short circuit in suburban furnace

[oxfletch]

I have just purchased a 1971 25' airstream, and had a dealer check it out (which I'm becoming less convinced was useful, but still). Seems to have a short in the furnace ... details below.

They said everything mostly works OK apart from the furnace (though I've since found some issues with the univolt, but still). First they told me the motor was bust and I needed a new one, it was a sealed unit and couldn't be rebuilt. I reluctantly told them to go ahead and replace it ... then they called me back and said the whole thing needed replacing - $600 + labour, and they couldn't get
parts for it. At this point I told them to forget it (and indeed it turns out parts do seem to be available ;-( )

Their conclusion was that it had a "dead short". Hmmm. After mucking around with it a bit, if I connect the thermostat wires, it seems to have 43 ohms resistance across the power lines. Without thermostat wires connected, it's an open circuit. I presume that's what they were referring to. Is that normal or not? Presume not ... AC motors are just coils that are meant to be a sort, but DC ... don't know?

I removed it OK, and it seems to have 4 external wires, 2 blue ones, which seem to connect to the thermostat wire (which they left disconnected), a yellow and a red. Coming to the furnace is a double grey wire (to thermostat) and a white and red heavier duty line (power, presumably). Furnace red <-> Trailer Red, Furnace yellow <-> trailer yellow. There's a knot in one side of the blue pair, and in one side of the incoming grey line, presumably marking polarity.

I removed the motor (disassembling far more than I needed to, Doh!), and connected it across a 12v source (2amp battery charger), and it spins fine.
Moreover, it's not a sealed unit at all - hmmm (dealer -1 more point).

So ... I guess my questions are:
1) is it a problem that it has only 43 ohms across it
2) are my assumptions above about wiring correct? (is there a schematic anywhere?)
3) what other tests can I run on it to see if it works?

It doesn't seem to have 12v power to it from the trailer, which presumably
means the fuse is blown (or I'm an idiot and have missed an "on switch"
somewhere?)"

[oxfletch]

OK, some more details and photos.

Schematic of what I think the wiring is like:
http://mbligh.org/pics/furnace/schematic.png

Wierd round thing (thermal switch ??) I really want to know what it is
(labelled 'E' in the schematic)
http://mbligh.org/pics/furnace/schematic.png

Front of the furnace ... what's the buttom on the bottom left corner for?
http://mbligh.org/pics/furnace/front.jpg

Presumably the main relay??
http://mbligh.org/pics/furnace/relay.jpg

(heat based?) mechanical switch for the gas?
http://mbligh.org/pics/furnace/gas_switch1.jpg

Some electric switch for the gas?
http://mbligh.org/pics/furnace/gas_switch2.jpg

Whole side view:
http://mbligh.org/pics/furnace/side.jpg

[TomW]

A dead short will blow a fuse, or pop a breaker depending on which one your furnace has. An open circuit will do neither. The first order of business is to determine which circumstance applies to your situation.

If your Airstream has its original furnace, electricity is only needed for the fan motor, and the gas valve solenoid. But the wiring can appear somewhat complex once the overtemp protection, and sail switch are wired in.

But the first step in your troubleshooting will be dictated by how much current the furnace is drawing, i.e. Is the fuse blowing/circuit breaker popping? Start your fuse search at the Univolt, and go from there.

Tom

[oxfletch]

Thanks. So the univolt looks to be riveted closed. The only way to access the fuses / breakers is to drill off the perforated blue metal cover box? That seems like a really bad design ...

[nickcrowhurst]

Oxfletch, before you start demolishing the Univolt, I humbly suggest you are on the right track by first trying to understand how each part functions. I don't believe your wiring diagram is correctly labelled. I started on the same mission as you a few years ago, so I may be able to assist, though a forum member with the same model furnace as you will be a better source. My furnace is of the electonic module type, but some parts will be similar. Forgive me if some parts are "teaching my grand-daddy to suck eggs". The first thing to find is the "always live" power wire to the furnace. This power wire goes to one of the 4 relay connectors.The relay ("A", I guess) is just a fancy switch for switching this power line. The relay connector opposite the "always on power wire" is the switched power out. The switching is done, not by a finger (!) but by a 12 volt signal from the thermostat. It is important to understand that this 12 volt signal is logically a completely independent source. Any old "always on" 12v supply goes, usually by a blue wire, to the thermostat. When the thermostat "calls for heat" (when the internal contacts close) the other (blue) wire from the thermostat now carries 12 volts, and this wire is connected to a third (coil) connector on the relay. The current in this wire flows through the internal coil in the relay and out to ground, via the 4th connector on the relay. When the current passes through the coil, the resulting electro-magnetic induction pulls shut the internal relay contacts, and power is supplied to the "switched power out" relay connector. This thermostatically switched power then goes to the fan motor, and , in a parallel circuit, to a sail switch, so that only when the fan blows hard enough is current allowed to continue to the "limit switch", which is an overheat cut-out, and hence to the electrical gas valve and the igniter. It looks as if you also have an over-heat cut-out system which cuts off the gas supply, as well as cutting off the power.
I suggest you connect a 12 volt supply to the two correct relay terminals (power in, and signal from thermostat), and methodically check each component in the correct sequence, using a voltmeter. You may find a defective sail switch, limit switch, relay, or gas valve. It worked for me! I'm sure a forum member will pop up soon with better knowledge of this model furnace.Good luck. Nick.

[oxfletch]

> Oxfletch, before you start demolishing the Univolt, I humbly suggest you
> are on the right track by first trying to understand how each part functions.

That'd be great if I could figure that out.

> Forgive me if some parts are "teaching my grand-daddy to suck eggs".

No problem, I know some electronics, but little about furnaces ;-)

> The first thing to find is the "always live" power wire to the furnace.

