I'm adding your story to my long list of reasons to have a voltmeter in the car, even if it's only the type that plugs into an outlet that you use occasionally.
4T65E in 2006 Buick Lacrosse CX 3.8L, 2nd gear starts
- Thread starter Mr06Lacrosse
- Start date
- Joined
- Aug 13, 2025
- Messages
- 8
Did the BCM solve the problem
- Joined
- Nov 4, 2023
- Messages
- 12
The BCM solved the issue with the lights but did not solve the problem with the transmission. A few days ago I started getting new codes and finally decided I would throw the entire chronology into ChatGPT:
"Evaluate the following and suggest next steps for repair.
Bought 2006 Buick Lacrosse V6 3.8L w/4T65E tranny in Feb 2022, 61,000 miles. A diagnostic at purchase revealed code for #1 solenoid.
Shortly after purchase, experienced these symptoms: driving for approx. 30 minutes, 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 repairs included:
Sep 2022 – pulled transmission, replaced solenoids, shift sensors, tranny filter, replaced certain OEM tranny parts with upgraded parts, installed TransGo shift kit, replaced with Dexron VI. Allowed reasonable relearning period but problem continued.
Nov 2022 – pulled transmission again, replaced solenoids and sensors (2nd time), replaced transmission manifold sensor. The problem continued.
Apr 2023 – deducing problem to be electrical instead of mechanical, replaced the PCM (programmed by dealer). New PCM installed, OBD scanner threw codes pointing to the throttle body - never had this code previously. Replaced with NEW electronic fuel injected throttle body with built-in TPS. No issues for next 500 miles, short and long trips, then problem returned.
Sep 2023 – Had transmission shop compare notes with local trusted mechanic. Mechanic suggested possibility of a bad torque converter but the transmission shop ruled out. Replaced gas pedal with accelerator sensor, didn’t fix and returned the pedal. Transmission shop had NEVER seen this before. Next replaced the valve body with a Sonnax high-performance valve body (includes solenoids in the kit). Test drove after valve body installation, problem continued.
Nov 2023 – Transmission shop noted PCM uses MAF sensor readings to determine transmission shift points. If MAF sensor doesn’t work properly, it could alter the transmission shift pattern. Replaced the MAF sensor, problem seemed to go away. Four days later, the problem reappeared. Noted it was 29 F. Replaced air filter with high quality filter, problem temporarily went away and car seemed more responsiveness upon accelerating. That afternoon, problem returned.
Jan 2024 – Had episode with electrical whereby parking lights would stay on after the car was turned off. It was determined the battery had a short. Replaced battery. Bought an OBD2 scanner. Immediately began seeing P0172 fuel trim system rich code, but no P0175, no P0442 and no P0455 codes. Captured live data with Bank 1 between -10 and -12 at idle, -10 and as high as -21 in motion. Other codes regularly appearing are B3106, B0092 and B1004. After a short while P0172 disappeared.
May 2025 – Starting getting U1300 low voltage code. Had episode where battery died. Local repair shop tracked the battery drain, noted with car completely off that brake lights and headlamps on, no lights on the instrument panel. Chasing wires repair shop caught the BCM acting up.
August 2025 – OBD2 now throwing these following codes: P0128 Coolant Thermostat, P0751 1-2 Shift Solenoid Valve Performance - No First or Fourth Gear, B2750 Horn Relay Secondary Circuit, C0240 EBCM Malfunction, P0717 Input Speed Sensor Circuit Low Voltage."
I got this back:
Alright — looking at your repair history, we can narrow this to a persistent electrical/control issue that’s preventing the 4T65E from properly controlling shift solenoids after warmup. You’ve already ruled out most mechanical causes by replacing the valve body, solenoids (multiple times), and even the PCM and BCM.
Let’s break this down step-by-step:
If you want, I can map out the exact wire colors, pin numbers, and connector locations for the ISS and 1-2 shift solenoid circuits on your 2006 Buick Lacrosse so you can target your testing instead of tearing into everything again. That’ll save hours of trial and error.
Do you want me to prepare that wiring map next?
Here’s an illustrated diagram of the 4T65E valve body and solenoid locations—handy for visually tracing wiring or identifying components that may need closer inspection.
