Of course, the title of this post is a very old saying but I felt is was appropriate here. Why did I choose this as the title — because the meaning is: If you can’t see the forest for the trees, you can’t see the whole situation clearly because you’re looking too closely at small details.
So why did I choose this as a title for a HVACR technical blog post? It comes down to being able to do proper diagnostics on a piece of equipment. Too often, the tech gets “hung up” on one thing that he misses the real problem.
Of course I have a perfect case in point:>). I had a call from a tech at a job site who had just replaced an ECM motor on a furnace. The original motor was bad because when he opened the module on the back, the capacitors were “blown”. (There had been some “nasty” thunder storms in the area and power outages). So, he got the new motor, put it in, and was running his tests to make sure everything was properly set up, and making sure all the speeds for heating, cooling, and continuous fan are properly set —Doing what he should be doing!
He test the motor in “fan on” and sets it up for low-speed. He sets the fan up for “cooling speed” to match the A/C tonnage. He set the “delay profile” for cooling also. Now he get to set up the “heating speed” so he has the proper temperature rise. He turns on the furnace, lets the burners run, and the motor never comes on and trips limit. Of course, the first thing he does is CHANGE THE CONTROL BOARD because he knows he has a new motor. He starts the heating sequence again and it does the same thing — no motor and the limit trips. So, he CHANGES THE BOARD A SECOND TIME. — Guess what? — the furnace does the same thing. — NO fan in heat but it works fine in cool and continuous fan.
Now he doesn’t know what to do so he calls for help! I get the call and he is explaining to me everything stated above and now he is asking –“could the motor be bad?” Did we program it wrong when we sold it to him? He is “HUNG UP ON THE TREES” and is missing the whole picture.
He is looking at the 2 parts he replaced and not the “system”. We know the motor works so we doubt that is the problem. He has replaced 2 boards (3 if you include the original board in the furnace) and the motor does the same thing every time. What do you think he missed here. What part of the FOREST (system) is he not seeing?
After he tells me everything he has done, I ask him what his control voltage wiring is AT THE MOTOR? From the board in the furnace there is a `16 pin wiring harness that each wire carries the specific command for the motor to operate at.
So, I have him unplug the harness from the motor and now we start to check the voltages at each pin.Since we know continuous fan and cool worked, we used those as our base line. Now we checked from pin 13, which is our W1 command to the motor, and Ground and guess what — no voltage present. We plug the harness back on the motor and now remove it from the board and check the same points on the output from the board. Guess what — power present at the board.
He never thought to check the harness because he was hung up on the motor and control board (“can’t see the forest for the trees”). He never though that something as simple as a harness would cause the problem — it had to be the board or the motor.
He got a new harness and everything works fine now. The point of this post is that WE HAVE TO DO COMPLETE DIAGNOSTICS. If we get hung up in the trees, we’ll never find the forest — or in this case — we got hung up on the big components and never thought it could be something simple like a wiring harness.
Service is something that takes patience. It needs to be performed in a logical step by step manner. When you think about this scenario above, you can see exactly what I mean. He missed the step between the control board and the motor — the harness — and got “hung up in the trees!”and then got “lost in the forest” At least he was smart enough to use his cell phone from deep in the woods and ask for directions out.