Ignition voltage at crank sensor is low

[Matthew Jeschke]

My crank sensor is getting 2 volts to low on it's ignition voltage pin and truck won't start now.

Background: My truck doesn't like to start. A few weeks ago a mechanic friend of mine road with me. He thought there was an issue with the crank position sensor. I swapped in a new one and it won't crank at all now. I got a crank position sensor DTC (forget which one). Unplugged the sensor and started probing voltages.

The crank position sensor gets ignition voltage from the PCM but does anybody know where the PCM get's it's ignition voltage from, the source? I need to test that connection.

When I turn the ignition on (with PCM connected) I get 10.2V instead of 12V at the crank sensor.

Connection from PCM to Crank Sensor C1 P12 to CKP Pin A is good. However, with ignition on it's 2 volts low.

C1 P19 Circuit Number 439 goes to Ignition 1 Voltage. I also assume this is routed in the PCM for Crank Position Sensor's 12 Volt Reference?

Another part of the FSM says the crank sensor gets regulated 12 V power. Maybe I have a bad voltage regulator in the PCM?

I see the camshaft also uses a similar 12 Volt Reference, maybe the same regulator if that's the case? Then I could check voltage for the cam sensor to see what it's at with ignition on. I think it's C2 (Red) Pin 39 Circuit 631. Assuming as much as Circuit 633 is the crank position sensor and 632 is a low reference.

Testing power pins in C1 and C2 to PCM:

Everything checkout. The harness seems good. I get 12 V (same as battery) per wiring diagram below. All the grounds pass continuity check and say 0 Ohms on my power probe.

 

[exp500]

I can't exactly follow your test results per your diagrams. Pass means what? Battery voltage?
What I do see is a question on C-11( both sides) and next C-1 pin 19. What is Crank PS pin B to ground ohms key off ? And pin C voltage?
CKP is hall effect sensor correct? So the trigger would be increased voltage x amount above low if I understand it correctly.
Check CKP wiring for flex damage.
After re-reading your post I understand your question about PCM internal regulation. Don't know.
Is this another "different PCM" ? Thinking again all problems related?
Look at differences between 1 and 3 wire CKP as some are NPN others PNP, so the reference signals important to the transistor fed from CKP pin A.

 

[Matthew Jeschke]

I removed the new crank sensor suspecting it's bad... For completeness sake, I've gone down this rabbit hole quite a ways... I probed with ohm meter and compared...

Sensor that truck ran with (the old one)

Original Sensor ~
A to C 5k Ohms
A to B 420k Ohms
B to C 420k Ohms

The new sensor is different ~
A to C 5k Ohms (same)
A to B 5.7k Ohms (different)
B to C 8.1k Ohms (different)

Lower image is ssimilar to our sensor (three pin). Although it's not showing the coils? The lower one is using a pull up resistor to pull the signal to 5 volts when the circuit is completed.

 

[Matthew Jeschke]

I can't exactly follow your test results per your diagrams. Pass means what? Battery voltage?
What I do see is a question on C-11( both sides) and next C-1 pin 19. What is Crank PS pin B to ground ohms key off ? And pin C voltage?
CKP is hall effect sensor correct? So the trigger would be increased voltage x amount above low if I understand it correctly.
Check CKP wiring for flex damage.
After re-reading your post I understand your question about PCM internal regulation. Don't know.
Is this another "different PCM" ? Thinking again all problems related?
Look at differences between 1 and 3 wire CKP as some are NPN others PNP, so the reference signals important to the transistor fed from CKP pin A.

Thanks for chipping into my post! I appreciate the help.

Pass means two things, two tests, I got battery voltage and a good continuity check.

Pins C1 - 19 and 75 were with ignition on. They both got bat voltage as expected.

What is Crank PS pin B to ground ohms key off ? And pin C voltage?

Using my power probe with ignition off...

Pin C (12 volt reference) zero volts, ground (chassis).

Pin B shows ground connection to battery (chassis), 0.1 ohms.

I also probed Pin C... Shows ground connection (I think probe triggers the PCM or something ~ Maybe a pull down circuit in PCM). When I switch to ohms it shows not connected (but clicks back and forth trying to autorange ohm meter). I also tried this with PCM disconnected and can see there's no issue in yellow wire to PCM. I'm assuming behavior is normal for PCM when probing? but is only one I've ever tested.

