Driven to learn more

2006 Chevy Trailblazer 4.2 Crank no start (most of the time) No RPM on scanner. Problem continued to get worse. Wouldn't start for me brother, he gave it to me, trailered it home and parked in my shop. It started in my shop so I suspected a wet condition was the beginning of the problem (dry in my shop) After moving it out of my shop then trying again wouldn't start. moved back into the shop, wouldn't start. I started testing as I learned from Paul. Actually started a couple of times but would die.
Fuel pressure 58 psi
Ohmed CKP=700
Backprobed 2 wire CKP with C2 disconnected, Ohmed=700
Continuity tested both wires individually=no open
Voltage drop tested all grounds after removing and cleaning=all good
2.5v on low reference purple wire from CKP (should be 0)
CKP making voltage (I don't currently have a scope but measured with Power Probe 4)
Suspecting PCM problem

I wouldn't feel comfortable changing the PCM until I knew more about the crank signal.
 
The Crank Sensor is a Variable Reluctance Sensor. It produces an AC sinewave. In order for the PCM to "see" the signal, the amplitude of the AC sinewave needs to be, at least, 0.5 -1.0 VAC. 
The best way to verify a good signal is with a labscope. Since you don't have a scope, you can set a volt meter to AC and read voltage while cranking. If you are not producing 0.5 - 1.0 VAC, check the air gap between the sensor and the reluctor wheel. The wider the gap, the smaller/less voltage, the signal will be.

I don’t have a scope but the power probe for is supposed to be able to measure ac current although not as accurately as a scope…but it is what I have. It said I was getting AC current although. I’d love to rule out the PCM too. I’m pretty new to electrical troubleshooting but have been following any instructions that I can find. From what I am understanding the 2.5v on the reference wire indicates a problem causing the computer to not recognize the signal given from the CKP sensor. Again, I’m new and trying to be a student and taking things step by step and slow so I don’t mess something up. If more info would help I’d be glad to give you the story of the problem. Don’t wanna take advantage of anyone’s time
Thanks
Mark

Also wanted to apologize for any late responses. I work 12 hour rotating shift and right now I’m on night shift.

the power probe for is supposed to be able to measure ac current although not as accurately as a scope…but it is what I have. It said I was getting AC current

It's not current that I am interested in. Current should be VERY low. What is important is the amplitude of Voltage, not Current. 

From what I am understanding the 2.5v on the reference wire indicates a problem causing the computer to not recognize the signal given from the CKP sensor.

Are you reading 2.5v with the sensor connected, disconnected, or both?
Connect an incandescent test-light to Battery (-). With the probe of the test-light, touch the wire that has 2.5v. Does the voltage sustain? Or, does it drop to 0v?

 

Ok, I’ll redo the CKP test with a DVOM.
I grounded the low reference wire to a known good ground and that brought it to 0v. I turned the engine over and the scanner picked up a bunch of new codes including modules losing comm with pcm. After clearing codes and disconnecting ground the codes went away and I got the old codes back for abs problems
I am curious about what would pull down the 2.5 volts. That was what led me to thinking the PCM wasn’t pulling to ground. Is there another module that would be suspect causing this problem? I did check the camshaft position sensor and it Ohmed as good

I just rechecked what I did and it was the voltage that I checked, not current. It was around.3-.5. Sorry for the wrong info. I’m new and still learning (not brand new by any means, but new compared to a lot of you guys.

it was the voltage that I checked, not current. It was around.3-.5.

I think that is bit low. The amplitude of the signal depends on a few factors:

One of which is the air gap between the sensor and the trigger wheel. Check to make sure that you do not have a "rust jack" at the sensor. Sometimes, rust can build up under the sensor mounting point, increasing the air gap. 

Another factor is speed. The faster the trigger wheel spins, the higher the voltage will be. Make sure that your battery is fully charged. Maybe, use a charger with "boost" mode. 

A scope waveform would be much more informative, if you can get your hands on one.

 

Awesome! Thanks for that info. I’ll try to get that checked today. We have between 3’ and 4’ of snow coming so I will be a little busy with that and 12 hour day shift Monday Tuesday Wednesday. Sure appreciate your help! Hard to find the guided help I need for this. I joined Scanner Danner because I want to learn every aspect of automotive electrical trouble shooting and your willingness to help makes it easier. Thanks again
Mark

Do you have a recommendation for a scope?

Any scope is better than no scope. 

The uScope, from AESwave.com , is a small, single channel scope that can be purchased for about $200. It's a very good scope for the price, but it is very small. 
Snap on scopes are worth considering, and are usually readily available to purchase, used. They have 2-channel and 4-channel scopes.
I have no experience with Hantek, but many people choose them because they are more affordable than some other scopes. 
Autel has a decent 4 channel scope, but you have to have an Autel scantool. Same with Topdon.
The ATS eScope is an automotive-focused scope. It has some robust features for advanced diagnostics and pairs, nicely, with the eScan scantool. It comes in 4 and 8 channels. 

But, hands down, the best scope for Automotive is the Picoscope 4425, or 4425A. 

If you get a single channel scope, you will immediately wish you had a 2-channel scope. If you start with a 2-channel scope, you will soon want a 4-channel scope. A 4-channel scope is suitable for most cases but, sometimes, you need more. 

Again, any scope is better than no scope.