2008 Jetta 2.5, voltage at CKP signal wire with sensor unplugged

I am working on a 2008 Volkswagen Jetta 2.5 with a crank no start condition, and P0322 code. I see no RPM signal on the tach or with my scan tool while cranking. From what I can tell, pin 1 is 5v ref, pin 2 is signal wire, and pin 3 is ground. With key on I have 5v at pin 1 and 5v at pin 2 and good ground with the connector back probed and the sensor connected. There is no change in these values when I unplug the sensor. I think there should be no voltage at pin 2 with the sensor disconnected. There is no continuity between pin 1 and pin 2, so those two wires do not appear to be shorted together. 

Scoping the signal wire while cranking, I see the 5v signal get pulled down to about 4v in a square wave pattern, where I think it should get pulled down to zero (scope pic attached). I am guessing this means the sensor is functioning properly, but it is not able to give enough wave form amplitude to the ECM to provide a crank signal because of the 5v supplied to the signal wire.

Any thoughts on how to proceed?

Whether or not there should be 5v on the signal wire when disconnected depends on if it's a pull up or pull down circuit design. It is easier when there is a change when unplugged, but that's OK.

I'd be good to get a known good wave form to compare, but it's unlikely that the 1v change you're seeing is normal, so I think you're on to something here. I'm thinking either the sensor is bad or maybe the airgap is not correct, and isn't able to be pulled down all the way.

Thanks for your reply. I have watched one YouTube video by a Volkswagen specialty shop stating that there should be no voltage at the signal wire with the connector unplugged. Do you have any suggestions on how to verify whether this is correct, or to determine whether it is a pull down or pull up circuit?

You could disconnect the wire from the ecm side to see if you loose the 5v. That would tell you if it's shorted to 5v somewhere in the harness.

I did check for continuity between the signal wire and the reference wire and found no continuity. I suppose the wire could be shorted to a different 5v wire. Is there any likelihood that such a situation could be caused by a fault internal to the ECU?

lmg866 wrote:

With key on I have 5v at pin 1 and 5v at pin 2 and good ground with the connector back probed and the sensor connected.



 

How are you testing the ground? I would connect a second scope channel to see a waveform of the sensor ground, along with the sensor signal.

Scoping the signal wire while cranking, I see the 5v signal get pulled down to about 4v in a square wave pattern

This suggests a pull-down design.

I suppose the wire could be shorted to a different 5v wire.

can you pull the 5 volts that is on the signal wire to ground with an incandescent test light connected to battery negative? Or, does the test light, dimmly, glow?
 

Thanks for the reply.

I simply verified continuity with a multimeter. I will try a separate channel on the ground wire while cranking. What should I be looking for?

In the incandescent test light example, it seems like I am trying to load the circuit some. What would a bright or dim result suggest?

I will try a separate channel on the ground wire while cranking. What should I be looking for?

You want to see the voltage on the ground wire remain less than 100 mV (0.1 volt).  If you see a square wave with an amplitude of more than 100 mV, you have a bad ground.

In the incandescent test light example, it seems like I am trying to load the circuit some. What would a bright or dim result suggest?

If the 5v on the signal wire is a bias voltage, and can be pulled down to ground, this would suggest that:
1) This IS a pull-down circuit design
2) The senor is bad, and cannot pull the signal down. Or,
3) You have a bad sensor ground.

If the test light glows, you have a short to power.
 

Thank you for your help; Chad. A lot of terms I have seen kicking around make a lot of sense applied as you’ve explained them here.

Before I saw your reply, I ended up buying a new sensor. This solved the issue. Next time I’ll know more about what to do with the test results I saw. Thanks again!