Blower wont turn on makes buzzing sound

Hello,

I have an unknown A/C / gas heater roof top unit. There are no data plates anyplace on this unit from the components it looks like it was made in the late 1980s.
Here are the parts I have replaced:
1. Rebuilt the 24 volt relay board that the thermostat connects to. I know it now gets 24 volts to the ignition module and the A/C will now come on.
2. I had to replace a bad Honeywell gas controller with a Robert Shaw universal ignition module.
3. Have replaced the blower motor and run cap.

Current result:
Good news the heater now comes on works great.

Bad news is that while the blower motor works fine for the A/C it will not turn on for the heater. It will move slowly and the relays make a buzzing sound.

Bad transformer maybe?

Any thoughts?

Thanks!

Greg

 

It sounds like a blower motor wiring problem. Is the blower motor a multi-speed motor? Does the heat annd cool mode use the same speed on the motor? What voltage does the blower motor operate on?

Ken

 

The A/C and heater use the same motor. It is a single speed motor on each leg I measured 120VAC. I did some testing today and have some interesting results:

Here is the layout. 2 brown wires from motor to run cap. 1 Yellow from motor to yellow wire. 1 black wire to relay block to black wire.

Today I jumper wired the relay so it is "on" and I had the following results. 1. The heater came on (like it already does) 2. The blower motor came on (since it was jumpered past the relay). 3. The heater will work for about 5 minutes then the 25VAC circuit breaker will trip.

Do you think I have a problem will my gas valve? Could it be drawing too much juice on the 25VAC side?

argh...........

Thanks for any help or ideas you can toss my way.

 

I'm still not sure if the motor voltage is 110 or 240 because apparently you didn't measure at the the entry point with a voltmeter. That isn't too big of a deal though and I am gathering that the blower motor is single speed. That sounds a little odd but I'll take your word for it. The 24 volt circuit breaker tripping is the first documentable problem. I would pursue this first because it may lead you to the real problem. If the incoming line voltage is low, the current will increase on the 24 volt circuit and that may trip the circuit. You may also have a short circuit in tha 24 volt wiring. It would be good to check the voltage on the 24 volt circuit and even check the amp draw at the same time. I would advise against blaming an unknown part and replacing all kinds of stuff before you nail it down.

Ken