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2008 STS Platinum
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Discussion Starter #1
2008 STS Platinum 3.6L Direct Injected & Automatic Transmission
125,000 miles plus

DTC U0101 - Lost Communication With Transmission Control Module

On about 4/18/15, I was coming back to work during lunch time and when nearing a green light, seen it change to yellow and I stepped on the gas:tisk:
(I don’t normally do that - it really makes very little difference)

The car did downshift when I stepped on the gas.

All of a sudden, I lost motion power and then it seemed every system in my car was reporting as failed in the amber Driver Information Center. (Power Steering, Brakes, and StabilTrak)
The last message i seen was "STABILITY SYS READY" (This was one of the first systems to report as failed)
Then a Red light came up and I had to stop.

When trying to accelerate from the red light, it was as if the transmission and TCM were on a long distance phone call and the conversation was not going well.
It was trans slip city!!
I tried to avoid as much slipping as possible and managed to reach about 30 mph eventually. Work was only about 1 mile away.

Got back to work and placed the shifter into Park and shut the engine OFF. Then a bell went off and the Driver Information Center was telling me to place the car in park.
Cycled the shifter from park to drive a couple of times and still same message.

Disconnected the Battery at the positive terminal and left it that way.

( At this point I did not want to investigate the problem I just wanted to eat lunch and get back to work.)

Went back out about 3 hours later and reconnected the Battery.

Everything was fine in the STS world...until later on in the following days..

It seems that on cool mornings (low 70'sF) and when I drive without accelerating quickly/downshifting/manual shifting that there are fewer issues.

(However, on one of those mornings I seen the LCD screen restart yet the radio kept playing without interruption.)

It seems that the problem occurs most frequently after I accelerate, like from a stop light. Sometimes it takes a while to occur after the acceleration.

I actually have to pull off the side of the road and disconnect the battery to clear the problem.

I did check my DTC CODES and U0101 Lost Communication With Transmission Control Module does seem to be consistently coming up each time.

It seems that the automatic shift from 3rd to 4th gear is around the time the trouble occurs.

Also, I don't drive my 2008 STS like the typical driver. I am used to normally manually shifting the automatic transmission just avoid the usual bogginess.
I have noticed it changing it behavior during shifts - shifts later

Before I had any problems, I did change the transmission fluid and filter myself in November of 2014 when the car had 117,194 miles on it.
I cleaned the debris (the fluid was pretty dark with real fine particles) from the pan and all areas I could reach. I even used an old filter spout with a hose on it to
I'll admit that I think that this was the first time it was ever changed! The car had about 45k miles on it when I bought it,

I wonder if I should pursue this problem as an electrical connection issue like a connector?
or
as a mechanical transmission failure - like a solenoid acting up due to debris in the transmission?

I am wondering if the transmission is seeing a fault condition that it is not expecting and locking-up / freezing?
Connector issues also?
This might explain the failure to detect when the car is in park?

:rant2::rant2:

I you guys have any thoughts on this please let me know!
THANKS FOR ANY HELP OR EXPERIENCE THAT YOU CAN PASS ON!
:suspense::crybaby:


DTC U0101 - Lost Communication With Transmission Control Module
2008 STS Platinum 3.6L Direct Injected & Automatic Transmission
125,000 miles plus


MAX in Florida..
 
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I also have 2008 3.6L DI. It happened to me once. I disconnected the battery and touched the leads together. All was well and it never happened again.
 

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2007 STS-V
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193 Posts
Since resetting the system (and it is a system, think smart phone) I would suggest it is an electrical problem associated with the overall power bus which is temperature sensitive. Check the power and ground cables to the battery all the way to their destination point. Especially the main power cable from the battery all the way to the lug in the fuse box under the hood. This costs nothing and could yield results. Anything loose needs to be tight.
 

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2008 STS Platinum
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Discussion Starter #5
Hi Guys,

Thanks for your input!

I actually connected a pigtail to the cigarette lighter and monitored the 12VDC using an AC/DC Digital Data Logger. (The Data Logger voltage range was not ideally suited for the task but was good enough.)

Basically I discovered that the 12V was jumping up to 18VDC! See image. You will also notice that that the voltage went to 0V. This was me doing the Battery Disconnect so I could restart the car and get off the shoulder of the road!

I am making an assumption here - but I am thinking that the cheap (Transpo- Yes its Chinese) voltage regulator that was replaced in the alternator about a year ago gave up the ghost. I don't know why it is that I must learn this lesson over and over, but in my experience there is no better quality than OEM Alternators. In a car like this 2008 Cadillac, there is about $10k worth of electronics & computers that get power from the battery & alternator. DO NOT CHEAP OUT on alternators or Batteries!!
DataLoggerCapture_less.PNG
 
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