4T65E in 2006 Buick Lacrosse CX 3.8L, 2nd gear starts

M

Mr06Lacrosse

Joined
Nov 4, 2023
Messages
10
New member here. I found this website and forum in a search of issues with a 4T65E transmissions. Whatever help, guidance and referrals you can give me are truly appreciated.

A lot here but will try to be brief as possible.

I have a 2006 Buick Lacrosse V6 3.8L w/4T65E tranny, now at ~77k miles, bought Feb 2022 with ~61k miles. A diag before purchase threw a #1 solenoid but I bought the car anyway. Shortly thereafter, after driving for ~30 minutes, I experienced progressively longer (and clunky) delays downshifting from 2nd to 1st until the car only started from a stop in 2nd. Manually shifting to 1st had no impact as the car would still start up in 2nd. Had a similar issue on highways, stuck in 3rd and not advancing to D (overdrive).

Successive visits to a local reputable transmission repair shop included the following:
  • 1st visit (Sep 2022) – pulled the tranny, replaced solenoids, shift sensors, tranny filter, certain OEM tranny parts with upgraded parts, installed a TransGo shift kit and replaced with Dexron VI. Allowed for a reasonable relearning period after the repair but the problem continued.
  • 2nd visit (Nov 2022) – pulled the tranny again, replaced the solenoids and sensors (2nd time) and replaced a transmission manifold sensor. The problem continued.
  • 3rd visit (Apr 2023) – deducing problem to be electrical instead of mechanical, replaced the PCM (programmed by dealer). New PCM installed, their scanner threw codes pointing to the throttle body - never had this code previously. My 3.8 requires an electronic fuel injected throttle body with built-in TPS. Hard to find but located one in China.
    • Once installed for the next 500 miles, short and long trips, no issues, car drove beautifully. Then, the problem returned.
  • 4th visit (Sep 2023) – had transmission shop compare notes with a trusted mechanic. Mechanic suggested the possibility of a bad torque converter but the transmission shop ruled out. We hadn’t replaced the gas pedal yet (which has an accelerator sensor). Did that, it didn’t fix and we returned the pedal. Transmission shop had NEVER seen this before. They next replaced the valve body with a Sonnax (includes solenoids in the kit). Test drove after the installation, problem is still there.
  • 5th visit (last week) – transmission shop in reviewing their materials noted the PCM uses air flow readings to determine transmission shift points. If the mass air flow sensor doesn’t work properly, it could alter the transmission shift pattern. We replaced the mass air flow sensor, drove for over an hour with the scan tool attached and the problem seemed to go away.
Four days after replacing the mass air flow sensor, I drove my daughter to school this morning and the problem reappeared. It was 29 F this morning. My air filter hadn’t been changed in about 10k miles but wasn’t hugely dirty and I could see light through it. I wondered though if maybe THAT could be the problem. Picked up an STP high quality filter, installed and the problem appeared to go away. Even noticed better responsiveness upon accelerating. Drove to work, went to lunch, came home – no problem. Ran an errand after that and the problem came back. Manually shifted down to 1st and started from a stop at two traffic lights in 2nd.

So now, the problem that only occurred after the car had been driven for a period now shows up shortly after I pull out of the driveway and is intermittent.
I'm taking back to the transmission shop on Monday and genuinely don’t know what my next steps should be. Any advise, assistance or referrals you can provide would be again greatly appreciated.

Thanks so much and I hope to hear from you.
Matt
 
Mr06Lacrosse said:
My 3.8 requires an electronic fuel injected throttle body with built-in TPS. Hard to find but located one in China.
  • Once installed for the next 500 miles, short and long trips, no issues, car drove beautifully. Then, the problem returned.
Click to expand...
I'd look at that first off. Chinese parts are often junk. Check for codes again and look for glitches in live data while opening the throttle.
 
Thanks atikovi. I actually still have the old throttle body we took off originally. Worth a shot and I'll mention it on Monday.

