Hi,

AC unit will not turn on. The model is 24ana736a300.

The error shown on the Infiinity t-stat is "No 230V to unit" So, I got out my multi-meter.

I confirmed that 240v is going to unit from exterior disconnect.
I confirmed that the contactor is out (not depressed) on contactors - both hi and low cool contactors.
I confirmed then when call to cool comes, no 24V goes to either contactor by putting the multi-meter probes on the contactor side coil contacts
Regardless, I checked the ohms on the contactors and both read about 17.
I confirmed incoming (from furnace) 27V to AC system board and comm lines seem to be working because it is throwing the error back to the t-stat.
After a few minutes of call to cool, the amber LED starts flashing code 47.

I'm pretty sure it's a bad system board, but is there anything else I should confirm first?

One odd thing, with the common and hot wires to the contactors are removed from the contactors and tested the wires, they are showing 27v. When I connect them back to the contactors, the voltage drops to zero. Why?

I'm a skilled homeowner, but not an HVAC expert. I don't want a $1000 service call and looking for some guidance before I decide whether to buy a used board.
thanks
Mike

 

Your condensor has two contactors ?

 

Correct. This unit has two contactors. It's a two stage AC, so there is a contactor for hi stage, another for low stage.

 

You checked the contactor resistance and it was around 17 ohms on both contactors. That would suggest both contactors are ok.

When you remove the contactor load you measure 24vac on the wires.
When the contactor is reconnected..... if you also measure 0v at the board... then it would appear that the board is defective.

on edit:
Trying to find a wiring diagram. That board is outside in the condensor. Is there another 24vac transformer near that board that could be failing ?

 

http://dms.hvacpartners.com/docs/100.../24ana7-2w.pdf
This is a link to the current diagram.

I take it that you are using a conventional thermostat instead of an Infinity communicating stat?

I have been on a call after the board was replaced and we still had this fault code popping up.

I would check the resistance across pins 1-4 and 2-3 on the model plug.

If it is slightly corroded it can throw off the resistance and do strange things with the operation. (I have seen the system think it has a heat pump when it was a standard condenser.)

I would also check if the incoming Y1 to C would bring in a contactor by turning off the service disconnect to the outdoor unit and connecting these wires directly to a contactor. ( I always use a contactor from a shelf in my truck for this test )

Replacing stage 1 contactor stopped the intermittent problem for us but if your contactor never tries to pull in it may very well be the board.

This is the older 2006 diagram...

 

Thank you, Houston. Great intel! I'll check the resistance on the model plug and also make sure the incoming can activate the contactors (or get a fresh one).

I am using the Carrier Inifinity T-stat which uses the comm lines. Not sure if that makes a difference in your recommendations.

thx
Mike

 

If you are using an Infinity stat you have no analog Y1 input at the condenser.
A and B are communication and C and D are 24 volt power. So C and D should be able to pull in a contactor. I have connected C to analog C and connected D to analog R with a jumper from analog R to Y1 to temporarily test in the past.

We have installed a transformer at the condenser to supply power to C and D many times but usually because of a lack of 4 conductor wires.

I would also check the last 10 system events at the stat and verify that the stat can read the model and serial number of the condenser. (only possible with an original board)

 

I connected incoming C and R to the contactors and they pulled in as expected.

I then connected incoming C to analog c, r to analog r, and jumper to y1. Nothing.

I think the system board is KIA. I'll update the post after I get a new one.

 

I would double check the wires on VR,and L2. (wouldn't hurt to also check VS and L1 while in there)

 

Ok, problem resolved. I ordered and installed a replacement AC control board. It still didn't work and threw the "No 230v at unit" error.

I then went back to diagnosing from square one.

So, a fuse on the disconnect had failed. This was the first thing I checked three weeks ago and I think I must have misread my multi-meter and failed to recheck.

$13 dollars for new fuses, I now have a happy wife and a bruised ego.

Plus I have an expensive spare board to remind me of my ineptitude.

Cheers!
Mike