Trane blower won't stop!

[doitmyselfer]

I have a 16 year old Trane, model number TWV736B140A0. The blower runs continuously and will not stop unless power to the unit is disconnected. I am very confident I can replace the defective part if I can determine what needs replacing. Does anyone have experience with this model Trane? I have included a link with a picture of the unit. Thanks in advance for any help. http://www.homerepairforum.com/image...s_001_w550.jpg

[HayZee518]

when you shut off power the blower stops. when you turn it back on does the blower start again? I mean right away? if it does then the blower contactor has welded contacts or the control coil is burnt and has expanded holding the crossarm in the closed position. in this case (either) change out the contactor or at least the control coil if it was the latter.

[doitmyselfer]

Yes, it comes back on instantly, the moment power is restored.

[HayZee518]

then I would check the blower motor contactor. Find the control box and look for a brownish to burgundy colored relay. Could be marked in white ink, Arrow-Hart or Allis Chalmers on the cover. Usually rated 30 amps open style with a 24 volt operating coil. The compressor contactor looks the same only slightly bigger "physical" size but still operates off 24 volt. I'd have to see the control diagram to see how the blower is connected in the circuit to troubleshoot it further as there are high pressure and lower pressure limits in the system.

[doitmyselfer]

Well I finally got time to take the cover off the furnace and locate what I believe you are telling me is probably bad. I have included a few pictures of the relay? I cant find this part number anywhere on the internet. It doesn't even come up on Honeywells site. Any suggestions? http://www.homerepairforum.com/image...s_007_w550.jpg

[doitmyselfer]

Here is the other pic, http://www.homerepairforum.com/image...s_012_w550.jpg The Honeywell part number is R8222U 1071

[HayZee518]

the first picture of the contactor looks about right. You can use just about any manufacturer, just arrow-hart is small enough to fit and still have the ampacity to do the job. Look for the amp rating, voltage it controls, and the coil voltage. with this information go to the grainger site or to your nearest electric jobber.

[doitmyselfer]

I went to Graingers today and purchased a new relay. It looks identical except it has two additional quick connect terminals and all the specifications are the same except for one. The new relay has DPDT switching, and the spec sheet in the furnace says the old one is DPST. Anyhow, I installed the PC board on top of the relay the same as it was on the old relay (it slides over two of the terminals on the relay). Flipped the breaker on and the fan immediately started. So I am assuming the relay was not the problem, unless the difference in relay switching type or the additional unused terminals is an issue. Any thoughts? Thanks

[Hube]

Are you sure the T stat's "fan on" contact/lever is not the cause?
Some brands of T stats will "freeze after long usage of 'fan on setting'

[doitmyselfer]

Well I haven't checked it with a tester but I have moved the switch back and forth several times when I first noticed the problem. The fan switch is in the auto position and the mode selector switch is in the off position on the thermostat. Fan comes on immediately when power is restored. I found on the stat sheet inside the unit that it has a 80 second delay. Waited much longer than that when checking the new relay. It never quit running.