My check engine light came on today at the same time my tachometer went out down to zero and has stayed there, so I guess it's broke. I guess my question would be is the tachometer going out what caused the light to come on?

 

Gotta get the computer scanned for the code(s).

 

Some time has passed since but my rough guess is the knock sensor. It drives the tach and when it's broken also triggers the CEL/MIL. And it's common to break.

 

Wait, the knock sensor drives the Tach??
Never heard of this.
Thought it was the crankshaft sensor or may be the camshaft sensor (although I don't think it is so) that gives the ECU the info on engine rpm and thus the tach.

 

Sorry, my bad (shooting from the hip ...)
What I actually meant is the ignition failure sensor.
Of course the tach signal comes from the engine ECU but the signal source used to create the tach signal is the ignition, not cam or crank sensors.

When your rev counter/tach isn't working and you have CEL/MIL on, very likely you can see

P0320 Ignition failure sensor open/short

stored.

 

Hmm, still doesn't seem right to me
But, hey if it works for you then who am I to say differently

 

+12V get supplied thru ign.fail.sensor to coils. PCM pulls coils to GND for spark (via power transistor in coil head). Ign.fail.sensor monitors 12V line current pulsating and outputs this to tach (trigger circuitry generating square 'wave') and via wire splice back into PCM. This way PCM can trigger CEL/MIL.

I haven't invented this, I can just read circuit diagrams ... (and by that I have to correct myself up there: tach signal doesn't come from/thru PCM but from sensor directly. Still the result is the same: Broken tach in most cases means broken ign.fail.sensor)

 

From your diagram, when the signal from the ECM goes high then the transistor is pulled down and the collapsing magnetic field in the coil then causes the spark to jump across the spark plugs.
The pulling down to GND is then seen by the Op amp in the Ignition failure sensor whose output then drives the base of the transistor high and the collector to go low
This signal feeds both the ECM and the Tach on the instrument cluster

But the ECM doesn't know when to fire the coils unless there is another sensor telling the ECM when it is the right time to fire the spark plugs
So even though the ignition failure sensor is connected to the tach on the dash, there is another sensor as well as the ECM that can also give similar symptoms of a failed tach

It needs a little more testing to be sure


Edit: Is the engine still running? Or has it stopped?

 

spark timing is determined by cam & crank sensors, ign.fail.sensor provides 'ignition success' feedback to ECM and drives tach.
What is to be discussed about that ? When engine runs fine and all that fails is the tach what else than ign.fail.sensor is the prime suspect (and it is known to fail ...) ???
Easy to diagnose, too. Read codes, get P0320, wham !
Or put piece of wire instead of sensor (read the appropriate pins from the schematic) to feed coils and if nothing changes and engine still runs the same way as before ...

Maybe one day the OP will report back here. Until then, I'm out.

 

Sorry for not reporting back with the solution, been quite ill for a long time. I replaced the Ignition Failure Sensor and it fixed the problem.