After a thunder storm my roof top air no longer worked. Checked all fuses & brakers, the 3 amp fues in the analog control kit # 3107541.009 that controls the furnace & air condioning was all that was blown. I replaced the fuse. Now the furnace works but the air does not. I have disconected all items from the control box one thing at a time untill all items are disconected. The fuse still blows when i set the thermostat for air conditioning, even with nothing connected to the control box. Is there a way to confurm that the control box is or is not the problem. LC in ohio
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Re: dometic air & analog thermostat control kit
Had one doing similar recently, and after a thunder & lightning storm. Nothing worked, but kept blowing 2 amp fuse in thermostat, when anything tried, heat or cool. Pulled plastic control box down from inside upper unit (AC) made sure breaker was off, took of plastic cover and smelled inside, and could smell burnt plastic smell. Replaced, control box, that got fan only working. Tried another thermostat , everything worked as it should. Turns out the lightning, or power surge, or whatever, shorted out the thermostat, and control box at same time.
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That unit has a thermostat I believe and 12 or 24 volts to power it. The 3 amps tells me it may be either line or low voltage, see manual. the fuse blows due to a short in the control circuit, a transformer if low voltage.
If you look at control box at roof unit, they usually use a separate set of wires for furnace. See what colors they are attached to at harness to thermostat. Some just follow other harness, and come out at thermostat. (bypass control box) Once you've determined which 2 they are, join them together & see if furnace starts, as it should. If it does, thermostat is faulty. If not, pull outside cover off & check voltages at furnace. I'm guessing by furnace model that your unit is approx 30 ft , but if it's a park model, you may have 24 volt system on furnace or 12 volt. Another way of confirming is to go to back of furnace where air intake grille is inside your unit, and locate 2 blue wires going to furnace & remove whatever wires are attached to them & connect them together. Furnace should start up.
The 3 amp is 12 Volt protection for the control board. If that fuse is blowing instantly, then you have a short in the 12 Volt system somewhere. More than likely it is somewhere between the Thermostat and the control unit. Check to see if you have 12V at the thermostat. If so, then disconnect the red wire from the thermostat and the red wire at the control unit. At the control unit side, use an Ohmmeter to see if you have continuity with ground. If so, that wire is the culprit. If not, reconnect the red wire and continue to the next wire. Do this until you find the wire that is shorted to ground. Of course, the ground wire will not be checked.
Have you tried removing the RH wire connected to the thermostat and try it ? This might rule out furnace as short in system. If it does the same, put RH- (heat) back on and try removing RC-(cool) , if it doesn't pop the fuse, you may have isolated the problem.?