Correct CKP is a hall effect sensor. I haven't scoped the waveform (truck isn't running).. But I think it amplifies the passing of the encoder wheel teeth on the crankshaft via magnetic effect. I'm assuming they built it so it opens and closes a switch to allow 12 V reference voltage through as the teeth pass by.

Internal regulation - yeah I cannot figure out what they mean by 12 V Reference. I bench tested my PCM and a spare I have both behave the same. What they refer to as the 12 V Reference both the camshaft and crankshaft sensors, the voltage is slightly reduced from battery (or ignition voltage). At my bench if I supply 12 V to PCM I get 11ish at the sensor. If I supply 14.6 I get 13 something at the sensor. Go figure. Reference to me means 12V regardless of input level (aside from off / 0V).

"Different PCM" I've tested two at my bench both behave the same behavior.

My post just prior to this one I discovered the new sensor to be electrically different than the old one. Maybe they changed the design and or this new sensor is faulty ?

I'm going to put in the old sensor and see if I can get truck to start.

 

[exp500]

Thats why I asked about CKP pin B to ground. Lower image you posted hall effect.

 

[Matthew Jeschke]

Yup Pin B goes to chassis ground. Despite saying it's an reference in PCM. I'm a bit nuanced about terminology, my background is engineering. Which Reference would lead me to believe it's isolated circuit (not directly connected to chassis ground) but rather a regulated circuit inside PCM.

Got the truck working, seems all this was an exercise for nothing. Turns out I had a bad part. The new sensor was defective. Seems to be more and more the case these days with car parts. I got a GM OEM replacement sensor, go figure.

 

[Jimmyy]

Where did the "new" sensor come from? Rockauto, Amazon or part store? I just a bought a AC Delco one from Rockauto but haven't installed it yet.

 

[Jimmyy]

 

[Matthew Jeschke]

Where did the "new" sensor come from? Rockauto, Amazon or part store? I just a bought a AC Delco one from Rockauto but haven't installed it yet.

It came from eBay. I thought the seller was a "dealership"... However, it was still defective. I've learned my lesson on eBay parts. Never again, although I did buy this the better part of a decade ago and it's been sitting on my shelf lol

You can ohm your pins like I did (previous post in this thread) and double check before you install yours. I assume the reading would be very similar for your sensor and mine if you have a GMT800 generation vehicle, which 2006 should be.

 

[rockola1971]

I was gonna say the new sensor ohm readings were wildly different than your original and the new part was bad or not the correct part. The regulator for sensor power is inside your PCM. The raw input power to your regulator comes in on Pin 19 and/or 20 at the PCM. The sensors need stable voltage to give valid and stable reading to the PCM. If they just used battery voltage you would get way too wild of voltage swings with lights, AC, tow package being used, etc. Even the difference of alternator output at 35mph in town vs cruising interstate at 70mph would be swinging to wildly for sensor signals to be valid for the PCM.

 

[Matthew Jeschke]

@rockola1971 thanks, makes since... As observed the "reference" voltage for cam and crank sensors is approximately 9/10ths of ignition voltage. I'm assuming the PCM is using a 5V analog to digital converter to read the sensor (or a edge triggered interrupt). Long story short, it seems to be fine when I plugged in the old sensor. My hard crank is due to something else.

I'm thinking it's "idle airflow" and or "Cranking VE" tables in calibration? Asking tuning questions online is a tricky thing. So many experts with no experience in the tuning forums...

Any ideas what might be causing the hard crank... I'm almost positive it's something in the PCM calibration / tune. I've went through:

- Cam & Crank sensors
- Throttle adjustment screw
- Entire fuel system including Evap (injectors, rail, pressure regulator, fuel filter all verified good with gauge set).
- No vacuum leaks
- No ignition issues (coils are all good)
- No electrical issues (harness is good, battery, alternator, & starter cables good)
etc. etc. etc.

I do have a custom ground cam from Roger Vinci. It runs SUPER SMOOTH once engine has started... plenty of info on that to tune. However, cranking / startup tuning I've not been able to find much reliable info on.