Trane furnace BLU 135F948B2

[Lisa]

The furnace won't turn on.  Isn't the thermostat - I crossed the wires.   Furnace guy came yesterday, said he had 120 volts on one side of the limit control, 0 on the other - I needed a new limit control which they'd have to order.   So I figure I can go get the limit control and pop it in myself, save the cost of another repair visit.

Guy at the parts store tells me to break out the brass bridge between the two sides because I've got low voltage on one side - which I do - those wires trace to the transformer.  So I break out the bridge, install the limit control and it doesn't work.

I buy a multimeter.  There is 121V between the hot fan load wire and the wire coming out that side of the limit control, that side is good, the fan goes on when I push the button on the control.

There is 0V between the two wires on the load limit sign (the ones that go to gas valve and ignition) but there is 1.1 V between each one and ground.

There is a hot wire and a neutral wire leading to the transformer.   There are two wires coming out the other side and both are hot, 1.1V to ground.  

Does the transformer need a neutral wire out?  I can't find any place where the igniter-valve circuit goes to neutral - it goes from one transformer wire to the other with the limit control and thermostat on it as switches.

When I disconnect the thermostat and don't cross the wires I'm still getting 0.7 V between both wires and the ground on the load limit side.  

There is also a sort of box clip containing wires from both of the out-wires of the transformer - I haven't managed to open it yet and I can't see the connections inside - but that could be another place things have gone wrong, I suppose.

[AccApp]

You should get at least 24v between the two secondary leads from the transformer. Sounds like the transformer needs repalcing if you are only getting 1.1v out of it. Does the transformer have 120v going into it?

What kind of "furnace guy" does not carry a fan/limit with him in the winter?

[Lisa]

A guy from a reputable established company, unfortunately.

When I cross the wires that go to the load limit side, I still get nothing.    So the problem isn't the limit control.  Both wires to the igniter are 1.1V to ground,  0V to each other, when I do this.

When I disconnect the load limit wires - no circuit - the igniter wires are still both 1.1V to ground.  

Now what I'm getting from the transformer is 29V between black and grey wires, 9.3 V between black and ground, and 1.1V between grey and ground.

In one of the wires leading to the limit control, there is a clear shrink wrap section with what looks like a resistor connecting the wire - a little cylinder with one pointy end - what is this thing, and could it have anything to do with the problem?

I just spent $98 on a limit control I don't seem to have needed, and $140 on a repairman who doesn't seem to have diagnosed the problem.  So I don't want to buy a transformer unless I'm really sure it's the transformer.

[hvacdrd]

"Does the transformer need a neutral wire out?" - The thermostat terminal board is the easiest place to pick up the transformer secondary(output voltage) common - the terminal is labled "C"

After verifying that you have 120vac on the primary(input voltage) side of the transformer (usually the black and white wires) check for 24vac from "R" to "C" at the terminal board.

Some systems have a 3A fuse installed in the low voltage circuit, if so you will need to check that first. I would like to think the "furnace guy" would have checked, but if he doesn't carry fan/limit control on his truck....

As AccApp states the transformer output is 24vac, 1.1v reads to ground are not relative. The transformer has 4 wires - 2 on one side, 2 on the other - what are the colors?

If you can post a wiring diagram or picture of the controls/transformer we can point you in the right direction.

[AccApp]

[user=20480]birdg[/user] wrote:

In one of the wires leading to the limit control, there is a clear shrink wrap section with what looks like a resistor connecting the wire - a little cylinder with one pointy end - what is this thing, and could it have anything to do with the problem?

It is most likely a thermal fuse which is open. Cut the power and use your meter to check it for resistance. If it is open find out why it opened, it is a safety device. Don't simply bypass it and walk away.

Sounds like the transformer is OK.

[Lisa]

The transformer has 120 Vac on the yellow wire in, a neutral white wire in.  On the other side, there are a black and a blue wire with 29.3 V between them.   There is only 0.3 V at the wires to the thermostat.

The fuse has a wildly varying resistance - mostly from 50-200 ohms.

There is a voltage of 28.2 across the fuse.

Maybe I shouldn't have done this, but when I measured current in parallel to the fuse, the gas valve opened and the igniter sparked.  (And I seemed to have burned out the fuse on the ammeter part of my multimeter.)

So there is a problem with the fuse.   Can I just replace the fuse, or is there likely to be some problem that caused the fuse to open up in the first place?

I am attempting to attach a wiring diagram.

CIRCUIT2.bmp

[Lisa]

To be more accurate, the pink wire is actually a dingy white with black stripes on the transformer side of the fuse, and dingy white with red stripes on the limit control side of the fuse, if the stripes are supposed to mean something.

