What is the 12V supply? Can it generate enough current? Are you using PWM?
I have attached a “napkin sketch” of a diagram.
The 12V power is supplied from a car fuse box. Previously, I had connected from the fuse box, through a DPDT switch and into the motor. It worked just fine. I am not intending to use PWM. The motor is essentially a car window motor.
Is that 12V current going to the motor via the Vin header?
Take care on the current draw - Vin is connected after the reverse polarity protection diode, so you will see some voltage loss there, and the diode is only rated for 1A.
Otherwise, provide a link to the motor shield.
The 12V power supply is not going into the “Vin” slot. It is going to the power input jacks on the motor shield. I have nothing connected to the “Vin” pin on the Arduino/motorshield.
Here is a link to the motor shield: http://www.seeedstudio.com/wiki/Motor_Shield_V2.0
I then connected the setup using 12V external power supply to the motor shield (with no motor connected) and saw 9v at the motor jacks, reversing polarity as intended.
If by “saw” , you mean “read on a DMM” , are you aware that using a DMM to measure a PWM signal is only going to give you the average voltage because there is no steady state voltage since PWM by definition means switching from Vcc to GND ?
Comparing your steady state power supply voltage with the average voltage on the motor terminals is all but useless (pointless). If you put a scope on the motor terminals you will “see” the signal.
When I hooked up a 12V (0.6A max) motor, i’m seeing reversing polarity as intended, but only at 2.7V.
This statement tells us practically nothing since you have not given the specs for the 12V power supply and we don’t know that it can source 600 mA. It would help to see a vendor link or datasheet for the motor but more importanctly , what do mean by 12V power supply (exactly) ?
I do not own a scope. However, My DMM is capable of measuring frequency. I read on the DMM 490Hz. I was under the impression that if I did not use all 4 terminals (see above link), and only used one channel (2 terminals), that the MC would not produce a PWM signal by default. Since I am reading 490Hz, this means that it must be a PWM signal, correct?
I wish I had a datasheet for the motor I’m intending to drive. I called the supplier and he could not provide me with one.
I appreciate the help guys!