trane furnace blu108e936b1 blower stays on continuously

[mwfarrar1]

This is an old 1983 trane gas furnace that the blower stays on all the time. I checked the thermostat and the fan runs in every setting. I am thinking a bad relay from what I have read on the internet. Any help would be appreciated. I have never worked on a furnace before except to replace the thermostat a few years back.

[dkpd1581]

Take the face off of the T-stat if it is a digital programmable and see if the unit still runs the fan.  If it does, turn off the power to the air handler (with the t-stat face still off) wait a few seconds and then turn it back on.  If the fan comes on again, suspect a stuck relay.  If the T-stat is one that is other than a digital you can turn the Fan off and the Mode to off as well.   You can also remove the R terminal from the sub base and see if the fan continues.  These are non-technical ways to check for the problem

The technical way to test and confirm a bad relay requires a meter set to volts.  With the unit powered up, check for 24 VAC from the G terminal to Common at the control board or where the T-stat wires enter the furnace case and are wire nutted to the unit's low volt wiring.  If you have 24 volts with the T-stat turned off and Fan off, you will have to check for a wire to wire short in the 18/8 pairs between the T-stat and Air Handler or a poor terminal connection at the T-stat or board.

You can check the fan relay itself (if it is in fact a relay not located on a control board) using Continuity on your meter across the N.O. contacts with the unit off.  If there is continuity on otherwise N.O. contacts, then they are stuck and the relay should be replaced.

Let us know....best of luck.

[mwfarrar1]

Thanks for the info. I believe I found the problem to be the fan limit switch. I went back and looked at a post that talked about the limit switch. With the power on, I turned the dial and got the fan to shut off.