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I have been troubleshooting Camshaft Position Sensor no-start issue in this thread for my 2007 Chevrolet Aveo. Long story short - in one of the answers it was suspected that camshaft position sensor is not getting stable 12V, so as a test it was suggested to connect Battery+ directly to the sensor's camshaft position sensor's Orange pin and car started successfully. Now, it surprisingly also starts without connecting the Orange pin directly to the Battery+ anymore.

However, I want to root cause this issue properly so that it does not happen again while I am driving.

Today I did a test where I measured voltage on Sensor Harness'es Orange wire with and without "main relay" plugged in when key was in "ON", position:

Without main relay

With main relay

As can be seen the voltage without relay is 2.4V, which I think is wrong. It should have been 0V. My questions are:

  1. Based on this test, can I conclude that "main relay" most likely is actually OK?
  2. Since I see 2.4V without main relay, then does it mean that there are two power wires with different voltages shorted together after the main relay?
  3. If so, then can that information of knowing 2.4V can be used to pinpoint the other wire that is shorted with the Orange wire?
  4. The obvious way to me seems to open all the splices in the fusebox after "main relay" power wire and see from which wire the 2.4V are coming and then just keep going forward until wire with 2.4V is isolated. Of course I would need to restore splices to the original condition after that. Any other better tips on how should I approach this problem?

P.S. May or may not be related - I have red Airbag light on for more than a year. Chevrolet dealership did not know how to root cause, because there "were no SRS codes". Could it be that AirBag is using 2.4V and has shorted somewhere with Camshaft position sensor power wire hence both issues? Though the timing does not match when both issues started to manifest themselves.

Update#1: I added load (resistor and yellow LED light) between sensor harness'es orange wire and Battery -. Voltage dropped from 2.4V to 1.98V, and the LED did light up (the alligator wire from battery- to the breadboard changes color to white and is not captured in the picture):

Not stray voltage

Update#2: Voltage measurements when key was in "ON" position ("Voltage between Battery Minus and" column should be correct. However, in "Voltage between Orange and" I may need to redo some measurement, because I think somewhere there I should see 0.4V and 10.1V):

enter image description here

If my interpretation is correct, then here is how last test with both fuses and relay removed looked like. Since there was continuity only between EMS1(R) and EMS2(L) then I assumed that those pins must be the ones facing Main relay.

enter image description here

I guess what this test suggests is that EMS1(L) and EMS2(R) and something else with voltage >2.4V is shorted together? Also, there was 109KOhm resistance between EMS1(L) and EMS2(R).

Update#3

With the battery disconnected the 107KOhm resistance between EMS1 and EMS2 changed to infinity once I unplugged the upper cable to PCM as well:

enter image description here

Also, with key in ON position and Main relay, EMS1 fuse and EMS2 fuse unplugged I got following voltage measurement between Battery Minus and CMP Orange wire:

  1. Upper cable and lower cable to PCM plugged in - 2.4V
  2. Upper cable to PCM plugged and lower unplugged - 11.81V
  3. Upper cable and lower cable to PCM unplugged - 0V.

So my latest 2 questions are:

Does this mean that PCM is malfunctioning and it is the reason why I see those 2.4V on CMP Orange wire even with EMS2 fuse removed? Or there is still possibility of something else causing that 2.4V on Orange sensor wire (e.g. after unplugging PCM the hypothetical 3rd circuit was not getting power hence it could not cause voltage leak on EMS2 circuit)?

Why with lower cable to PCM unplugged, but upper one plugged, I suddenly start to see 11.81V instead of 2.4V or 0V?

enter image description here

Update#4: With the key in "OFF" position and both PCM cables unplugged I connected Battery Plus directly to EMS1 terminal (that is facing harness, not main relay) and then measured voltage between Battery Minus and EMS2 fuse terminal that is also facing harness. As can be seen the voltage is 0V which I believe proves that there is no direct leak between EMS1 circuit and EMS2 circuit.

enter image description here

Errata:

  • In the past I incorrectly called CMP's "Orange" wire as "Pink" wire. It is actually Orange for my notchback model and presumably Pink for hetchback model.
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  • Perhaps you want to draw this point to ground with a temporary 1KOhm resistance and measure again?
    – Martin
    Oct 22, 2017 at 20:10
  • Attention: do not allow shorts, as the car battery has dangerously much current.
    – Martin
    Oct 22, 2017 at 20:21
  • @Myself you mean to rule out possibility that 2.4V that I see without main relay is stray voltage? I added picture that IMHO demonstrates that it is not stray voltage.
    – user389238
    Oct 22, 2017 at 20:48
  • Try pulling the EMS 1 fuse to see if the voltage is bleeding in from another circuit. The Main relay powers both the EMS 1 and 2 fuses. Maybe try unplugging the PCM as well. I'd probably pull the EMS 1 fuse and unplug everything on the EMS 2 circuit and see if the voltage is still there. That way you know it's not a shorted component and it's a wiring issue.
    – Ben
    Oct 23, 2017 at 22:20
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    I would check it with the key on as well. I’d still be hesitant to call it a pcm without a known good circuit to test against. If I run into an aveo with similar specs i’ll run a few tests. In the meanwhile I’d suggest you clean any engine to frame and the g104 grounds. And swap out the main relay. I’ll see if I can post an answer that makes sense later on. Check for any corrosion in the under side of the fuse box and maybe spray some contact cleaner in there.
    – Ben
    Oct 27, 2017 at 10:59

1 Answer 1

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i'm really surprised! you have really a good knowledge in diagnostic techniques, any way upon to my experience that car has faulty ecm motherboard , you have to make sure in case if you disconnect the ecm you will loss " ROM" the keep alive memory or not because the same thing with camaro , if not you can disconnect ecm and see voltage will drop to zero at orange terminal camshaft with main relay pluged , if yes >>>replace ecm , if no >>>>replace harness

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