Does testing a connection with voltmeter risk damaging the circuit ?

[vacman]

I am scared to even test the connections in my air handler. I want to hook up the probes of my multimeter between the R and G terminals on the control box, with the thermostat in the 'fan' position and the power on.  Any chance that will cause a problem?

Also, I read somewhere that you should never connect the R and C terminals since it would blow a fuse or damage circuits. Is that true, and also, if I were to simply connect R and C with a voltmeter, to test it, would that be just as bad as connecting (jumpering) R and C ?

 And another question. I keep reading that jumpering R and G bypasses the thermostat, but does it bypass the control board too?   

[Porcupinepuffer]

Make sure to set the meter to read AC. Don't mistakenly put it on resistance or you could damage your meter testing voltages.
Yes, connecting R and C is dead shorting the connection and you would hope it blows a fuse before something else more important. Yes, you can safely probe R and C with the voltmeter to read the voltage. That's fine, and normal use of the meter. Definitely not the same as shorting R and C.

No, jumping R and G does not bypass the control board, but it does bypass the thermostat to rule it out.

The thing you should be afraid of is accidently touching something with your hands that's 120v when tinkering with the unit.  Make sure you're well aware of where your hands are and try to use one hand only when possible.

[vacman]

Make sure to set the meter to read AC. Don't mistakenly put it on resistance or you could damage your meter testing voltages.
Yes, connecting R and C is dead shorting the connection and you would hope it blows a fuse before something else more important. Yes, you can safely probe R and C with the voltmeter to read the voltage. That's fine, and normal use of the meter. Definitely not the same as shorting R and C.

No, jumping R and G does not bypass the control board, but it does bypass the thermostat to rule it out.

The thing you should be afraid of is accidently touching something with your hands that's 120v when tinkering with the unit.  Make sure you're well aware of where your hands are and try to use one hand only when possible.

Thanks for answering all my questions, Porcupine! 

I still don't understand how using the tester doesn't short out the connection. I guess it is due to something in the tester itself.

If I set my voltmeter on AC volts, and set it to a number higher than the volts I am encountering, all will be fine?

I want to test the voltage at my motor, as suggested, but I can't get the wires off the motor, and the other possibility for testing is a plastic connector 'upstream' that looks like it will break if I try to open it. So I want to insert the probes into the connector which is on the motor. 

I have attached a pic of the motor connection.   

Does it matter that I locate 'positive' and a 'negative' spades on the motor connection ?


 

I am assuming the red wire is positive and the purple wire is negative.

[Porcupinepuffer]

Think of everything that runs off electricity as needing 2 wires. So all our every day items make a connection between those 2 wires and work fine because they have a measure of resistance to limit the current. But when you short something, you're just bridging a direct connection between those 2 wires, which allows an endless amount of current until something lets go since there isn't much resistance in shorting right across something with a big piece of wire. So the voltmeter is reading the voltage in a similar way you would power something with those 2 wires.

Yes, choose the a/c setting on the meter that can read over the maximum of what you're going to test.

I would yank those 2 wires right off the motor to test them for voltages. One is a neutral and one is hot. Not polarity sensitive. I would put them back on the motor the same way they were located.

[vacman]

I can understand your explanation of short vs test. Thanks.

The wires on the motor don't want to come off, and I don't want to break the connectors or something off by applying too much force.

I might be able to open a plastic connector 'upstream' that houses just those 2 wires. It looks brittle, but I can remove a bolt and get it loose, then maybe it will come apart easily. Will try that next.

It's the connector below the big yellow warning sign, with the horizontal purple wire and red wire going into it.