Another interesting Diag & Repair

Dec 19, 2013
Fort Worth, Texas
2014 Chevrolet Impala, 2.5L/6T45E, 183,xxx miles, Customer complaint......
*Trans falls out of gear
*PRNDL display toggles quickly between Park & Drive
*Service Stability/Traction Control System message
*Service Steering System message
*Door Locks toggle quickly
*Check Engine Lamp illuminated
The above symptoms appear randomly according to the owner.

Car has been to a couple different shops before the owner was referred to me by a transmission shop that I do programming for. Good news is.....The parts cannon wasn't fired off on this one.

*ECM DTC's.....U0101, Lost Communication with TCM
*TCM DTC's.....None
*BCM DTC's.....U0100, Lost Communication with ECM
*PSCM DTC's....U0101
*EBCM DTC's...U0101
*IPC DTC's.........U0101
Basically every module in the car except the BCM is losing comm with the TCM & The BCM is losing comm with the ECM.

First things first.......
*Clear DTC's & Scanned all modules, Everything good.
*Did find a loose Main Hot that feeds the Underhood Fuse Box, Had evidence of it being very hot as it melted the plastic cover & a loose Positive Battery Cable at the Battery during my initial inspection, Charged the Battery as it was a little low, Load tested battery & Checked charging system voltage.....All good.
*Man....This is going to be an easy one, Right......Found 2 bad connections in as many minutes ;)

Time for a test drive, It's after work & I run a few errands which is about 20 miles, Took my daughter to work which is another 10 miles. Decided to run out to Summit Racing about 34 miles from my daughter work. About half way back & about 70 miles into this test drive.....It goes nuts & it doesn't just fall out of gear, It violently slams into a lower gear (Manual 1st I believe) almost squeals the front tires it slows the car down so fast. It downright startled the crap out of me.....All the other symptoms the owner stated would happen when the trans acted up.....Of coarse I was on the frontage road of I20 by this point trying to limp this thing back to the shop at lower speeds, Had to get back on I20 to get past Loop 820 onto a US287 frontage road.
It would cycle in & out of the issue at random times, 2 minutes between, 5 minutes between, 1 minute, 10 minutes etc etc.....It's about 8pm when I pull up to the shop & I've had my fill for the day.

I did have my scanner with me & caught the TCM not communicating several times while it was acting up, It does/did communicate in-between it's little tantrums.

The next mourning, I found the same exact DTC's stored in memory, Cleared them again & had my Dad (Retired mechanic with 50+ years experience) drive it around, Told him to keep it close to home. He drives all over Northside, Riverside, Stockyards, & Downtown Ft. Worth for about 5 hours without a hiccup.

He brings it back & I rack the car......With it running, I wiggle test about every Ground, CAN Bus, Trans connector, Pulled the underhood fuse block & wiggle tested those connectors. Also had my Pico DLC breakout box inline with the Scanner & watched for any anomalies in the Data Bus.
Backprobed B+, Ignition Hot, & Ground at the TCM & watched the voltages with my Picoscope while wiggling the Harness, Fusebox connectors & the Fuses themselves.
Also kept an eye on TCM module voltage via Scan Data.

The TCM would communicate the whole time with no Communication DTC's.

At this point.....The vehicle has ran so long it was about out of fuel. So I shut the engine off & decided to check Amperage draw & signature on the B+ & Ignition Hot feed to the TCM at the 2 fuses in the UH Fusebox with the Key-On.
Everything was fairly normal for about 2 minutes....Then the Amp draw on both circuits fell off a little. "Okay"......Then I glanced over to the Scanner, No communication with the TCM.
*Started the vehicle back up, Communication resumed.
*Let it sit for more than a couple minutes with the Key On, Engine Off....Would lose comm with the TCM.
*If I kept a Battery Charger on the battery & maintained above 13.5vdc.....Communication would continue uninterrupted with the TCM.
*The TCM would go to sleep at voltages under 13.5vdc while hot & I could pretty much track the issue to Module Temperature from there. The next mourning....It would communicate all the way down to 10.3vdc. I ran it a little & the threshold rose to 12.7vdc.

A Blank Remanufactured AcDelco TCM for this runs near $700 my cost....Then another $40 to GM to program it via TDS. So I pulled an old trick that I learned on 6L80E's......
Use a TCM with the same OS within +/- 1 year, The VIN is not stored in the TCM!! While I wasn't 100% this would work on a 6T45E, It DID.
Bought a entire low mileage 2014 6T45E delivered for $275 & robbed the TCM out of it, Swapped the TCM & all is good.
Thread of the day award if not the week. Excellent work.

First year of that generation which was the last one since these are no longer being made. Last year was its final year of production. Good looking vehicle.
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That is some awesome troubleshooting, we get down to component level like that on Submarines, it is rare to it happen in the land of cars these days.
That 2014 Impala gets to live another day!

Do you figure it was heat or was it moisture or corrosion that got that factory TCM?

I'm surprised a used 6T45E is so cheap! That's good to know.
I will admit that I would have drove it about 10 miles after finding the loose connections and shipped it down the road. Good job in sticking with it.

It has always been multiple concern repairs that I've struggled with.
You rock, man. I rather flat-bed a vehicle over to you than take it to a dealership (parts replacers). Nice job. Hopefully I won't need you in the near future, but good to know you're around if I do.
See... I would have just replaced the TCM due to the symptoms. :alien:

And you would've been correct, And I agree to a point given the symptoms. I have to know WHY & prove it out to myself as I haven't ran across this particular issue. This isn't a pattern failure that I'm aware of??
That 2014 Impala gets to live another day!

Do you figure it was heat or was it moisture or corrosion that got that factory TCM?

I'm surprised a used 6T45E is so cheap! That's good to know.
The TCM is internal to the unit & no corrosion or moisture was present at the TCM connector. The Speed Sensors & Range Sensor unplugged from the TCM without the lock tabs breaking......They break VERY easily if the unit had been continuously overheated.

I'd be guessing at the actual reason for failure.

The 2.5L DI Ecotech is a POS, In fact this car has a wreaking yard engine in it. My "Yard" guy said they buy these for the Engines & Body parts.....Can't hardly give the transmissions away. The 6T45E doesn't fit 3.6L cars.