Trane model# TWG025A140B1 blowing fuse when throwing heat on

[bcippola]

Hello all,

TRANE MODEL# TWG02 5(or S) A140B1

           SERIAL# 2064 I (or 1) TW6V

I am a household appliance repair tech and know hardly anything about heating and air. A friend that is very low on cash asked if I could come by and check out his upstairs ac/heat unit. I was completely dead. Starting from the beginning:

Approx 2 months ago he self-installed a digital thermostat and AC worked great. Last week he used heat for 1st time since installing thermostat and worked fine for several days. Yesterday he went to turn heat on and nothing. I checked power at thermostat an all is ok. Went in attic and popped panel of AC unit and blown 5 amp fuse(car fuse looking?). Replaced fuse and when he turned heat back on it went popalop and fuse blown again. He is taking a pic for me today and will update if needed. Any thoughts on where to start.Trying to save him some cash but don't want to cause more damage either.

Thanks

BC

[bcippola]

As a follow up I think I will head over there tonight and re install old thermostat(non electronic) that worked fine and see if ok.

[dkpd1581]

The unit looks like AC with Electric Strip Heat.  Can you confirm that this is AC/Heat Strip or if its a Heat Pump with with Electric Backup.

First, disconnect all the wires to the sub base of the T-stat and then do the same where the 18/8 wire goes into the air handler. 

 Now with all wires disconnected and nothing touching check for any shorts to ground on all wire pairs at the air handler.  Correct any that are found.

Next check to see if there are any wire to wire shorts...you will have to go round robin and check each wire against all the remaining ones.  Correct any found.

Visually inspect all the insulation on the wires and looks for the tiniest breaks, scrapes, and nicks.  Also look at the ends that are stripped.  A sloppy terminal job will cause the fuse to blow when the connectors get mushed together behind the T-stat head or in the air handler cabinet.

Put the wire terminals back on the air handler correctly and do the same at the T-Stat.  Don't go by what your friend did because he may have wired it wrong originally and this is causing the issue.

Since this is a digital stat, go into the Builder's Configuration Menu and make sure the stat is properly configured.

If the unit is Heat Strips only, jump the unit out at the air handler and observe the strip heat contactor.  If it is going bad it may be stuck open and the VA draw on trying to close an otherwise stuck contactor will cause the fuse to blow.

Continuing further, power off the unit and check for any shorts to ground on each side of the contactor 24VAC terminals.  Afterward if none are found, Ohm out the coil.  If you get an unusually low reading (below 7 Ohms) the coil may be partially shorted and blowing the fuse.

These are some starting places.  Check the 18/8 wiring and the T-stat configuration first.  Chances are good that's where it is at.  Let us know.

[bcippola]

I re-wired old thermostat for him and ac functioned fine. When we went to throw heat on a loud buzzing noise came from what looks like a relay or something.This buzzing part is located in upper right of picture"to the right of 5 amp fuse" in previous post. The electronic board actually sits flush on this relay??? I am guessing it is. I dont want to screw up his unit any more than he did but he has no money and I am trying to help a friend out.Any thoughts.

Summary:  He wired in new digital thermostat himself and it kept blowing fuse when heat was put on.  I rewired old thermostat and now heat doesnt come on and loud buzzing occurs at location where board sits and some sort of relay? Again scroll up for detailed pic.

I rewired his old thermostat Trane Baystat239a with following wire guidelines:

g-green

o-orange

x2-black

y-yellow

t-brown

r-red

w-white

b-blue

u-not used

f-not used

[dkpd1581]

Yes that is in fact the control board attached to the coil contacts of the relay (Trane's idea of being cleaver).  With them doing that, it does make it more difficult to troubleshoot.  You never mentioned the results of your short to ground or wire to wire tests; bottom line is this:

1. You have a direct short to ground in the t-stat wiring

2. You have a wire to wire short somewhere

3. The relay is going bad

4. The board is going bad

Trane has been known to have board troubles because of where they are located in the case makes them prone to water/condensation damage in the AC mode.  You can pull it out and inspect the board looking for corroded tracks on the underside or corroded components and connectors.

This has a Heat Pump wiring diagram to help you out (page 12) and a full schematic (page 13):

http://www.trane.com/webcache/notax/22-1695-04_04012008.pdf