I've got a 1999 Chevy blazer, 4.3l with 135k miles. I'm getting a P0339 code and a no-start condition. There is 10volts on the sensor wire from the crankshaft position sensor no matter where it is reading the reluctor wheel, even if the sensor is not mounted but is still plugged into the wire harness. I've tried 2 different new sensors resulting in the same readings. My video shows this at: https://youtu.be/ZfIkFPqDOu4

Thanks for your assistance!

 

I assume that the engine cranks but will not run? Have you checked to see if you have spark during cranking? Assuming no spark:

What is the voltage on the pink wire with the sensor plugged in and again with it unplugged? reference to battery ground.

Where are you measuring this constant 10V and is this with the sensor installed, and connected and hand turning the engine at the crank pulley?


George

 

Cranks- Yes
Spark - Yes
12.06v, both plugged in and unplugged (back probed).
Ground checked by connecting red meter wire to positive on battery and black meter wire to ground (middle) on harness. Showed 12.06v.
10v is on the wire (back probed) going to the ECM, installed and while turning the engine over by hand. Video is here:

 

If you have 1" of strong blue spark at a regular cadence during cranking then that is not why your truck wont start and your crank sensor has not failed. You need to look at fuel pressure and injection function. Have you performed the complete fuel pressure test? Same question on injector firing?

George

 

Truck attempts to start, at times, and runs roughly for 3-5 seconds before dying. Not sure what "complete fuel system check" consists of. Fuel pump wasn't working, changed along with filter. Getting 60 psi at fuel rail for 2 seconds with KOEO. Smokes when roughly running for the 3-5 seconds. New plugs, wires, distributor. If CPS is showing 10v, no matter where the reluctor wheel is, randomly dropping to 0v for 3-4 seconds before going back to 10v, wouldn't that be the reason for the P0339 (Crankshaft Position Sensor A – Circuit Intermittent) code? This is all new to me though. 🥴

 

The most important part of the fuel pressure test is that it holds 55 psi at 10 min after KOEOff.

I thought it was a hard no start with spark. There are a number of possibilities for this code and problem:

Poor power at the the sensor
Poor wire condition on the two signal wires between the sensor and the PCM
Electromagnetic interference
Damaged sensor
Damaged reluctor wheel
Misaligned reluctor wheel from crankshaft end play

I would start by looking for clean power and solid wiring between the sensor and the PCM.

George

 

Good power as in steady, repeatable 12+ voltage? How can I check the wires' integrity between the sensor harness and the ECM? Original CPS showed signs of rubbing the reluctor. Tried a new, cheap CPS and got these readings. Thought the new, cheap CPS was faulty...bought a better ($60 AC Delco) sensor. Same readings as the cheap one. Measured and installed one shim on new sensor. Reluctor doesn't show damage and looks to be aligned in center of opening. Sensor gives same voltage readings (10v) even when not installed, but still connected to harness. I'll check the fuel psi like you suggested.

 

The crank sensor has 3 wires:

Pink: +12v from ECM1 fuse
PPL: Signal ground from VCM
Yel: Signal from sensor to VCM

The first test is +12v measured from pink to ppl. This says that the 12v power is making it to the sensor and that the VCM supplied signal ground is at least working. There are more detailed tests for these two under a load if we get stuck

The next test is the square wave pulses that should be getting sent out between the yel an ppl wires as the engine is rotating. Best would be an O scope while cranking but since most diy'ers don't have one we need a more basic way to see if the sensor is outputting a square wave signal with a duty cycle. This more basic measurement cannot be done at full engine cranking speed because the meter will just show an unchanging average voltage. You need to watch DC volts while hand turning the engine at the crank pulley with a breaker bar. A slightly more sophisticated method is to measure duty cycle with your meter if equipped as the engine is cranked which should read between 40-60%

If these show nothing then we need to check the intergrity of all 3 wires, and the reluctance wheel quality, alignment and spacing.

Which of these have you done so far?


George

 

I have checked the 12v coming to the sensor (good) and the ground (good). I have not been able to check the wave pulses. The blazer starts now (possibly was flooded during previous starting attempts...?). It will idle smoothly, but misses at rpms above 2000. I checked was the Cam Retard to make sure the distributor was close. It was +1, so I adjusted it to 0. Still getting only P0339 and P0341 codes. I have some files from a Snap On scanner I borrowed, but am unable to attach because of the file type (.SCM). Maybe there is something in the files that can provide some answers to someone who know what to look for. That obviously isn't me... 🥴

 

The truck has to have a functioning crank sensor input to the engine computer to run. The fact that you still have the crank sensor code says that there must be a marginal or intermittent problem. Let try a few things:

Watch the sensor power at the pink wire when you try to raise RPM's and it runs crappy

We need to check the integrity of the two sensor signal wires (putple and yellow): resistance and check for shorts to power and ground and inspect the harness for corrosion. Your going to need to locate and disconnect the connectors at both ends (sensor and computer). Attached is your wiring diagram, let me know if you need help with this.

George