amperage be >1amp. This will not damage the transformer if you do it quickly, but it will save a lot of fuses. The only fuse blown when I go on a service call like yours, is the fuse blown when I get there. I’ll have the problem solved and I’ll see the problem gone with my meters before I will put a fuse back in the unit. No point in going through all of those fuses unless you like the little “flash” 🙂 [this message was edited by Gatorguy on Mon 31 Jan 21:48] Michael-E “Re(3):Heat Pump blows 5 amp fuse..” , posted Tue 1 Feb 00:42: ______________________________________________________________________ Thank you for the reply. I removed the wires that run to the Compressor from the Control Board. I also removed the wires from the Compressor side at the same time. Using the Ohm meter, I checked for continuity between all wires, and found none. Having all wires disconnected, I didn’t find a short to ground. I wasn’t sure if you meant for me to leave all wires connected at both ends except the Yellow (Y), and then place one probe to ground and the other probe to each one of the other wires (excluding Y) and see if a short to ground was found.. If you can, please let me know what you intended. Thanks! [this message was edited by Michael-E on Tue 1 Feb 00:47] Gatorguy “Re(4):Heat Pump blows 5 amp fuse..” , posted Tue 1 Feb 21:49 ______________________________________________________________________ If you found .1 ohm resistance across the coil of the contactor, you’ve already found your problem. I was trying to tell you to pull all of the thermostat wires loose at each end (condensing unit and control board), separate them to where all are just floating in free air space with nothing touching them. Set your meter to around 200k ohms and put one lead to a thermostat wire and the other lead to the equipment ground. Your meter should show infinity. If you received an ohm reading at all, there is a short from that wire to ground. The other test is to put one lead on one thermostat wire and the other lead on a different thermostat wire. Again, your meter should read infinity. If you receive an ohm reading at all, the two wires are shorted to each other. If you had a situation where the yellow wire (compressor contactor) is shorted to the common wire (usually blue or brown), you would be blowing a fuse each time the yellow wire is energized. Hope that helps. Michael-E “Re(5):Heat Pump blows 5 amp fuse..” , posted Thu 3 Feb 01:32 ______________________________________________________________________ Well, after removing the contactor, I rechecked it. This time, I noticed that my meter was not moving the decimal so I used a different meter. This [alternat] meter gave me a reading of 15.8 Ohms. Adjusting the settings for the first meter (after refering to the users manual — it’s one of those high-tech types), I again checked the contactor. This time, the reading alternated between 15.8 and 16.0 Ohms. So, the contactor is apparently good; leaving me to continue looking for the cause.. Thanks for the clarification on the wire-to-ground testing. I conducted the test, but didn’t find a short. Following your advice for the Yellow (Y) wire… I followed two yellow wires that went inside the compressor cage and down to a small fitting on one of the small copper tubes. One of the two yellow wires was sitting against the cage and also against another, yet smaller copper tube. Upon closer inspection (yes, I removed the top of the cage) I found that the yellow wire had apparently rubbed so much on that small copper tube, that it had rubbed a hole in the insulation of the wire — causing a short to ground to occur. I repaired this wire and relocated it so it wouldn’t touch anything. Reconnected all wires, both inside and out, crossed my fingers and flipped all breakers on…. After moving the SYSTEM switch to the HEAT position, the unit came on and is now working again. Thanks again for all the feedback. The information provided helped to increase my understanding of heat pumps and the logic behind the low-voltage side of things. You guys are great!! And what an awsome web site / forum! I’m definitely going to bookmark and pass on to others. Gatorguy “Re(6):Heat Pump blows 5 amp fuse..” , posted Thu 3 Feb 20:23 ______________________________________________________________________ Sounds like you found either a pressure switch or a coil sensor that is in the low voltage circuit. I have found several instances like you have described where a wire has rubbed the insulation off on a copper line and shorted. I’m glad you found it. You did something that a large percentage of service techs (at least the ones around here) wouldn’t be able to find.