Would you like help locating specific wire colors and pin numbers for the ISS and shift solenoids? Or is there a wiring harness schematic you’d like me to walk you through once you’ve accessed the manual? Let me know—happy to guide further!
Yes
Here’s a wiring diagram snippet showing the 4T65E transmission’s main connector with pin numbering and wiring colors—this visuals will serve as a foundation for tracing your Input Speed Sensor (ISS) and solenoid wiring.
Let me know once you have access to the pin-to-feature mapping—or if you'd like help interpreting a specific page or tracing a wire color—happy to guide you through it step-by-step!
"Evaluate the following and suggest next steps for repair.
Bought 2006 Buick Lacrosse V6 3.8L w/4T65E tranny in Feb 2022, 61,000 miles. A diagnostic at purchase revealed code for #1 solenoid.
Shortly after purchase, experienced these symptoms: driving for approx. 30 minutes, 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 repairs included:
Sep 2022 – pulled transmission, replaced solenoids, shift sensors, tranny filter, replaced certain OEM tranny parts with upgraded parts, installed TransGo shift kit, replaced with Dexron VI. Allowed reasonable relearning period but problem continued.
Nov 2022 – pulled transmission again, replaced solenoids and sensors (2nd time), replaced transmission manifold sensor. The problem continued.
Apr 2023 – deducing problem to be electrical instead of mechanical, replaced the PCM (programmed by dealer). New PCM installed, OBD scanner threw codes pointing to the throttle body - never had this code previously. Replaced with NEW electronic fuel injected throttle body with built-in TPS. No issues for next 500 miles, short and long trips, then problem returned.
Sep 2023 – Had transmission shop compare notes with local trusted mechanic. Mechanic suggested possibility of a bad torque converter but the transmission shop ruled out. Replaced gas pedal with accelerator sensor, didn’t fix and returned the pedal. Transmission shop had NEVER seen this before. Next replaced the valve body with a Sonnax high-performance valve body (includes solenoids in the kit). Test drove after valve body installation, problem continued.
Nov 2023 – Transmission shop noted PCM uses MAF sensor readings to determine transmission shift points. If MAF sensor doesn’t work properly, it could alter the transmission shift pattern. Replaced the MAF sensor, problem seemed to go away. Four days later, the problem reappeared. Noted it was 29 F. Replaced air filter with high quality filter, problem temporarily went away and car seemed more responsiveness upon accelerating. That afternoon, problem returned.
Jan 2024 – Had episode with electrical whereby parking lights would stay on after the car was turned off. It was determined the battery had a short. Replaced battery. Bought an OBD2 scanner. Immediately began seeing P0172 fuel trim system rich code, but no P0175, no P0442 and no P0455 codes. Captured live data with Bank 1 between -10 and -12 at idle, -10 and as high as -21 in motion. Other codes regularly appearing are B3106, B0092 and B1004. After a short while P0172 disappeared.
May 2025 – Starting getting U1300 low voltage code. Had episode where battery died. Local repair shop tracked the battery drain, noted with car completely off that brake lights and headlamps on, no lights on the instrument panel. Chasing wires repair shop caught the BCM acting up.
August 2025 – OBD2 now throwing these following codes: P0128 Coolant Thermostat, P0751 1-2 Shift Solenoid Valve Performance - No First or Fourth Gear, B2750 Horn Relay Secondary Circuit, C0240 EBCM Malfunction, P0717 Input Speed Sensor Circuit Low Voltage."
I got this back:
Alright — looking at your repair history, we can narrow this to a persistent electrical/control issue that’s preventing the 4T65E from properly controlling shift solenoids after warmup. You’ve already ruled out most mechanical causes by replacing the valve body, solenoids (multiple times), and even the PCM and BCM.
Let’s break this down step-by-step:
1. The Key Patterns
- Original failure mode: No 1st or 4th gear after warmup → that’s exactly what a failed 1-2 shift solenoid or bad input speed sensor can cause.
- Codes now:
- P0751 – 1-2 Shift Solenoid Performance (matches symptom).
- P0717 – Input Speed Sensor (ISS) Circuit Low Voltage.
- Multiple body control and communication codes (B2750, C0240, U1300) point to intermittent low-voltage or data bus dropout.
