Has anyone ever changed an engine harness? - Pontiac G6 Forum
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post #1 of 7 (permalink) Old 10-15-2018, 09:54 PM Thread Starter
2007 G6 'vert
Join Date: Jan 2009
Posts: 15
Unhappy Has anyone ever changed an engine harness?

I'm faced with a nasty problem.
My G6 GT convertible (3.9L) has eaten 2 transmissions in 70K miles (35K each) with gentle driving.
The problem: When starting to roll from a stop the car hesitates, then there is a bang, a feeling like you were hit from behind, and the car then rolls normally. Do this enough times and the front input clutch explodes. Completely random although more common when the car is cold and load is light. Sometimes 2x a day, other times I can go for 2-3 weeks and it doesn't happen.
First transmission replaced by GM. Second by an independent shop.
Changed parts include ECU, TCU, shift solenoids, EPC solenoid, valve body, TPS sensor.
The independent transmission shop has given up, blaming the car for destroying the 2 transmissions.
They are probably right.
I really find it hard to sell/junk a car with 70K miles in otherwise perfect shape.
So I suspect the only thing left is to replace the wiring harness. They are still available new (surprised me!) but I'm not sure that I can do this with the engine in the car.
Has anyone tried one? Or have any advice?
Thank you in advance!

2007 G6 GT Convertible
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post #2 of 7 (permalink) Old 10-22-2018, 08:13 AM
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If you remove anough stuff around the engine (coil pack, starter, cats, and heatshields) you can definitly get to the whole wiring harness with the engine still in, if that helps any.


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post #3 of 7 (permalink) Old 10-22-2018, 07:59 PM
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Not sure on the 3.9 but on the 3.5 it's not to hard to get to the harness for the transmission
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post #4 of 7 (permalink) Old 10-23-2018, 07:24 AM Thread Starter
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Thumbs up

Thanks for the feedback.
I spent some time with a flashlight and could easily see most of the connections so I went ahead and ordered the harness. Waiting for it to ship, supposedly it's still available.
In the meantime I found that using the manual shift for the first couple of miles (and then putting it into Drive) works fine and there is no stumbling or banging.
I was reading the operation modes of the transmission and saw that activating manual shift forces a high line pressure and that temporarily solves the low line pressure issue.
Has to be a bad pin or wire in the harness that expands when hot and fixes the bad connection (wherever it is).
At the rate that winter is coming in I'll probably end up putting the harness in when spring comes as I usually lay this car up for the winter. I'll post again and let everyone know how the harness replacement goes.
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post #5 of 7 (permalink) Old 10-26-2018, 06:29 AM
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So I’m not crazy. I have been having this issue randomly since I bought my G6 four years ago. It seemed completely random as to when it would happen but I discovered using manual mode seemed to solve it. Thanks for the info I’ll run threw my harness and see if anything odd jumps out at me.
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post #6 of 7 (permalink) Old 08-07-2019, 09:06 PM Thread Starter
2007 G6 'vert
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Posts: 15
So it's been 9 months and I finally have an update.
For anyone else who needs the info, the correct wiring harness for a G6 GT Convertible with the 3.9L V6 and the 4 speed 4T65E transmission is part #15791210.
It fit perfect, all connectors are correct and it has all the miscellaneous clips, retainers and such.
I installed the harness while replacing the lower intake manifold gaskets which were botched by the local GM dealership (who then wanted $1100 to fix it because the 10K mile warrantee was up).
I did not have to remove the exhaust manifolds and most routing was common sense, the location of the clips gives you some guidance.
I found one broken sensor, appears to have been broken when GM changed the first transmission in 2013 (by the amount of dirt in the break) it was a temperature sensor.
I'd say it took two days, one for the rear two harnesses and one for the front two. No special tools but a lot of working by feel to get the crank position sensor and knock sensors connected. And take the starter out, it gives you a great window to thread the front lower harness through.
After the repair the engine runs great, no check engine light and smooth as ever.

But the transmission is still screwing up when I leave a traffic light. Next is to replace the transmission harness that goes from the top of the transmission into the solenoids. Despite requesting all new wiring and solenoids I found the old connector and harness left in place. Another drop of the transmission...
I'll post another update when ready for anyone else who has this issue. As you can tell this is not my primary transportation.

2007 G6 GT Convertible
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post #7 of 7 (permalink) Old 08-08-2019, 10:49 AM
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congratulations on your solution. well done.
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