This is a Lennox UHG4-75A-2. The model is 80UHG. Inside blower tries to restart (hear several clicks and occasional "chunk" sounds before the fan starts. It is not dead yet but the clicking is getting more frequent before the fan kicks in.
If I turn the thermostat a/c off and switch to fan, the fan comes on immediately.
Was thinking either the motor, cap or board. Swapped out thermostats anf the problem remains. Have not disassembled anything yet. Any thoughts or advice?

If I turn the thermostat a/c off and switch to fan, the fan comes on immediately

If that is the case the motor is probably good and the fault lies in the low voltage area of the unit. Something is preventing the full voltage from activating the blower relay right away when there is a call for cooling. I would measure the low voltage output of the transformer for starters and the work my way through the low voltage control system. It could be as simple as a loose or dirty connection causing this to happen.

Clicking or chattering of a relay/contactor is usually caused by a disruption of the low voltage powering the relay or contactor.

If I turn the thermostat a/c off and switch to fan, the fan comes on immediately

If that is the case the motor is probably good and the fault lies in the low voltage area of the unit. Something is preventing the full voltage from activating the blower relay right away when there is a call for cooling. I would measure the low voltage output of the transformer for starters and the work my way through the low voltage control system. It could be as simple as a loose or dirty connection causing this to happen.

Clicking or chattering of a relay/contactor is usually caused by a disruption of the low voltage powering the relay or contactor.

I checked the transformer and it was putting out 26V. I also noticed that the fan worked fine in the Heating cycle or on Fan only cycle. Cap checked out OK. Tried to check the resistance on the motor but they did not add up (I'm not sure I am doing this right anyway). Noticed the Cool cycle was using the High Speed while Heat was using Low speed. Switched Cooling to the Medium Speed connector and still continue to have the problem.

I noticed with every click (attempt to start) the blower wheel would turn maybe an inch. After several attempts, the blower would fire up and then die out before reaching the set temprature. As sok as it dies the clicking begins ane it ties to restart. Any ideas?

http://www.hvacparts.ca/LennoxPDF/80UHG.pdf

The above diagram if correct for your unit might be handy for you to keep.

Based upon what you have said and tested the blower relay on the circuit board is not working properly when called to work in the cooling mode. It seems to work OK when you manually switch the thermostat to fan on but it has problems when you call for cooling and the fan is automatically to come on when the outside unit does for the cooling process.

Try this. Turn the thermostat down to call for cooling then switch the blower to ON instead of being on automatic. Lets see if the blower comes on without problems when there is a call for cooling and the blower switch is ON at the thermostat and NOT on automatic.

While you might have 26 volts at the transformer that does not mean you have 26 volts to power the control system. It is possible one of the devices is drawing too much current and causing a problem back at the blower relay that is attached to the circuit board in the unit. In other words if the contactor outside or some other control in the low voltage part of the system is drawing too much current then it is very possible that the voltage available at the blower relay is 20 volts or less.
Now it is also possible that the relay itself is going bad but it is best and cheaper to test all other possibilitys first then replace the circuit board.

Another thing you might do is to make sure ALL low voltage connections are clean and tight with no damage. That is from the thermostat to the inside unit then to the outdoor A/C condensing unit. Many times I have found loose or dirty connections and when dealing with such low voltage and amperage it does not take much to cause a problem.

Now I appear to have problems with the blower by itself and as part of the cooling cycle. I noticed that when the fan stops working the ACB Cool voltage drops off. When the fan come on again the voltage is back to 120v. The Transformer out to the board stays constant at 24V while this is happening.

When I direct wire the blower it works fine.

I have not checked the outside low voltage, but the inside seems fine. I'm suspecting the board now. What do you think?

Thanks for the link. Yes I had a copy, which has been very helpful.

If the blower motor runs when directly connected then the problem has to be else where. If you have checked everything else then it would point to the board. They are not cheap but if you need one go for it.

Thanks for the suggestions hvac1000!

As suspected, the board swap out worked like a charm. Wish I could have found a loose connection instead. But nice to have the a/c back at 100%.

Glad you got it going