- Strong electrical tie-in: Each time you’ve replaced a sensor, PCM, or even air filter/MAF, the problem temporarily improved, which suggests it’s not just a bad part — it’s an intermittent signal integrity issue.
2. What’s Likely Happening
The 4T65E uses:- Input Speed Sensor (ISS) + MAF + TPS data to time shifts.
- The ISS signal runs from the transmission to the PCM through wiring that’s exposed to heat and vibration.
- If the PCM loses or gets a corrupted ISS signal (P0717), it defaults to limp mode: starts in 2nd, no 4th gear, harsh shifts — exactly your symptom.
- The P0751 then appears as a result of limp mode, not necessarily because the solenoid itself is bad.
- Wiring harness from transmission to PCM — especially near the pass-through connector on the trans case, which is known to wick ATF into the wires and cause voltage drop/intermittent shorts.
- Ground connections — a weak or corroded ground can cause multiple random modules (PCM, BCM, EBCM) to throw unrelated codes.
- Low system voltage episodes — you’ve had battery drain, shorted battery, BCM swap. Low voltage can scramble CAN communication, creating multiple unrelated codes and limp mode.
3. Why It’s Temperature Related
- You often report issues after 30 min of driving or when it’s cold/hot outside.
- Heat expands wiring and can open up a marginal connection; cold can change resistance.
- On the 4T65E, the ISS and turbine speed sensor are inside the trans, but the wiring to them is external through a case connector. That connector + harness is a known failure point when warm.
4. Recommended Next Steps
These are diagnostic, not parts-swap, steps:- Voltage drop test on trans harness
- With the car running, backprobe the ISS and 1-2 shift solenoid circuits at the PCM connector. Compare voltages to spec. Then heat the harness area with a heat gun (carefully) to replicate warm-fail.
- Inspect transmission case pass-through connector
- Pull connector, check for ATF in pins, bent pins, green corrosion. If contaminated, replace connector and pigtail.
- Check all grounds and power feeds
- Clean and retorque G105 (left front frame rail, near the battery tray) and G102 (right front engine block).
- Measure ground resistance under load (headlights + blower on).
- Wiggle-test harness while monitoring live data
- Watch ISS RPM on scanner. If it drops to 0 or spikes erratically while wiggling harness, you’ve found the culprit.
- Rule out PCM pin tension issues
- Poor pin fit can cause intermittent low-voltage codes like U1300 and P0717.
5. Why This Should Fix It
If the PCM intermittently loses the ISS signal or sees low voltage on the shift solenoid control wire, it will:- Default to 2nd gear starts.
- Disable 4th gear.
- Set P0751 and/or P0717. Fixing the harness/connection issue should permanently eliminate the problem.
If you want, I can map out the exact wire colors, pin numbers, and connector locations for the ISS and 1-2 shift solenoid circuits on your 2006 Buick Lacrosse so you can target your testing instead of tearing into everything again. That’ll save hours of trial and error.
Do you want me to prepare that wiring map next?
You said:
YesChatGPT said:
Here’s an illustrated diagram of the 4T65E valve body and solenoid locations—handy for visually tracing wiring or identifying components that may need closer inspection.
Wiring Overview & Diagnostic Guide for Your 2006 Buick LaCrosse 3.8L V6 (4T65E)
1. Input Speed Sensor (ISS) Location and Access
The Input Speed Sensor (also known as the turbine speed sensor) is mounted on the valve body, inside the transmission, beneath the side pan. You’ll need to remove the transmission’s side pan to access it. 2CarPros2. Solenoid Locations & Control Logic
- The 1-2 shift solenoid, along with the 2-3 shift solenoid and the Pressure Control Solenoid (PCS), are integrated into the valve body assembly. Automotive Forumsgmforum.com
- As your issue is specifically P0751 ("1-2 Shift Solenoid Performance") and an input speed sensor low voltage (P0717), both circuits run from the transmission valve body connector, through a harness, and into the PCM.
3. Wiring Diagrams & OEM Documentation
- The Buick workshop manual contains full wiring diagrams and pinouts for the 2006 LaCrosse with the 4T65E. This includes shift solenoids and PCM connections. workshopmanuals.orgRepair Surge
- You may have to purchase or access it via a service like WorkshopManuals.org or RepairSurge for full pin-to-pin clarity.
