I have a Honeywell System. Back in November of 2014, I replaced the Circuit Board with one by Honeywell from Amazon (1014460). The blower motor was replaced over 2 years ago. Last month, I have replaced the Transformer. (No part # available at time of writing). I also replaced the cap on the board as well. (97F9002). My thermostat is about 5 years old. All caps for my a/c system are no older than 2 years old.

When I switch from Heat to AC, my circuit board fuse immediately pops. I am unable to use either until the fuse is replaced. I haven't tried replacing the circuit board fuse and putting it on AC to see if the AC would kick on that way. However, I haven't been able to determine why the fuse keeps blowing. Nothing was replaced last year and it worked great during the summer.

Any suggestions to try for troubleshooting? (See below for another suggestion)

Research: All the research I have found regarding my issue tends to be the other way around. People seem to have the issue with AC to Heat. However, I want to test to see if it is my thermostat as that has been suggested to others. How would I test if that is a possibility?

Schemes BRAND ICP Model N8MPNO75B12A1

Transformer 44504 jard 4031f

  • What is the make and model of the furnace/air handler? What is the make and model of the thermostat? How is the thermostat wired to the furnace/air handler? What is the make and model of the condensing unit? How is the control wiring from the condensing unit connected at the furnace/air handler? Including a clear photo of the schematic for the furnace/air handler, would be helpful. – Tester101 Feb 22 '16 at 22:08
  • Why have you replaced so many parts? Have you looked in the condensing unit for obvious signs of damage, corrosion, etc.? Do you own a multimeter? Do you own an ammeter? Do you know how to use those devices? – Tester101 Feb 22 '16 at 22:10
  • I'll get that information in about an hour or two. I appreciate the request for detail. Let me see if I can answer some questions in the meantime. Do you need the schematic on the back of the wall that comes out? Just want to make sure I find the correct information. – traveler84 Feb 22 '16 at 22:33
  • My wife's uncle is an HVAC tech however he is not available in the short future before it starts to get warm again. A lot of parts when he would assist such as the capacitors would be replaced just as the first line of testing. I haven't checked the condensing unit yet because it doesn't seem to reach that far. The blower motor never kicks on which prompted me to think it was in the furance possibly. The old transformer was reading higher so I went ahead and replaced it. I do own a multimeter with limited knowledge on using it. I have one with the push plugs. – traveler84 Feb 22 '16 at 22:38
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    The caps in the unit are usually under 150uf shorting out with a screw driver is the normal way if no bleeder resistor. A shorted cap can also cause the problem. With power off after discharging the cap Use your meter in ohms mode and put leads on the cap. you should observe a change in resistance (the cap charging). Reverse the leads and you should see the discharge. The contactor may have a diode in the coil winding if it is being driven with AC and the value could be around 500 ohms 1 way and show 10X the other. If your contactor is DC driven it may be as low as 6 ohms in both directions. – Ed Beal Feb 23 '16 at 0:18

Without knowing much about your system, I'd recommend starting by checking the contactor in the condensing unit (outdoor unit).

Before you begin

Before you open up the condensing unit, make sure to turn the power off at the breaker, pull/turn off the serviceman switch, and verify power is off.

Once you've opened the unit, you'll want to properly discharge the capacitor(s). You don't want them to accidentally discharge through you.

Check the contactor

Using an insulated screwdriver, or pair of pliers, push the contactor plate in. You'll want to make sure the plate moves freely, and is not sticking or getting hung up.

If you're really careful, and comfortable doing so, you can run the system with the condensing unit open. Then you'll be able to see if the contactor pulls in.

Here's a good YouTube video that shows why the contactor not pulling in is a problem.

Next, set your multimeter to test resistance. Carefully remove the leads from the coil terminals, then test the resistance of the coil by touching one probe on each of the coil contacts (where you removed the wires). Make sure the resistance matches the manufacturer's specifications.

If the resistance is really low, the coil is likely shorted out. If the resistance is infinity, the coil is likely broken. In both cases, the contactor will need to be replaced.

Isolate the problem

To try and determine where the problem is, you could disconnect the control wires feeding the outside unit. Then switch the unit to COOL, and lower the set point until the thermostat calls for COOL. If the fuse blows with the outdoor unit disconnected, then the problem is likely on the control board.

Unfortunately the control board doesn't appear to be serviceable, so you'll have to replace the entire board if that's where the problem is.

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