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No diagnoistic protocol, no tach, no speedo. Looses comm…

Customer Question
No diagnoistic protocol, no...

No diagnoistic protocol, no tach, no speedo. Looses comm with scanner after issue starts. Car has started cold but after running for 5-10 minutes will start having intermittent tach. Once tach/speedo stops working trans stays in second gear(if driving) and if car is shut off, it will not restart but TCM and ECM powers up when turning key back on, fuel pump primes but no start. DIC will still go into engineering mode and dash self-test shows everything working. Seems to be a data line issue or BCM. Not sure how to check from here...

Mechanic's Assistant: How old is the transmission? Have you checked the brake switch?

Brake switch works

Mechanic's Assistant: What is the model of your '10 Pontiac?

09 G8GT

Mechanic's Assistant: Are you fixing your vehicle yourself? What have you tried so far?

"No diagnoistic protocol, no tach, no speedo. Looses comm with scanner after issue starts. Car has started cold but after running for 5-10 minutes will start having intermittent tach. Once tach/speedo stops working trans stays in second gear(if driving) and if car is shut off, it will not restart but TCM and ECM powers up when turning key back on, fuel pump primes but no start. DIC will still go into engineering mode and dash self-test shows everything working. Seems to be a data line issue or BCM. Not sure how to check from here..." Have tried different TCM and ECM with no change.

Mechanic's Assistant: Anything else you want the mechanic to know before I connect you?

Can't think of anything else...

Submitted: 5 months ago.Category: Pontiac
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Answered in 12 hours by:
11/15/2017
Pontiac Mechanic: Lou P., ASE Certified Technician replied 5 months ago
Lou P.
Lou P., ASE Certified Technician
Category: Pontiac
Satisfied Customers: 5,740
Experience: I have 15+ years gm experiance and am very knowledgeable with systems and operations and diagnosis and have access to factory wiring diagrams and tsb's
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Hello, my name is***** and i will do my best to assist you.I am going over your story now and will reply back soon with some info,thanks!

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Pontiac Mechanic: Lou P., ASE Certified Technician replied 5 months ago

so what we do with communication issues is we disconnect modules from the circuit to see which disconnected module allows the car to function normally.

do you also have access to a code scanner?

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Customer reply replied 5 months ago
Got no codes and scanner will not connect when the issue is present. OBD2 port has power....
Pontiac Mechanic: Lou P., ASE Certified Technician replied 5 months ago

ok, what kind of scanner are you using?

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Customer reply replied 5 months ago
I use my HP Tuners which is a scanner and editor for tuning the vehicle. It also has VCM controls...
Pontiac Mechanic: Lou P., ASE Certified Technician replied 5 months ago

ok, so it cannot communicate with the whole car.

ok so that is going to be a setback.

let me get you the wiring diagram to what modules are tied into each other , then you can either disconnect the fuse to the module, unplug the module or pull the diagnostic line out of the module connector to tell if its the issue, also you need to physically inspect the diagnostic line as if it hits ground it can cause issues.

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Customer reply replied 5 months ago
Yeah, I am going to need to ohm out the data lines....so.....when the issue is not present, the vehicle will start and drive normal with scanning functions. After 5-8 minutes, tach and speedo will drop to 0 and I loose data connection. The vehicle will stay running and trans will operate in default gear. I can not manually shift and it will not go up or down gears while driving. Once the car is shut off, it will not restart. I would GUESS that it is a data line going from ECM thru the BCM to the trans but that is a guess..... everything appears to stay powered up and did not find any fuses blown anywhere in the engine compartment, cabin, or trunk where the battery is.
Pontiac Mechanic: Lou P., ASE Certified Technician replied 5 months ago

ok, so how long does the car have to cool off before it runs normally again?

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Customer reply replied 5 months ago
I have a 3-lead 25 ft jumper harness for ohming out wiring. It would be this evening before I would be around the car....
Customer reply replied 5 months ago
Last night, it never would let me restart it. I changed the ECM and TCM to see if that would change anything. Right now, the valve body is down and TCM is just hanging loose to test another one but got nothing. I'll have to put the TCM back on the valve body, reinstall, and add fluid before I can restart... But If the scanner connects it will start. I Could jump the starter relay to make it start but that does help with anything since it still has no communication...
Pontiac Mechanic: Lou P., ASE Certified Technician replied 5 months ago

ok, so mu thought is temperature affects it.

so if a module is getting hot it is causing an internal open.

so we can use that to heat up and cool down a module to test.

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Customer reply replied 5 months ago
Data stream appears to be only going out with no return data since fuel pump will prime and everything powers up with key -on.
Customer reply replied 5 months ago
If it powers up, I can try to leave "key-on" to see if it is time sensitive as it warms up. I have seen wiring have high resistance that finally fails once warm.
Customer reply replied 5 months ago
BCM is the only thing I can't test since I can't program one to try.
Customer reply replied 5 months ago
I have unplugged and plugged back in the BCM just to see if that did anything....
Pontiac Mechanic: Lou P., ASE Certified Technician replied 5 months ago

well unplugging the bcm would be good enough.

what about the traction hydraulic module?

sorry for the delay on the wiring diagram

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Customer reply replied 5 months ago
I have both ABS fuses out at the moment. No effect. ABS module is still plugged in.
Pontiac Mechanic: Lou P., ASE Certified Technician replied 5 months ago

diagrams

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Customer reply replied 5 months ago
I have also visually checked as much of the harness as possible and messed with the wiring. No effect...
Pontiac Mechanic: Lou P., ASE Certified Technician replied 5 months ago

ok, other then the diagrams I wont be able to assist much further, I will open this up , hopefully another has experienced this with this model and can assist,

Lou

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Customer reply replied 5 months ago
Diagrams will help. Just ordered a GM G8 service manual from helminc.com
Customer reply replied 5 months ago
Ok, Thanks!
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Lou P.
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Lou P., ASE Certified Technician
Category: Pontiac
Satisfied Customers: 5,740
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Experience: I have 15+ years gm experiance and am very knowledgeable with systems and operations and diagnosis and have access to factory wiring diagrams and tsb's

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