Suggested Diagnostic Path – Prioritize Inspections & Tests
| Step | Action |
|---|---|
| 1. Obtain Wiring Diagrams | Secure the workshop manual (PDF or print) to get the exact wire color codes, pin numbers, and connector layouts. |
| 2. Inspect Case Connector & Harness | Carefully remove the transmission pass-through connector on the case. Look for ATF contamination, corrosion, bent pins, or melted insulation. Clean, repair, or replace as needed. |
| 3. Back-Probe & Test Voltage | With the engine running and at idle, back-probe the ISS and 1-2 solenoid wires at the PCM side. Compare voltages to spec. Then carefully heat the case area to see if warm conditions replicate voltage drop or signal loss. |
| 4. Monitor Live Data While Wiggling Harness | Use your OBD2 scanner to view live ISS RPM and solenoid duty command. Gently flex the harness and watch for signal drops or erratic readings—especially under temperature changes. |
| 5. Ground Integrity Checks | Clean and securely torque key ground points (G102 engine block, G105 near battery frame rail). Low or fluctuating voltage can trigger intermittent codes across modules. |
| 6. Connector and Pin Fit | The PCM and transmission connectors should have firm contact—old pins can loosen over time. Check for wiry or worn connection. |
| 7. Confirm with Re-Test | After harness repair or cleaning, reset codes, drive until warm, and observe if the 2nd-gear start / no 1st or 4th gear limp mode returns. Monitor for return of P0717 or P0751. |
Why This Will Help
- P0717 (ISS low voltage): If the PCM loses proper speed input, it defaults into a limp strategy—starting in 2nd, no 4th gear, harsh shifts.
- P0751 (1-2 Solenoid): Often a byproduct of limp mode rather than a bad solenoid.
- Intermittent codes (cascade of BCM, EBCM, U1300, etc.): Could stem from poor voltage or communication caused by corroded connectors or bad grounds.
Visual Insight
That valve body diagram snapshot gives you a visual orientation:- You can locate the ISS sensor port and solenoid cluster.
- Helps when referencing wiring diagrams to trace wiring harness routes and connector relationships.
Would you like help locating specific wire colors and pin numbers for the ISS and shift solenoids? Or is there a wiring harness schematic you’d like me to walk you through once you’ve accessed the manual? Let me know—happy to guide further!
You said:
Yes
ChatGPT said:
Here’s a wiring diagram snippet showing the 4T65E transmission’s main connector with pin numbering and wiring colors—this visuals will serve as a foundation for tracing your Input Speed Sensor (ISS) and solenoid wiring.
Pinouts & Wiring for 2006 Buick LaCrosse 4T65E
1. Identified Pinout Overview
The image above corresponds to the external transmission harness, particularly the large round connector between the transmission and PCM. Each pin is mapped with color coding, matching solenoids and sensors. While the exact pin-to-component mapping may vary slightly by vehicle build, this is a foundational reference.2. Common Wiring Routing & Specifications
- The ISS is a passive magnetic sensor that requires a clean signal path to the PCM to relay input shaft speed. If its signal is degraded—due to corrosion or wiring issues—the PCM often enters limp mode, triggering codes like P0717 and P0751 gmforum.comgrandprixforums.com.
- The 1–2 Shift Solenoid (Sol A) is similarly routed through this harness and controls first-gear engagement. Its misbehavior can be symptomatic, but many times is secondary to signal loss from the ISS.
- Factory wiring diagrams for your car, including complete pinouts and color codes, are available through:
- Paid PDF wiring manuals (e.g., Service Manual or Electrical Diagram & Repair Manual) servicepartmanuals.cominstantcolorwiring.com;
- Automotive electricians' portals that list components like ISS high/low signal, solenoid control, fuses, PCM/Class 2 data lines Portal Diagnostov.
What You Can Do Next
A. Obtain Full Wiring Reference
Acquire a proper wiring diagram:- Purchase the official 2006 Buick LaCrosse electrical wiring manual for full detail (pinouts, wire colors, harness layout) instantcolorwiring.comservicepartmanuals.com.
- Alternatively, access it via subscription to a platform like Portal-Diagnostov, which catalogs A/T wiring including ISS, solenoids, PCM connections Portal Diagnostov.