It actually concerned me when we did a search for the throttle body. No domestic parts retailer carries them for a 3.8. You can find the 3.6L all day long. The only company I believe that keeps them in their production run is a Canadian manufacturer and of course, those runs are scheduled few and far between.
 
Thank you atikovi. When I was in the market earlier this year I only saw a few used on eBay, elected to get a new one even if it was Chinese. To see one for $140 w/free returns is excellent and will surely keep in mind.
 
Update for the group here.

I read several references to Dave, a tech at Triple Edge Performance and a primary source of knowledge specifically on the 4T65E. Great guy, a few emails back and forth, he thinks as we do the problem is outside the transmission and likely the wiring. If the PCM is commanding the correct gear, the transmission is doing its job. He's also seen several occasions where a wire within the main wire harness under the air filter housing is damaged, either from a poor crimp on a terminal, wire break inside the insulation, etc. It doesn't throw a code but gives the appearance the solenoid is at fault.

We did swap throttle bodies, put the old one back on after a thorough cleaning. After this, pulled the wire harness connector to the transaxle case, cleaned the connector on the case and the harness, applied some dilectic grease and reinserted. Did the same actually with the connector and cable leading to the throttle body. No long drives yet but in four days, one isolated instance with a small delayed downshift to 1st coinciding with a delayed upshift to 2nd. I haven't yet been on the freeway to test between 3rd and D.

The transmission shop has a connection to ATSG and they indicated they would reach out to discuss. Meanwhile, I broached with GM in an attempt to locate TSBs or history of customers with this same issue. A rep said I need an authorized GM servicer to perform a diagnostic before they could do anything. I explaining that with over $3,000 in transmission repairs, even aftermarket parts purchases would have provided royalty to GM several times over the cost of a diagnostic. She then recommended I go to my local authorized servicer and see if they would help. I proposed if I had the diagnostic done, what would they do? She said they could give concessions but wouldn't go into specifics. A 17-year old car, no remaining warranty on ANY part of the vehicle, what are you gonna do - a 2% discount on a brand new GM vehicle
854d1243097187-12v_clap.gif
.

I did visit my local GM dealer, talked to a parts manager who accessed some TSBs which I took back to my transmission shop for their thoughts. Highly unlikely, but if the PCM wasn't programmed to the latest firmware update, it could cause an issue. Next steps with the transmission shop are to target new harnesses. An AC Delco replacement transmission wire harness is $60-$65. Some engineers with whom I work suggested to also replace wire looms, apply some heat shrink or heat tape over exposed wiring. I'll report back once we get this done and have had a chance to test drive.

Thanks again to everyone for the assistance and support.
 
About the only thing mechanical that can cause intermittent wrong/2nd gear starts is low Actuator Feed Limit (AFL) pressure. A Sonnax VB surely would've taken care of AFL concerns

You stated that it was setting a Shift Solenoid A/1-2 DTC.....Is it still setting this DTC?

I would back probe the main trans connecter at Pin A, Light Green wire with a Power Probe so I could see it switch High & Low.

Low/Ground = Solenoid On.
High/12vdc = Solenoid Off.

Then when it does act up, Assuming the circuit stays High......Ground the circuit with the PP....It should downshift to 1st, This will confirm the issue is outside the transmission.

The main input for the unit to be commanded to 1st gear is the VSS, If vehicle speed is Zero.....The TCM should command 1st gear no matter other inputs!
**Some models do command 2nd gear starts with the shifter in Manual 2nd**

F0MoVNn.jpg
 
Clinebarger,

Thanks for the detailed reply back on this. The last time the scan tool was connected up and we had the delayed downshift, it didn't throw ANY codes.

