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Z07 07-10-2009 09:31 AM

Diagnostic map for AC
I'm trying to help my brother out. His house has two HVAC units one serves the lower floors and one serves the upper floor. They are identical Whirlpool models circa 1985.

The units failed to cool about a year apart from each other so this is an old problem.

The heating portion of both units works flawlessly.

When cool is selected and the temp is set below ambient the blower comes on but there is no sound what-so-ever from the compressor units located out back. These units have two motors each. One turns the fan while the other turns the compressor and neither unit's motors make any noise. I pulled the capacitors and tested them and they were within spec. I doubted myself and took them to a shop and they verified that although their cases were encrusted with rust they were within spec. I did not see any evidence of ot taletell sign of electrical arcing or melted wires etc.

One of the units was serviced by a pro two years ago and received a new compressor.

What I need is a guide. I would like a diagnostic map to follow to pinpoint the problem to attempt a DIY repair.

Thanks for reading my post.

hvaclover 07-10-2009 03:25 PM

Did you check to make sure there is power to the units?

kenmac 07-10-2009 03:36 PM

A good V.M. can help along the way trying to find problems

Yoyizit 07-10-2009 04:22 PM

Post a schematic; a generalized fault tree for this task would be huge.
You measure voltages and currents, we interpret.

hvaclover 07-10-2009 04:40 PM


Originally Posted by Yoyizit (Post 300094)
Post a schematic; a generalized fault tree for this task would be huge.
You measure voltages and currents, we interpret.

Whata 'ya mean "WE", white man?

Plumber101 07-10-2009 04:52 PM

Me know how Kemo Sabe

Z07 07-10-2009 05:16 PM

I only checked voltages at the contactor's terminals. I got 105-108ish AC at the two "hot" wires red and black. I was confused by the CTC line so I checked it for 12 DC 24 DC and 24 AC I didn't get anything in the "call for cool" terminals of the contactor but that doesn't mean I tested those two terminals properly. I was tempted to manually close the contactor briefly but I didn't do it. What would happen if I did close the contactor with a well insulated "stick" pushing the spring-loaded arm down to the AC contacts?

hvaclover 07-10-2009 05:37 PM

It is supposed to 220 to 240-ish. That low a reading you got would only be 216v across both supply legs.

Check for supply voltage , Check for blown fuses or tripped breakers. Could have a burned off conductors in the ac disconnect.

qbert 07-10-2009 06:21 PM

contactor not closing, I would think control power. 24v coming out to condenser?

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