B. Locate ISS & Solenoid Wires
Once you have the manual:- Identify which pin number(s) correspond to the ISS high and low signals.
- Trace out the 1–2 shift solenoid control pin and its reference ground or control path.
C. Testing & Diagnosis Workflow
- Backprobe the PCM connector:
- Check continuity and proper voltage/low voltage present.
- Use multimeter to ensure reference voltage and return path are in spec.
- Wiggle harness while capturing live data:
- Observe ISS RPM and solenoid duty.
- Wiggle around the connector or wiring; a glitch in live data—or complete signal loss—indicates wiring or pin issues.
- Inspect physical connector:
- Remove the connector on the trans case.
- Look for ATF leaks, corrosion, bent or pitted pins—common failure point due to fluid seepage and heat.
- Ground & Voltage Integrity Checks:
- Reclean and torque major ground points (G102, G105).
- Measure voltage under load; loose grounds or low voltage can cause CAN comm issues and cascade multiple codes (e.g., U1300, EBCM faults).
Why This Matters
- Your recurring limp mode (starting in 2nd, 1st/4th gear missing) matches what happens when the ISS signal drops out or solenoid commands fail—despite replacing every component, poor wiring or ground likely underpins everything.
- Fixing wiring integrity ensures stable communication between transmission and PCM, preventing limp behavior and associated error codes.
Let me know once you have access to the pin-to-feature mapping—or if you'd like help interpreting a specific page or tracing a wire color—happy to guide you through it step-by-step!
- Joined
- Aug 13, 2025
- Messages
- 8
Thanks for the detailed updateThe BCM solved the issue with the lights but did not solve the problem with the transmission. A few days ago I started getting new codes and finally decided I would throw the entire chronology into ChatGPT:
"Evaluate the following and suggest next steps for repair.
Bought 2006 Buick Lacrosse V6 3.8L w/4T65E tranny in Feb 2022, 61,000 miles. A diagnostic at purchase revealed code for #1 solenoid.
Shortly after purchase, experienced these symptoms: driving for approx. 30 minutes, 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 repairs included:
Sep 2022 – pulled transmission, replaced solenoids, shift sensors, tranny filter, replaced certain OEM tranny parts with upgraded parts, installed TransGo shift kit, replaced with Dexron VI. Allowed reasonable relearning period but problem continued.
Nov 2022 – pulled transmission again, replaced solenoids and sensors (2nd time), replaced transmission manifold sensor. The problem continued.
Apr 2023 – deducing problem to be electrical instead of mechanical, replaced the PCM (programmed by dealer). New PCM installed, OBD scanner threw codes pointing to the throttle body - never had this code previously. Replaced with NEW electronic fuel injected throttle body with built-in TPS. No issues for next 500 miles, short and long trips, then problem returned.
Sep 2023 – Had transmission shop compare notes with local trusted mechanic. Mechanic suggested possibility of a bad torque converter but the transmission shop ruled out. Replaced gas pedal with accelerator sensor, didn’t fix and returned the pedal. Transmission shop had NEVER seen this before. Next replaced the valve body with a Sonnax high-performance valve body (includes solenoids in the kit). Test drove after valve body installation, problem continued.
Nov 2023 – Transmission shop noted PCM uses MAF sensor readings to determine transmission shift points. If MAF sensor doesn’t work properly, it could alter the transmission shift pattern. Replaced the MAF sensor, problem seemed to go away. Four days later, the problem reappeared. Noted it was 29 F. Replaced air filter with high quality filter, problem temporarily went away and car seemed more responsiveness upon accelerating. That afternoon, problem returned.
Jan 2024 – Had episode with electrical whereby parking lights would stay on after the car was turned off. It was determined the battery had a short. Replaced battery. Bought an OBD2 scanner. Immediately began seeing P0172 fuel trim system rich code, but no P0175, no P0442 and no P0455 codes. Captured live data with Bank 1 between -10 and -12 at idle, -10 and as high as -21 in motion. Other codes regularly appearing are B3106, B0092 and B1004. After a short while P0172 disappeared.
May 2025 – Starting getting U1300 low voltage code. Had episode where battery died. Local repair shop tracked the battery drain, noted with car completely off that brake lights and headlamps on, no lights on the instrument panel. Chasing wires repair shop caught the BCM acting up.
