Is it expected to see 2.4V on EMS2 circuit even with EMS2 fuse removed?

[hans]

I have 2007 Chevrolet Aveo notchback model. My car once stalled on road and did not start anymore because of p0340 code (Camshaft Position Sensor). I replaced Camshaft Position Sensor, but the car still did not start. Then one of the Mechanics StackExchange members suspected that CMP sensor is not getting stable 12V and that I should try to connect sensor power pin directly to Battery Plus terminal and try to start the car. Surprisingly, now the car starts even without me having to connect Battery Plus to CMP power pin anymore.

While the car starts, I want to root cause the issue properly so that it does not happen again.

Remaining symptoms that I think may have something to do with the original issue I faced are that electrical wiring after EMS2 fuse is connected to some other electrical circuit because I see there 2.4V even without EMS2 fuse when Ignition is in "ON position.

My questions:
1. Could someone measure voltage between their cars Battery Minus and harness facing EMS2 fuses terminal with the EMS2 Fuse removed and key in ON position to see if you get 2.4V or 0V? I think it should be 0V, but just want to be 100% sure that I am focusing on the right problem.
2. Has anyone seen similar electrical issue when P0340 code showed up with "crank, but no start" condition? What was the solution (did you change PCM, find a fluid leak on some contacts that caused electrical leak between different electrical branches)?

FYI:Here is my thread on mechanics.stackexchange with pictures and on what I have done in more details:
https://mechanics.stackexchange.com/...ain-relay-is-t

[JB_Fraser]

Mine is a 2005 HatchBack , which is a type 1 fuse/wiring layout so I can't give you any comparable readings since I don't have an EMS1 or EMS2 fuse.


On a side note, i think you should attack this from a different angle and look at what 'you did' to get the car to start.
The 12v jumper you connected to the CMP sensor gave power to the CMP but may have supplied power to something else since a lot of these are 'branched' together after the fuse.
I think you need to trace back your jumper point and the wiring(s) these share and narrow down from there.


As for the voltage measured with fuses/relay removed, a lot of the inputs to the computer have 'pull-up resistors'. So if you have a pull-up on an input that runs through a variable resistor sensor then you will see some voltage present.


Intermittent issues are the worst to trouble shoot and hopefully it was just a loose connection you managed to 'jar back into place' while you were trouble shooting.


I admire your gumption to try and nail down the issue, hopefully this helps!
Good luck


Cheers

[hans]

I connected Battery + not to the harness that plugs into CMP, but directly to the CMP sensor pin. By doing so could I still somehow powered up something else that lead my car to suddenly start?

I think your theory about pull up resistor "providing" those 2.5V with EMS2 fuse removed could make sense (at least there *was* continuity from EMS2 branch to the one of the PCM pins). In that case I may indeed be focusing on the wrong thing. Though why GM designed circuit in such way - what's the purpose of such circuit besides monitoring healthiness of EMS2 fuse and Main relay? I think it would still be great if some forum members with the same 2007 Aveo Notchback could double confirm this for me to clear any remaining doubt.

Assuming that those 2.5V are expected, then I may have to fall back to the theory where Main Relay was faulty, because:
1. when car did not start the voltage on EMS2 branch slowly grew from 2.5V to 12V. And at the time of cranking again fell from 12V to ~2.5V.
2. when car started, perhaps, main relay opened up completely. Though, not sure how likely it is for relay to be broken in a such way.

Maybe I should try to reproduce this issue again and then try to swap main relay to see if it solved the issue?

[JB_Fraser]

Okay, my ‘assumption’ that you back probed the power connection.
So the connector was removed and you supplied a know source voltage to the power pin ?
Where the signal and ground connections jumpered to the harness (power isolated as known good supply and signal and ground connected to the harness) , I may be making assumptions again.

My thought originally was that you inadvertently supplied power to the branch circuit that this sensor was connected too, but from your response:
1.This is not what you did.
2.There were other testing/changes that were done.

Since you had a ‘crank no-start’ condition, dis-connect the connector for the cam position sensor and try and start the vehicle (I can’t say for sure if this signal is a requirement for the 2007 model) but my guess is that it will start.
if it does start, this would pretty much guarantee that it was something else during your trouble shooting that happened to resolve the issue.
If it doesn’t start (if it requires crank and cam) then it stays as a possibility that it may have been an issue.

If it inadvertently happens again , you need to check after each attempt so you can try and isolate what may have been the corrective measures.


Cheers