No trouble whatsoever for a few days, then yesterday, started the car, and got an air bag light. Backed out of a parking space, put in 3rd (I always drive in 3rd unless on highway), car started in 1st, advanced to 2nd. First stop I made, the car had a slight delay from 2nd to 1st. This is NOT the first time I've seen this, but to your point, and after reading some similar issues online wondering if I have a grounding issue. On a Jeep, as I understand the air bag system is connected to the PCM and other electronic control units. If I have a short, I'm thinking it could be indicative of what I'm seeing and quite possibly damage the PCM if nothing is done.

I'm going to print this off and take back to the transmission shop. Thanks so much again for taking the time to respond to me here.
 
Mr06Lacrosse said:
Clinebarger,

Thanks for the detailed reply back on this. The last time the scan tool was connected up and we had the delayed downshift, it didn't throw ANY codes.

No trouble whatsoever for a few days, then yesterday, started the car, and got an air bag light. Backed out of a parking space, put in 3rd (I always drive in 3rd unless on highway), car started in 1st, advanced to 2nd. First stop I made, the car had a slight delay from 2nd to 1st. This is NOT the first time I've seen this, but to your point, and after reading some similar issues online wondering if I have a grounding issue. On a Jeep, as I understand the air bag system is connected to the PCM and other electronic control units. If I have a short, I'm thinking it could be indicative of what I'm seeing and quite possibly damage the PCM if nothing is done.

I'm going to print this off and take back to the transmission shop. Thanks so much again for taking the time to respond to me here.
Click to expand...
clinebarger is absolutely the best transmission go-to guy here and arguably, anywhere on the various message boards. Not to mention a fantastic diagnostician in general.
 
The_Eric said:
clinebarger is absolutely the best transmission go-to guy here and arguably, anywhere on the various message boards. Not to mention a fantastic diagnostician in general.
Click to expand...

No doubt. I took a pic of his response, texted it to my transmission guy, went down there today to discuss and they corroborated what he said. The air bag sensor as I understand isn't tied in electrically to the PCM on my Lacrosse as it is on a Jeep. But we did follow the wiring coming off the front air bag sensors and noted a section common with wiring leading from the transaxle connector and branching off to the air filter housing where the PCM is.

I need to buy some heat resistant wire loom but the next step is to go through the connections to the ground and look a little closer for corrosion.
 
So, first off, I hope everyone had an enjoyable holiday season and Happy New Year to all. I still don't appear to have all the right solutions but seem to be getting closer.

The quick updates are we worked through the electrical and all connections to the transaxle, grounds, PCM are intact. Thanks yet again to @clinebarger for the helpful tips. We did have an interesting episode with the electrical, however, where the parking lights would stay on after the car was turned off. We went down a few rabbit holes but eventually found the battery had an internal short. Interestingly, for a short period the downshifting issue disappeared but eventually came back and has continued intermittently.

So, to try and capture the issue myself, I bought a Launch OBD2 Diagnostic scanner. Thus far I've only had it about a week, ran daily diagnostic reports and captured a few data streams on short drives.

The one DTC that comes up every time is P0172 Fuel Trim System Rich. I'm not seeing any other related codes like P0175, or P0442/P0455 which as I understand are evaporation leaks. Over the last few days, I've made 3 captures of the live data and attempted to draw some correlations. Reviewing the Long Term Fuel Trim - Bank 1, at an idle the value is between -10 and -12. In motion, it ranges usually from -10 and as high as -21. So, I'm pretty confident if nothing else I'm definitely running rich. I need some help to troubleshoot and need to correct, but now wondering if running rich can also mess with the readings from the MAF sensor or vice-versa and contributing to my intermittent problem. Recall, I did replace the MAF sensor a few months ago, drove for a few weeks only to have the same problem recur.

The other codes that regularly show up are B3106 Keyless Entry Data Link Parity Error, B0092 Passenger Presence System Passenger Detection Error and a B1004 Radio Error for Keep Alive Memory Reset - none of which I'm sure are contributing to my transmission issue.

