My fan center transformer failed, I replaced it with a new one, and it has failed too. It seems the transformer burns out when the thermostat is moved to the cool setting.

There must be a short in the low voltage wiring, however I'm unsure of what I should see across the different L.V. wires when testing with a VO meter. I connected the LV wiring to the new fan center just as they came off the old one.

The old unit had been in service for 18 years, worked fine last weekend, then suddenly failed, as with the new replacement. Any help or insight would be appreciated. I'd hate to burn up another $50. Unit fooling around with wires I'm not sure about. Thanks for your help.

Regards,

Tom

I suspect the wire is shorted. Tracing it may be problematic. Maybe others would have a better idea.

The voltage on these systems is usually 24 volts, this can be confirmed by reading the label on the control transformer. If you know how to use a continuity tester, then a simple method to find which wires are shorted would be to label each wire with numbers, letters etc. disconnect all the wires and test between each wire.

Since each end is clear, you should not see any reading, or low ohms if a ohm meter is used, across any pair of wires.If you do see a reading this will indicate a short.

The next trick is to find the problem area. Could be pinched or peirced any where along the entire length of the cable. You will need to inspect every inch of the cable for damage.

The trick is there is no trick, only patience.

If there are many obstacles or other reasons the fault is difficult to find, many times we just replace the entire cable to save on dead time.

Check first that you are dealing with low voltage, or be sure to know the voltage you are working on. If for some reason the volts are higher than 24 volts, then more detail is needed, and caution on your part.

Thanks for the feedback, but I guess what I'm asking what I should see (using a meter) on the far end of the LV thermostat wires as they are wired.

The system is definitely 24 volts, so I'm looking for a short through the LV thermostat wiring between the fan control relay and outside unit or the fan control relay and/or the themostat itself. These meter readings would be from the end of the LV wiring that would normaly be connected to the fan control relay transformer, which of course is burned and now gone.

I will certainly look at each "pair" of wires (especially the "cool" wires) in an open (disconnected from the far end) status, but would like information regarding the indication/meter reading I might see as they are connected at their "far" ends right now. It's strange that this could simply turn bad so quickly, and without an obvious cause. Thanks again for any insight and/or opinions.

Regards,

Tom

Your information on you last post is very hard to follow at one point you say (These meter readings would be from the end of the LV wiring that would normaly be connected to the fan control relay transformer, which of course is burned and now gone.) Are you talking about a transformer or a fan really? Also I am not sure what a "Fan Center Transformer" is.

As tkrussel was telling you how to check the wires you may not have understood to disconnect the wires from the circuit board at the air handler and also at the thermostat. And then after numbering them on both end of the wires tie wire number one and number two together and then go to the other end and use your ohm meter. If you get any movement of the meter other than a slight wiggle the wires are shorted if not tie wire number one to wire number three and so on till you find out that you have a short or you don't have a short... if you do find two wires shorted you might be lucky enough to have an extray wire in the group of wires going to the thermostat. If you do then use the extra wire to replace one of the shorted wires, just remember to use the new wire in the same place that you disconected the shorted wire from.

You need to make sure you are replaceing it with the proper transformer. If you have a 70va transformer and are replacing it with a 40va tranformer it will not work.

You need to make sure you are replaceing it with the proper transformer. If you have a 70va transformer and are replacing it with a 40va tranformer it will not work.

I replaced three transformer on my air handler, and like you not sure why this is happening, I have a RUUD 5 ton 12 seer condenser unit.