I replaced the timing cover gasket in my 2001, 3.0. When I put the harmonic balancer back on, some teeth were broken on the back of it. Is this why my truck won't start?

 

Yes, that is a "tone wheel", there should be 36 teeth minus 1, 1 is not there on purpose.

When distributors were removed there still needed to be a method of maintaining a firing order for the engine.
This is done using the Crank Position(CKP) sensor and a tone wheel on the crank shaft.
Tone wheel has 36 teeth spaced at 10deg, 36 X 10 = 360deg(full circle)
One tooth is removed to mark when #1 cylinder is at Top Dead Center(TDC).

CKP sensor sends a pulse for each tooth passing by and then a gap for TDC, this is how the computer times spark and fuel injectors.
If there is more than 1 gap in 360deg rotation computer would not start spark or fuel injection, because it can't be sure of the timing.

Just FYI:
Computer doesn't need to know if #1 is on compression stroke or exhaust stroke because Ford uses a Waste Spark system, #1 spark plug fires on BOTH compression stroke and exhaust stroke, as do the other 5 cylinders, so spark on exhaust stroke is wasted, hence the name Waste Spark system.

Cam Position Sensor(CPS) was added after the CKP sensor, while it could be used instead of the CKP sensor it isn't as accurate since it turns at half the speed of crank, CPS is mainly used for fine tuning fuel injection timing.

Electronic speedometers also use tone wheels and sensors, as do ABS systems, no gears means no wear.

 

Thank you Ron, that helps a lot. I'll get the balancer replaced.