#13 LED blinking at startup

Project is to reverse engineer a device which uses an Arduino UNO with Adafruit v2.3 motor shield to run single stepper motor, and a single servo. It uses 4 push buttons to control motors and a small 2 x 8 video screen to monitor number of steps and direction. I have carefully reviewed the device and copied the wiring/hook-ups to new UNO and Motor Shield.
After mounting the motor shield to the UNO, 1 wired in the Video screen. I then plugged in a 12VDC power supply to the power plug on the UNO. The Vin jumper on the motor shield is installed and connecting the two pins. I have uploaded no code. I just wanted to check that everything getting power.
When I switch on power supply everything looks good. Both boards "On" LEDs are bright and steady. However, the #13 LED begins flashing rapidly a second or so after powering on. If I hit reset it all stops then starts back up again. I did not notice any burning smell or overheating of either board. The device I am copying does not do this.
This is my first time working with arduino and I would very much appreciate any information at all. It

It's possible that your Arduino came with the blink sketch loaded into it.

When you reset an Uno, the boot loader will give a couple of quick flashes. After that, the loaded sketch takes over. Unos usually come preloaded with blink which will blink de led one second on, one second off.

Thanks for the timely reply. The #13 LED is blinking much faster than one second on-off. I am going to wire in the stepper and servo motor later this morning and check voltages. I will post additional info.
Thanks again.

Try loading the blink sketch - it'll prove that you have good connectivity and remove whatever code is doing the flashing you observe.

That sounds like a plan.
I'll let you know how it goes.