I Like Fried Chicken and Fried Bacon but not a Fried PCM

Michael Mobile Technician Clinton, Utah Posted   Latest  
Discussion
Drivetrain
Electrical
2008 GMC Acadia 3.6L (LY7) 6-spd (6T75)
P0449 - EVAP System Vent Valve Control Circuit/Open

I was called to check out a 2008 Acadia that had a constant P0449 code. The customer had changed the Vent Solenoid with out any resolve. In testing we found that there was an open in between the valve and the PCM. Wiring diagrams are my hero! My idea was to jumper the wire to see if we could get the vent valve to work. In the process we fried the PCM. I went over the process several times. I discovered that when we were looking through the wires to find the correct one to back probe, two of the main connectors were not securely seated to the PCM. The theory is that when the Vent Valve was commanded on, the ground took whatever path it could since the prescribed path was not connected. Pop goes the weasel. Today we installed and programmed a used PCM. We got to the same point of testing. I was concerned about letting the smoke out of another PCM. We wired the vent solenoid through my Multimeter. This way we could watch the current flow at the same time pull the plug if needed. Maybe it would pop the fuse in my meter? The jumper wire worked fine. We were able to command the Vent on and off. The code cleared without a hitch.

I am really glad the PCM was only $40. I am also glad that we got one that was not from a Global A vehicle. Been there done that. Lesson learned. Check all connections before loading the circuit!

diag​.​net/file/flj560540…

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Geoff Diagnostician
Lahaina, Hawaii
Geoff
 

$40?!?! Where?

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Cuba Mobile Technician
Phila, Pennsylvania
Cuba
 

For that year & make, they go for about the same here in PA. Range is $35 to $50....

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Geoff Diagnostician
Lahaina, Hawaii
Geoff
 

That has to be used.

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Michael Mobile Technician
Clinton, Utah
Michael
 

car-part​.​com There are wrecking yards all over with similar pricing. Shipping to Hawaii will cost you a more.

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Geoff Diagnostician
Lahaina, Hawaii
Geoff
 

OH. Used from a junk yard. I thought you meant a new one. Yeah, we don't have junk yards here. Land is far too expensive.

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Eric Owner/Technician
Edgerton, Wisconsin
Eric
 

I'm lost, I don't understand your theory of how the PCM was damaged. SI shows constant battery power to and through the vent solenoid to the PCM. The PCM operates the vent solenoid by providing the ground. Why would a lack of connection, in other words no battery power from the vent solenoid to the pin on the PCM, cause the PCM to be damaged when the PCM internally grounded the pin? Wouldn't…

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Michael Mobile Technician
Clinton, Utah
Michael
 

Eric, The open was determined to be on the ground side of the circuit. So we have a fused wire coming to the solenoid and then to a connector which is not in the diagram. The white wire was back probed at the connector and made a jumper to the PCM. It was the white wire in the blue connector (labeled on diagram as X1) pin 55. The circuit is completed by commanding the vent valve to be closed…

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Eric Owner/Technician
Edgerton, Wisconsin
Eric
 

What SI were you using, looking in Identifix I see a connector on the ground side of the circuit labeled X305, is that the connector you were using? I understand the circuit operation but I still don't see how your explanation of how the PCM was damaged can work. In a correctly operating system there is always battery voltage at the pin 55 terminal, at least as long as the battery is…

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Anthony Technical Support Specialist
Kirkwood, Pennsylvania
Anthony
   

Hi Mike: Why not fuse your jumper wires? I had a Simpson meter, years ago. Fuses were $9/pop. (Pun wasn't intended but is appropriate.) BTW, I think that you're correct on what caused the damage. Terminal ends aren't designed to be switches. HTH, Guido

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Michael Mobile Technician
Clinton, Utah
Michael
 

Guido, A fuse may not have helped in this case. We did not add power or ground to the circuit, only replace the open part of the circuit. The current flow through the meter was just over 500mA. So maybe a 1A fuse would have saved the PCM. It depends on what path the electrons took. The meter I used has a less expensive fuse but it is not a common one.

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Eric Owner/Technician
Edgerton, Wisconsin
Eric
 

Guido, I agree on using fuses in jumper wires, all of mine have fuse holders in them so I can install the proper size fuse for the job. Please explain to me how Mike is correct with his conclusion that the disconnected terminal at the PCM for the canister vent solenoid caused the PCM to be damaged. Thanks

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Michael Mobile Technician
Clinton, Utah
Michael
 

Hi Eric, SI used was Motologic. It does not show a connector between the vent and fuse. It would be nice to have more sources of SI. Right now it is all I have available. Maybe I am not explaining the situation well. We are on the same page that the B+ to and from the vent circuit is always there when the circuit is open. We are on the same page that pin 55 on X1 is the driver that provides…

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Eric Owner/Technician
Edgerton, Wisconsin
Eric
 

Okay, that is a better (for me at least) description of the possible issue and one that I could see happening. I guess I missed the part about the ground for the PCM being the item under discussion. Even so, I have a problem with that diagnosis simply because of all the times I've found poor connections at PCM grounds (high voltage drops) and after cleaning/repairing them I've not yet seen a…

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Anthony Technical Support Specialist
Kirkwood, Pennsylvania
Anthony
 

Hi Eric: I took what he wrote to mean that the connector moved and the terminal made a connection with the spike from the connection taking out the circuit. Perhaps I misunderstood what he was attempting to state. Guido

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Eric Owner/Technician
Edgerton, Wisconsin
Eric
 

In his last reply to me he explained he was talking about the main ground, pin 73 in connector X2, for the PCM and not pin 55 in connector X1 for grounding the vent solenoid. That was the part of the story I missed. I'm still not sure how a bad connection at the PCM ground would damage the PCM when the vent solenoid was grounded. I've repaired a lot of bad PCM grounds on running vehicles and…

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Anthony Technical Support Specialist
Kirkwood, Pennsylvania
Anthony
 

Hi Eric: I saw that reply after I replied to you. I may be able to offer an example of how this can happen though. Take an older Tech 2. It took the same amount of voltage to turn the Tech 2 on as it did to turn the module on (approximately 9.5vdc). If you cranked the engine and the Tech 2 shut off, you knew to check the power, ground and battery of the vehicle. If you do that with a newer Tech…

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Eric Owner/Technician
Edgerton, Wisconsin
Eric
 

That is why I like my ancient version Tech 2, much more bulletproof than the newer ones. Sometimes us early adopters get lucky. :-)

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