Can Bus Voltage

Randy Diagnostician Glendora, California Posted   Latest  
Question
Driveability
2007 Audi Q7 3.6L (BHK) 6-spd (09D)
No Crank / No Start

Were does the can bus network get its voltage 

+1
Michael Mobile Technician
Clinton, Utah
Michael Default
 

The Bus voltage comes from the modules that are connected on the Bus. For instance, the PCM generates the CAN Hi and CAN Lo. You can connect a PCM directly to a labscope and see the voltages regardless of anything else on the network. The CAN microprocessor gets it's voltage usually from a 5 VDC or 3.3 VDC regulator on the mainbord of the PCM. If CAN is shorted to ground or power on both Hi and…

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Randy Diagnostician
Glendora, California
Randy Default
 

So if voltage is 2.4 and you should 4.5 hi and low and can communicate with all other modules except ecm . No what

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Randy Diagnostician
Glendora, California
Randy Default
 

o and new ecm

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Michael Mobile Technician
Clinton, Utah
Michael Default
 

You can disconnect ECM and see if the voltages come up. You should also be able to backprobe the wires at the ECU and make sure both Lo and Hi are present.

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Randy Diagnostician
Glendora, California
Randy Default
 

already did that

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Michael Mobile Technician
Clinton, Utah
Michael Default
 

So voltages are not changing with ECU disconnected? There is signal getting to the ECU? Would you want to try to communicate with ECU on the bench? Drop me an email with a picture of ECU and I will try to find a connection diagram. …

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Randy Diagnostician
Glendora, California
Randy Default
 

I can disconnect 94 pin connector at ecm wait two minutes and plug back in and car will start and run with no codes

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Michael Mobile Technician
Clinton, Utah
Michael Default
 

Does the vehicle stall after running or just not restart?

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Randy Diagnostician
Glendora, California
Randy Default
 

Runs just fine tell you cycle key

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Michael Mobile Technician
Clinton, Utah
Michael Default
 

Wow you got a challenge on this one. It seems that if it were a CAN issue there would be codes for lost communication with ECU. Relays? Drag test terminals to ECU?

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Randy Diagnostician
Glendora, California
Randy Default
 

There is codes for lost comm with ecm . And I did do drag teat

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Michael Mobile Technician
Clinton, Utah
Michael Default
 

The Wiring diagram shows a splice pack on the left and one on the right. My diagram does not say exactly where. If you can locate the splice packs then your could eliminate modules one by one that could be effecting the bus.

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Randy Diagnostician
Glendora, California
Randy Default
 

Found them already did that

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Randy Diagnostician
Glendora, California
Randy Default
 

Im going to retry tomorrow but pack probed in to gate way . Im thinking gate way iss

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Michael Owner/Technician
Cartersville, Georgia
Michael Default
 

Michael is right. voltage is generated from all of the modules on the network. If you are not seeing voltage then there is some bringing it down. Something on the physical level. Mike

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Rudy Technician
Montebello, California
Rudy Default
 

Did you do the ol' ohm test for the terminating resistors? (I know German vehicles dont always have the same 60 ohm values as other vehicles,but your service info should provide what you are looking for as far as that goes)

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Randy Diagnostician
Glendora, California
Randy Default
 

yep

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Rudy Technician
Montebello, California
Rudy Default
 

And?

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Randy Diagnostician
Glendora, California
Randy Default
 

all good

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