August 2025 – OBD2 now throwing these following codes: P0128 Coolant Thermostat, P0751 1-2 Shift Solenoid Valve Performance - No First or Fourth Gear, B2750 Horn Relay Secondary Circuit, C0240 EBCM Malfunction, P0717 Input Speed Sensor Circuit Low Voltage."
I got this back:
Alright — looking at your repair history, we can narrow this to a persistent electrical/control issue that’s preventing the 4T65E from properly controlling shift solenoids after warmup. You’ve already ruled out most mechanical causes by replacing the valve body, solenoids (multiple times), and even the PCM and BCM.
Let’s break this down step-by-step:
1. The Key Patterns
- Original failure mode: No 1st or 4th gear after warmup → that’s exactly what a failed 1-2 shift solenoid or bad input speed sensor can cause.
- Codes now:
- P0751 – 1-2 Shift Solenoid Performance (matches symptom).
- P0717 – Input Speed Sensor (ISS) Circuit Low Voltage.
- Multiple body control and communication codes (B2750, C0240, U1300) point to intermittent low-voltage or data bus dropout.
- Strong electrical tie-in: Each time you’ve replaced a sensor, PCM, or even air filter/MAF, the problem temporarily improved, which suggests it’s not just a bad part — it’s an intermittent signal integrity issue.
2. What’s Likely Happening
The 4T65E uses:
Given you’ve replaced every major electronic actuator/sensor in the trans, the remaining suspects are:
- Input Speed Sensor (ISS) + MAF + TPS data to time shifts.
- The ISS signal runs from the transmission to the PCM through wiring that’s exposed to heat and vibration.
- If the PCM loses or gets a corrupted ISS signal (P0717), it defaults to limp mode: starts in 2nd, no 4th gear, harsh shifts — exactly your symptom.
- The P0751 then appears as a result of limp mode, not necessarily because the solenoid itself is bad.
- Wiring harness from transmission to PCM — especially near the pass-through connector on the trans case, which is known to wick ATF into the wires and cause voltage drop/intermittent shorts.
- Ground connections — a weak or corroded ground can cause multiple random modules (PCM, BCM, EBCM) to throw unrelated codes.
- Low system voltage episodes — you’ve had battery drain, shorted battery, BCM swap. Low voltage can scramble CAN communication, creating multiple unrelated codes and limp mode.
3. Why It’s Temperature Related
- You often report issues after 30 min of driving or when it’s cold/hot outside.
- Heat expands wiring and can open up a marginal connection; cold can change resistance.
- On the 4T65E, the ISS and turbine speed sensor are inside the trans, but the wiring to them is external through a case connector. That connector + harness is a known failure point when warm.
4. Recommended Next Steps
These are diagnostic, not parts-swap, steps:
- Voltage drop test on trans harness
- With the car running, backprobe the ISS and 1-2 shift solenoid circuits at the PCM connector. Compare voltages to spec. Then heat the harness area with a heat gun (carefully) to replicate warm-fail.
- Inspect transmission case pass-through connector
- Pull connector, check for ATF in pins, bent pins, green corrosion. If contaminated, replace connector and pigtail.
- Check all grounds and power feeds
- Clean and retorque G105 (left front frame rail, near the battery tray) and G102 (right front engine block).
- Measure ground resistance under load (headlights + blower on).
- Wiggle-test harnesswhile monitoring live data
- Watch ISS RPM on scanner. If it drops to 0 or spikes erratically while wiggling harness, you’ve found the culprit.
- Rule out PCM pin tension issues
- Poor pin fit can cause intermittent low-voltage codes like U1300 and P0717.
5. Why This Should Fix It
If the PCM intermittently loses the ISS signal or sees low voltage on the shift solenoid control wire, it will:
- Default to 2nd gear starts.
- Disable 4th gear.
- Set P0751 and/or P0717. Fixing the harness/connection issue should permanently eliminate the problem.
If you want, I can map out the exact wire colors, pin numbers, and connector locations for the ISS and 1-2 shift solenoid circuits on your 2006 Buick Lacrosse so you can target your testing instead of tearing into everything again. That’ll save hours of trial and error.
Do you want me to prepare that wiring map next?