[hvacdrd]

AccApp has his game on...a thermal fuse is tripped from temperature not amperage (like the one in your meter). What is the location of this thermal fuse? Usually near the burners and trips if the flame rolls out for some reason.

[Lisa]

[user=11934]hvacdrd[/user] wrote:

AccApp has his game on...a thermal fuse is tripped from temperature not amperage (like the one in your meter). What is the location of this thermal fuse? Usually near the burners and trips if the flame rolls out for some reason.

So the fuse got too hot and all I have to do is replace it?  It is forward of the burners.

And I can assume that the flame rolling out was a one time fluke, and not some ongoing condition?  Unless it happens again?

When I ask at the store, I just ask for a 3 amp thermal fuse?  I'll take it in with me, but I'd like to look like I know what I'm talking about.  Do you think an electrical supply store would have it too - they're a lot closer than the furnace parts store.

[hvacdrd]

When I ask at the store, I just ask for a 3 amp thermal fuse?  I'll take it in with me, but I'd like to look like I know what I'm talking about.  Do you think an electrical supply store would have it too - they're a lot closer than the furnace parts store.

Negative - it is rated by temperature not amperage - look closely on the link and see if you can read a temperature rating on it. Probably in the range of 250-350F. Your local hardware is unlikely to carry one. 

Sometimes these tripping is a fluke, sometimes it indicates a larger problem. Possibilities include cracked or plugged heat exchanger, dirty burners, gas pressure problems, etc. Like AccApp stated this is NOT something you want to bypass as it is a safety device.

Is the fuse attatched to a ceramic tear-shaped block or just in-line with a wire?

[Lisa]

The fuse is in line with the wire, no ceramic block.  I don't think it has a temperature on it.  It says,

K

4227A

WEABU either 1090 or 109C

109 deg C is 228 F, so maybe that's a temp.

The furnace is 15 years old and the repairman did say rust concerned him, and we should have a CO detector,  so a crack is possible.  I will clean the burners off.

My plan is to replace the fuse, and if it fails again then to assume there is a larger problem and investigate.  Safe plan?

 The furnace failed ten days ago, and but started again after I tinkered and jiggered with things - for no particular reason i could see.   What's the likelihood of the limit control having had anything to do with this?  If the furnace got too hot and the limit control failed to  shut down the burners, then the fuse could trip.   Then there would have to be something else making the furnace too hot in the first place -  doesn't seem so likely that AND the limit control would fail together.

[hvacdrd]

Not likely they both failed at the same time. Each one serves a different purpose. The limit you replaced earlier - fan/limit control shuts off the burner in the event the heat exchanger gets too hot, lack of air flow usually caused from dirty filters, failed blower motor, etc.

The fusible link is a one time deal in the event the flame rolls out from causes discussed earlier. Replace it (they are only a couple bucks) and see what happens. Observe the flame at the beginning of the cycle before the blower starts and then look for changes it starts.

[AccApp]

I have seen many flues blocked by heavy snowfall which will then cause a thermal cutout to trip. By the time I get there the snow has cleared enough to fire up the furnace. I have seen a deteriorating flue liner break apart and block the the exhaust passage enough to trip a thermal safety device. Good luck, tell us what you find.

[Lisa]

Yeah.  It works.  The first time I switched it on, nothing happened, but the second time, oh yeah.   No flame roll out during ignition or the blower starting.

Maybe the fuse costs a couple dollars to those of you in the business - I get the feeling the parts stores around here charge more to amateurs than pros.  $25 sound right to you?  Still lots cheaper than a service call and the wrong part by far.

The flue is metal, and we haven't had enough snow to block it.  I cleaned the burners, and I got a CO detector.

Thanks, guys.   My husband's out of work, and throwing money away hurts right now.

Now I've got to see if I can  get the furnace repair people to give our money back.  Slim chance, huh.

[AccApp]

What did you do to fix it?

[user=20480]birdg[/user] wrote:

Now I've got to see if I can  get the furnace repair people to give our money back.  Slim chance, huh.

Try it, you got nothing to lose at this point, squeaky wheel gets the grease.

Was the original fan/limit switch no good?

[Lisa]

Can I test the original limt control without reinstalling it?  Because it was a pain - the repairman was saying "oh, s---" at it.   The spring clamps didn't want to let go or receive the new wires, and that plastic strain reliever - how are you supposed to open it once it's closed, or slide it back in the hole in the box?  I had to use pliers to widen the hole in the box and close it again.

I'm taking 7 classes and raising 4 kids so there's not a whole lot of time to play with the furnace.