OK, I think that's the red one labelled "positive?" on the right side of the
schematic.

> When the thermostat "calls for heat" (when the internal contacts close) the
> other (blue) wire from the thermostat now carries 12 volts, and this wire is
> connected to a third (coil) connector on the relay.

Yeah, I understand relays. Though in retrospect ... when there's no power
applied to the relay, I don't see why shorting the two blue (thermostat)
wires together would change the resistance, the relay shouldn't activate
if there's no power to it, presumably.

> This thermostatically switched power then goes to the fan motor, and , in a
> parallel circuit, to a sail switch, so that only when the fan blows hard
> enough is current allowed to continue to the "limit switch", which is an
> overheat cut-out, and hence to the electrical gas valve and the igniter.

Aha. Is the overheat cut-out / limit switch the "mysterious round thing" I
referred to? Labelled E (http://mbligh.org/pics/furnace/round_thing.jpg) ?
I don't see why it has 3 connections in that case, though maybe it's for
this:

> It looks as if you also have an over-heat cut-out system which cuts off the
> gas supply, as well as cutting off the power.

?

> I suggest you connect a 12 volt supply to the two correct relay terminals
> (power in, and signal from thermostat), and methodically check each
> component in the correct sequence, using a voltmeter. You may find
> a defective sail switch, limit switch, relay, or gas valve. It worked for me!
> I'm sure a forum member will pop up soon with better knowledge of
> this model furnace.Good luck. Nick.

Thanks, will try it. I guess the easiest thing is if I go buy a 12volt supply
from RS or something ... with a trip breaker in it ;-) Anyone got any idea
how much current these things are meant to draw?

Thanks very much for the help,

M.

[nickcrowhurst]

Originally Posted by oxfletch

>
when there's no power applied to the relay, I don't see why shorting the two blue (thermostat) wires together would change the resistance, the relay shouldn't activate if there's no power to it, presumably."

"Aha. Is the overheat cut-out / limit switch the "mysterious round thing" I
referred to? Labelled E (http://mbligh.org/pics/furnace/round_thing.jpg) ?I don't see why it has 3 connections in that case, though maybe it's for
this:
> It looks as if you also have an over-heat cut-out system which cuts off the
> gas supply, as well as cutting off the power."

Thanks, will try it. I guess the easiest thing is if I go buy a 12volt supply
from RS or something ... with a trip breaker in it ;-) Anyone got any idea
how much current these things are meant to draw?
M.

1. Oxfletch, the circuit to ground from the thermostat via the coil will still function, even if the main power supply is off, but I'm not sure if this is what you mean.

2. Yes, I suspect you are correct that this (E) is the limit switch, with 2 functions to cut off electricitty AND gas. Hence the extra wire.

3. I looked on the Suburban site, and the modern furnaces take between 6 and 12 amps, depending on the model. I would just use a car battery on the workbench for the power supply, with an in-line 15 amp fuse.

Have fun! Nick.

[tphan]

hi- hope you've got it solved, but if you do need to access the fuses in the univolt, at least in my '72 there is a door on the end you just open for access. It says in my service manual to NOT open the case, that there may be some high voltage going in there... good luck- tim

[oxfletch]

Aha! The two batteries were wedged up against the door, so I didn't see it. Thanks very much - saved me making some idiotic mistake. There appears to be one fuse blown, and one just plain missing for some reason I don't understand. Will play some more with that later.

Whilst I was in there, I found another mistake I'd made - the univolt didn't seem to be charging the batteries, so I'd connected an external battery charger (without first removing the batteries). Oops - no longer operated on battery power ... turned out I'd blown a 40amp fuse someone had added in. Doh. Fixed now ;-) I think I'll put a battery disconnect switch in there to just disconnect the batteries altogether (for storage, other idiot-moments, etc). As the batteries alone were 12.53 volts, and with the univolt on, there's 13.5 across them, I guess it was working after all, and just the panel controls that read amperes (always 0) and charge (always reads low-mid) are just wrong.

I hooked the furnace up to 12V 2A trickle charger, and the fan spins on it just fine, switching on and off correctly as I connect/disconnect the thermostat wires. I'm beginning to really not trust that dealer at all ...
mind you ... the connection TO the furnace doesn't seem to have 12V supply, but that seems like an entirely different problem to me (fuse or
whatever, presumably).

Thanks to everyone for your help ... I'll keep tinkering with it.

[CanoeStream]

Please stand back and take a look at the big picture. These are previous threads:
Suburban Furnace Recall
Furnace safety

For the relatively affordable price of a new furnace I would bet a dealer would not even look at doing the safety mods -- to say nothing of being able to get the correct (and safe) parts! Like the one post says, "Don't want to wake up dead."

Take care!

[oxfletch]

OK, will do - thanks. But the fact that what they told me was wrong seems to be wholly incorrect makes me unhappy, and not trust them. If they said "that furnace is an old design, and unsafe" ... fine. But they seem to be trying to tell me it doesn't work ... when it does seem to, AFAICS. I may just take it down there and ask them to show me what was wrong with it (I paid 'em already, after all).

I updated the schematic ... in response to Nick's comment that the schematic was incorrectly labelled ... not sure it was - just some of the crossovers were confusing, as some were connections and some weren't. I updated it:

http://mbligh.org/pics/furnace/schematic.png

Thanks to all for the advice. I guess I get to re-install it tommorow, and see if it fires up (never lit one before. eek).

[oxfletch]

Well, damn it. That piece of hose IS horribly old and cracked. Thanks very much for the pointer Canoe Stream.

[bilby05]

I have been following this thread, since I have about the same problem. I can not figure it out. Did you ever find out what was causing your problem? None of the wires, red or white, to my furnace have power, but I don't have any blown fuses.
thanks