AC Condensor Contactor Voltage issue wiith common

Fan runs but contactor does not engage and compressor does not start.

I am seeing 0volt across the contactor coil when cooling is on. Disconnected the contactor coil cables and measured the voltage. both common and red have 24vac. When cooling is off, it still shows 11v for either common or red wire. Since both sides of contactor coil has equal voltage, contactor does not engage.

Measured the voltage for COM in furnace board that is connected to condensor outside. it too has the same 24 volt with ground when cooling from thermostat kicks in.

What could cause COM to have 24 or 11v ? Is this a furnace board issue? THis is honeywell/carrier furnace.
Removed wiresfrom the 24vac transformer cable out and it too shows 24vac.
So transform does not seem to be the issue. Fan runs too. so I assume transformer is OK.

I have replaced 3amp fuse as it was blown. Contactor is also new.
hermostat Y and G shows continuity beep on multimeter. so when Fan is on, Y too has 24V. This could be a Thermostat issue.

Any ideas what might fix this issue?
Replace board and thermostat?

Thanks
Krish

 

Welcome to the forums.

How is the fan running without the contactor closing ?

You need 24vac to the contactor for it to close. If you have 0v with the contactor connected and some +/- 11-24 vac with it disconnected.... you may have a loose connection.

At the furnace there should be 24vac on the Y-C terminal. Start there and work your way to the condenser. Since you blew a fuse....there's going to be a possible wiring issue unless the old contactor had a shorted coil.

 

Fan is pat of the the furnace. Compressor is outdoor unit with contactor. COM and Y are going to the outdoor unit

 

There are two fans in the system. The outside condensor has a compressor and fan.
The inside furnace has a blower.

 

right. outside fan does not run too as that depends on Contactor.

 

Inside Blower works. outside fan does not work. Have not checked if heating works.

 

At the furnace..... with the condenser disconnected.... do you have 24vac between Y and C ?
Don't worry about the off voltage.

If you have the voltage at the furnace board..... reconnect the condenser. Is the voltage still 24vac at the furnace board ?

If yes.... measure it at the contactor. If it's not there.... you have an open wire or an open sensor..... like low pressure switch.

Don't measure from ground to common or red to ground. You need 24vac on the red and white.

 

Disconnected condensor wire connected to COM and Y at the furnace board.
Now measured COM and Y and it is 0 when thermostat is off or on.

Measured Voltage at between other terminals too (while thermostat is off)
COM to W - 12V
COM to R - 12V
COM to G - 0
That is why I was suspecting COM as R should be 24 and rest of the terminals should be 0 when thermostat is off.
G and Y have continuity beep too (even after removing condensor wire)
COM terminal is not connected to any wire after removing the Condensor wire.

 

Are you sure you're on the AC scale not DC ?

Right off the bat.... you MUST measure 24vac from R to C.
The R terminal is the 24vac source out of the furnace board and to the thermostat.

If you don't have 24vac from R to C...... you could have an open safety in the furnace.
You can check the AC transformer for 24vac out.

Remember the blower door must be closed to restore power to the furnace.
You could post the complete model number for the furnace too.

 

I have attached the photo multimeter setting that I used to measure.
It has AC (~) sign next to V.



Thanks
Krishnan

 

Carrier Model 58PAV090-14 Made in USA
Series indicates manufacture date as 1996

Tested heat today and it works fine too along with blower..
Pulled transformer wires beneath the 3AMP fuse and checked the voltage and it is a perfect 24V.

 

I taped the switch so that I can do the measurements with the blower door open.
Led light is on while doing measurements.
Do you see any Voltage between COM and ground?
Heater and Blower are working. So transformer output should be 24V on W and G for this to work.

Since condensor wire is physically connected to Y on the board, Y is unlikely to have safety switch as when thermostat turn on Y, contactor would get a 24V on one side.
To stop the contactor for safety. COM needs to have 24V.

COM voltage seems to the only way to create a safety switch.
If you have the manual for this model, please point it to me.
Thanks
krishnan

 

The safety switch on the blower turns off the AC which turns off the 24vac transformer.

If the furnace and blower are working correctly then you must have 24vac between R and C.
There should be no voltage measured between C and ground BUT C is not the same as ground.

Manual for your furnace.... hvacpartners/docs/1009/Public/01/58P,R-13SM.pdf

There should always be 24vac between R and C.
There will be 24vac between W and C when heat is called for.
There will be 24vac between G and C when in FAN ON or A/C cooling mode
There will be 24vac between Y and C when the system is in cool mode and calling for A/C.

 

Thanks for the link to the manual .
No luck finding any other switches like Pan switch for this COM.
After turning on Fan only, I see COM to R drop to 0 from 12.
COM has 24V to ground when turning on Fan. So there is no difference in voltage between COM and R.

It is a mystery how Blower and Heater still works.
Thanks

 

There is no switch in the C/common line. You MUST get the R to C voltage correct first.

Using the highlighted diagram below....... the R line is in red and the C is in blue. Those aren't the colors actually used.... I just used them for clarity. You'll see the R line goes thru the 9 or 12 pin plug to safeties in the furnace and back to the same plug. You have to follow the R line until you find the problem.