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justplumducky 04-24-2013 08:29 PM

Turn thermostat to COOL (mobile home split system), fuse on control board blows.
Mobile home, split system; natural gas, igniter & remote flame sensor and intregrated control board. Pic below maybe not much help, but better than nothing? Pic is of system switch (that's what the parts list calls it)/main power switch (#24), integrated furnace control board/#12 ("IFC" it's called maybe), tranformer (
#26), and limit switch (blower is controlled by the IFC board).

Attachment 69731
Complaint: No heating, no cooling. Only dead silence when t-stat turned to heat or cool (selector switch in AUTO).

Found blown 3A fuse on IFC. New one installed. Furnace now comes on, but blower comes on immediately instead of booster fan coming on immediately (don't think/pretty sure now that booster fan never/ever came on _ sorry, but didn't think about it till now). 30 to 45 seconds later (after main blower comes on), igniter comes on, flame lights up and room reaches set temperature and flame shuts down, followed by blower shut-down within 2-minutes.

Shut down furnace operation and switch t-stat to COOL. No noise from outdoor condenser unit - the 3A fuse blew instantly.

Since there's no other controls on this unit, except another UPPER limit switch on cabinet next to (left of) base of blower, it seems that the IFC board is the culprit? However, I should add that there is a Standard Blend Air II component attached to this system, which has not caused any problems over the years, and to date (the system that has a vent on the roof and motorized damper control in furnace closet near top of furnace cabinet - to bring in fresh air).

I'm mentioning this cause someone with lots of mobile home experience once recommended (a while back) disconnecting entirely the Blend Air system when the operating sequence of components started acting up. Can 't remember now, but maybe he was referring to problems experienced with the more complex Blend Air system (Blend Air III, possibly) that had its own control board not only at the furnace location, but on the roof as well, where the was also a blower.

There is a Control Box labeled "Fresh Air" on the front of furnace cabinet, that has only a switch for AUTO position or OFF. I momentarily turned it off - it didn't make and difference (while the blower was running prematurely), but I didn't leave it off and restart the furnace operation - I just turned it back on and left it that way.

Still a control board fault (furnace IFC board I mean), despite the Blend Air system, or should I disconnect the Blend Air entirely and see what happens?

beenthere 04-24-2013 08:35 PM

Could be a short in the low voltage wires running to the outdoor unit. Or the contactor in the outdoor unit.

justplumducky 04-24-2013 08:51 PM

Thx beenthere for jumping in. I did initially check for shorted low voltage wires in the furnace closet, but I told her I would come back and check the outdoor unit (low voltage wires) for a short, but what about the furnace operation - as long as I don't turn t-stat to COOL, furnace runs, although out of proper sequence: booster fan doesn't come (if it did, it would have had to be in sync with the blower coming on and I didn't hear it - no, standing right there, I think I would have heard it), and blower fan comes on immediately (but shuts down properly (within a minute or two after gas valves shuts off).

Are you saying a short in the low voltage wires to condenser unit could cause the out-of-sequence operation of the furnace also?

beenthere 04-25-2013 04:03 AM

On some furnace boards it could. They are sensitive to grounding conditions.

ben's plumbing 04-25-2013 06:03 AM

disconnect the 24 volt coil control at contactor ..see if it blowes fuse....if it does not ...bad contactor...ben sr

justplumducky 04-25-2013 02:30 PM

Ok ben~ & been~, thank you. I'll see what I can find and get back...

justplumducky 04-28-2013 12:31 PM

Going back tomorrow, hopefully, to look for a short in the cooling mode, but would be grateful for clarification on the path of the 24 vac power, related to this pic:Attachment 69988

Power leaves the control board on "R" terminal via red wire, goes to t-stat (switch in Cool mode/Auto), returns via yellow wire (Y terminal in t-stat) to control board & contactor coil simultaneously - has to go to control board simultaneously so blower relay circuit can be energized immediately along with condenser unit.

Green wire from t-stat carries power directly to the board's fan relay for the constant ON (position on t-stat) ventilation function.

In Heat mode, power comes back from t-stat on white wire (W terminal) to W terminal on the board which energizes the ignition sequence in the board, proves successful ignition to the board (or shuts it down if a problem exists), followed by board's fan relay turning the blower on after a predetermined amount of time. The predetermined amount of time is controlled by the board, since there is no Fan switch on this particular furnace.


beenthere 04-28-2013 12:56 PM

Pretty much correct.

justplumducky 04-29-2013 07:42 PM

Thx for your reply beenthere.

Going to detail what I did today back at this job so maybe it will help another newbie-type me.

Finally made it back to this job today, but in the mean time, owner had called me couple more times and said furnace was now working OK (proper sequence - combustion air motor on first, followed by successful ignition, then blower motor ON. )

One of the prior complaints was blower was coming on immediately upon a call for heat, then successful ignition about 30 to 45 seconds later (but shutting down in proper sequence).

Next complaint was blown 3A fuse (instantly) on control board when A/C was turned on (split system). All this in just a few days since I was there last.

When I arrived today and turned on furnace for Heat , 3A fuse blew again (this time on Heat function, not Cooling).

Following a seasoned tech's advice (before opening up the condenser unit and looking for a short), I put a clamp meter on the "R" wire at control board, disconnected Y and G (at board), bypassed board's 3A fuse (jumper wire) and turned on power, ready to cut power if high current showed up on clamp meter, to avoid damage to transformer. Only about .13 A showed up on this R-Y circuit (was a bit puzzled when it didn't show high current). Started thinking maybe the control board was bad. Same precedure on G circuit - no problem - Fan manually On was operating properly.

Then I re-connected "Y" at board while fuse was bypassed, turned on power with t-stat set to Cool and calling for same. With clamp meter still on "R" at board, Amps went to 9+ very quickly. I shut power off immediately before Xformer started to heat up. NOTE: If the experts don't have any problem with me posting this particular method of checking the circuits, I should add that newbie types like me should watch this video : on using this method.

Finally noticed what I thought to be an unnecessary 18 guage wire on common terminal at board. Last time this furnace as worked on (before me) was for Recall service to replace the heat-exchanger. When they put it all back together, the wire on "Y" terminal of t-stat was connected to the Common/ "C" terminal of control board. Connected it to the Y terminal of board, short circuit disappeared, furnace returned to proper sequence of operation (combustion air motor on first, then blower, after ignition) and remained that way for three complete heating cycles. Outdoor unit no longer blows the fuse - indoor blower came on immediately with outdoor unit also.

The Y terminal on board not having a wire from t-sat connected to it, was easy for me (newbie) to miss last time I was out there, cause the Blend Air/Fresh Air control box on front of furnace was connected to the board's terminals also, including a wire on the Y terminal.

A lesson well-learned for this newbie-type. Thx ben~ and been~

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