Blower will not turn on when a/c compressor comes on - tstat ok fuses ok

http://www.fixya.com/tags/honeywell_lr1620_thermostat_support

Turn of the system at the thermostat. If you turn off the blower, assuming you mean at the tstat, there's no need to turn off the pilot. If blower is running and won't stop, even if turned off at tstat, turn off power by any means necessary, and pilot does not need to be turned off. A "standing pilot" is made to stay lit at all times, running or not.

A/c compressor works fine and the blower motor works and shutdown right? it is the problem with the blower resistor or with blower motor relay. try to direct the resistor irrespective of the speed. hope the problem may be solved.

Your tstat is not the problem. I've not yet worked on an oil furnace. I'm way down south. But either way, if your tstat was calling for heat(heat on), and you pushed a reset at the furnace, more than likely that reset is a safety. OK to reset, but now what ever that safety is preventing from happening, has failed and can not be reset until repaired. The reset should be an automatically opening switch upon unsafe operation. And manually closed to "reset" the safety. That safety is connected to something that is not working properly. Hope this helps!

Bypass the tstat first but that's where I'd be leaning.

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For the ac to stop working either the compressor clutch is not getting power or the gas is too low so the low pressure switch has switched the compressor off to stop sucking in air through the leak. The blower is controlled by the fan switch which is a 3 positional switch that has a resistor for each position. These resistors have a habit of burning out so the fan will not work. The resistors are in a block located in the blower housing. So you have two faults to fix.

I can't stand the tempstar smart valve! Now I'm not sure what a dc90, but if you have a gas fire residential furnace I know what it is. The smart valve controls gas and flame proving only. The blower issue is probably one of the high temp safeties that has stuck. During a call for heat, your system verifies that all safeties are closed. If this sequence is not correct, then the unit assumes that the unit has overheated due to a high temperature safety issue, and brings the blower on to cool it down, normally for 90secs. Also if the unit is calling for heat when the power is turned off, still calling for heat when power is turned back on(like you said by throwing the switch), this will also bring the blower on for 90sec, then blower will/should turn off, and the heat cycle will start. So if your tstat is calling for heat(heat on and set temp higher than accrual temp), and you turn off power then turn power(main voltage) back on, this will bring blower on for 90secs. Now, the burners staying on is a different issue. I've not seen that in 15 yrs of service on a smart valve. There is probably 24volts feeding the "W" circuit, for some reason. On the control board there is a terminal block with R W G Y C. The W is your heat. This W receives a 24v signal from your tstat. Turn off power, remove W from the board, then turn power back on. If no heat then you've isolated your problem to the control wire coming from your tstat, or the tstat itself. If it continues to heat with W disconnected then its something in the furnace, circuit board? Hope this helps!

Sounds like a bad tstat. Not likely anything inside the furnace. If you have a digital tstat try just removing the screen and see if units shut down. If both do you know a relay in the tstat is bad.

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carrier gas furnaces will try to light three times then will usually go on a lockout for one hour you can bypass the lock out by resetting the power, turn furnace on, remove wires from gas valve, you will need a multi meter to check for voltage to valve, if control board sends a call to the gas valve the valve is bad, if voltage is never present during the lighting cycle your control board is bad

You have a voltage bleed somewhere or a short, whatever you want to call it. Do you have a control board on this unit where the relays are integrated into it or do you have the old type where the relay is separate? Anyway, you need to take a meter and take voltage readings to the motor when it's acting up and when it's running right. I have had the same problem with a Goodman unit before and it turned out to be the tstat! What I would do is remove the cover off the tstat and take a small jumper wire and touch the r terminal to the y terminal. If the motor starts right up, you know it's the tstat. If you have the same problem, then the only thing it can be is a low voltage short to the unit from the tstat, bad control board if you have one, or a bad relay. But if the unit is working fine in heat, it's probably not the relay. I hope this helps.