Thoughts and responses appreciated as always.
 
None of those will cause a intermittent 2nd gear start. 3800 Buicks never had dual bank O2 feedback, One upstream O2 sensor feeds back all 6 cylinders meaning P0175 isn't applicable.

You stated the PCM was replaced? This car doesn't have a PCM.....It has a ECM & a TCM, Which is standard for most all CAN bus GM vehicles.
 
Thanks CB for the prompt reply, again.

I have every confidence you're correct but my scanner tool doesn't seem to differentiate on a diagnosis and refers to PCM. Interestingly enough, my scans have no longer thrown a DTC P0172. Once the scanner links it immediately reads the VIN correctly, lists out the year, make, model correctly then goes through a few additional questions. First one is whether it has an air pump; I've always responded yes or "with air pump" and received the code. The first time I answered "without air pump" the P0172 code went away. I then disconnected the tool, reconnected, answered "with air pump" again and the P0172 didn't reappear. I did nothing in the interim to manually clear the codes. So, at this point, I'm uncertain what is going on and not sure whether I should feel good or not about not getting a code.

Anyway, so now that I've invested in the scan tool, what live data should I stream so when the 2nd gear shifts intermittently start again I will have captured the right data for an appropriate diagnosis? I can run all sorts of data on the automatic transaxle (AT), engine controls, etc. Inside the AT, it will read 1-2 shift time, 1-2 solenoid, 2-3, 3-4, transmission fluid temperature, input/output shaft speed, etc.

Thank you.
 
Thanks yet again, CB, and appreciative for the response.

On the circuit testing, I can only rely on what my transmission guy indicated he did. They said they took the external transaxle wiring harness and tested all the connections, wiggle and contorting the wire and indicated no issues. No testing performed on the internal if I recall correctly.

So, I did have an episode with the delayed dropdown from 2nd to 1st. It happened following lunch yesterday and driving from the house back to the office, only 5 minute drive normally. I didn't have the scanner hooked up initially but pulled over with the car running, hooked up the scanner and the diagnostic report through a BCM U1300 code which I'd never seen before. From what I researched the check engine light usually comes on with this code, which it did not. I also understand this code gets thrown in conjunction to other codes. In this case, the other codes present were ones I've seen before: B3106 (Keyless Entry Data Link Parity Error), B0092 (Passenger Presence System 1 Performance) and B1004 (Keep Alive Memory Reset, the radio). I then started recording all the transaxle data I could and drove for about 15 minutes. The issue persisted but strangely improved and eventually went away.

Reviewing the data, I isolated to the 1-2 and 2-3 solenoids, but also the commanded gear and the speeds. There are other roughly 30 other metrics for transmission temps and shaft speeds, TCC, pressure control, TP angle, and vehicle speed. To the question raised, in all cases here the vehicle speed dropped to zero.

Lastly, I can't find a way to export the data from this diagnostic record and thus will likely return the device. That plus since I bought it on Amazon, Launch USA won't support it - only China. Ridiculous.
 
So it commanded 1st gear every time you stopped?

A Scope is the correct tool here to eliminate/condemn the Control side of the equation, A Power Probe would be 2nd best as I outlined in a previous post.

GM T42 trans controllers aren't know to cause issues, I've replaced very few.....It's also used on all CAN 4L60E/4L80E/4T80E applications, But that doesn't mean they can't have issues.

Class 2 network DTC's don't alarm me on a CAN vehicle with the symptoms given.
 
I'm not a professional automotive technician, just a guy who fixes my own vehicles and a little bit for friends & family. But, I am an electronics technician with about 25 years experience. With electronic devices, whenever I see something with a bunch of seemingly unrelated symptoms (like your variety of codes), I like to verify a good, clean power supply. I know you said that the transaxle harness was inspected, but has anyone checked and even cleaned the various underhood grounds? Other wiring harnesses and connections? Maybe even testing for AC ripple?
 
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