What are the symptoms of a bad one? Will the engine still start?

 

I've never seen the sensor go without generating a CEL, so I'd think if the truck is not starting, but not coming up with a CEL, I'd start by troubleshooting elsewhere. But there are always exceptions...

 

That's what I was thinking as well. I've got a no crank condition and the service manual say's the following:
The ground circuit for the coil within the ASD relay is controlled by the Power-train Control Module (PCM). The PCM operates the ASD relay by switching its ground circuit on and off. The ASD relay will be shut–down, meaning the 12–volt power supply to the ASD relay will be de-activated by the PCM if:
1. The ignition key is left in the ON position. This
is if the engine has not been running for approximately
1.8 seconds.
2. There is a crankshaft position sensor signal to
the PCM that is lower than pre-determined values.

I'm getting momentary signal to ground, just long enough to engage the starter, it's starts to turn over then stops. So I figured maybe it's not getting a signal from the crank position sensor. But I'm not getting a DTC for it.

What would cause the PCM to stop providing this path to ground?