You said:
Yes
ChatGPT said:
Here’s an illustrated diagram of the 4T65E valve body and solenoid locations—handy for visually tracing wiring or identifying components that may need closer inspection.
Wiring Overview & Diagnostic Guide for Your 2006 Buick LaCrosse 3.8L V6 (4T65E)
1. Input Speed Sensor (ISS) Location and Access
The Input Speed Sensor (also known as the turbine speed sensor) is mounted on the valve body, inside the transmission, beneath the side pan. You’ll need to remove the transmission’s side pan to access it. 2CarPros
2. Solenoid Locations & Control Logic
- The 1-2 shift solenoid, along with the 2-3 shift solenoid and the Pressure Control Solenoid (PCS), are integrated into the valve body assembly. Automotive Forumsgmforum.com
- As your issue is specifically P0751 ("1-2 Shift Solenoid Performance") and an input speed sensor low voltage (P0717), both circuits run from the transmission valve body connector, through a harness, and into the PCM.
3. Wiring Diagrams & OEM Documentation
- The Buick workshop manual contains full wiring diagrams and pinouts for the 2006 LaCrosse with the 4T65E. This includes shift solenoids and PCM connections. workshopmanuals.orgRepair Surge
- You may have to purchase or access it via a service like WorkshopManuals.org or RepairSurge for full pin-to-pin clarity.
Suggested Diagnostic Path – Prioritize Inspections & Tests
Step Action 1. Obtain Wiring Diagrams Secure the workshop manual (PDF or print) to get the exact wire color codes, pin numbers, and connector layouts. 2. Inspect Case Connector & Harness Carefully remove the transmission pass-through connector on the case. Look for ATF contamination, corrosion, bent pins, or melted insulation. Clean, repair, or replace as needed. 3. Back-Probe & Test Voltage With the engine running and at idle, back-probe the ISS and 1-2 solenoid wires at the PCM side. Compare voltages to spec. Then carefully heat the case area to see if warm conditions replicate voltage drop or signal loss. 4. Monitor Live Data While Wiggling Harness Use your OBD2 scanner to view live ISS RPM and solenoid duty command. Gently flex the harness and watch for signal drops or erratic readings—especially under temperature changes. 5. Ground Integrity Checks Clean and securely torque key ground points (G102 engine block, G105 near battery frame rail). Low or fluctuating voltage can trigger intermittent codes across modules. 6. Connector and Pin Fit The PCM and transmission connectors should have firm contact—old pins can loosen over time. Check for wiry or worn connection. 7. Confirm with Re-Test After harness repair or cleaning, reset codes, drive until warm, and observe if the 2nd-gear start / no 1st or 4th gear limp mode returns. Monitor for return of P0717 or P0751. Why This Will Help
- P0717 (ISS low voltage): If the PCM loses proper speed input, it defaults into a limp strategy—starting in 2nd, no 4th gear, harsh shifts.
- P0751 (1-2 Solenoid): Often a byproduct of limp mode rather than a bad solenoid.
- Intermittent codes (cascade of BCM, EBCM, U1300, etc.): Could stem from poor voltage or communication caused by corroded connectors or bad grounds.
Visual Insight
That valve body diagram snapshot gives you a visual orientation:
- You can locate the ISS sensor port and solenoid cluster.
- Helps when referencing wiring diagrams to trace wiring harness routes and connector relationships.
Would you like help locating specific wire colors and pin numbers for the ISS and shift solenoids? Or is there a wiring harness schematic you’d like me to walk you through once you’ve accessed the manual? Let me know—happy to guide further!
You said:
Yes
ChatGPT said:
Here’s a wiring diagram snippet showing the 4T65E transmission’s main connector with pin numbering and wiring colors—this visuals will serve as a foundation for tracing your Input Speed Sensor (ISS) and solenoid wiring.
Pinouts & Wiring for 2006 Buick LaCrosse 4T65E
1. Identified Pinout Overview
The image above corresponds to the external transmission harness, particularly the large round connector between the transmission and PCM. Each pin is mapped with color coding, matching solenoids and sensors. While the exact pin-to-component mapping may vary slightly by vehicle build, this is a foundational reference.
2. Common Wiring Routing & Specifications
- The ISS is a passive magnetic sensor that requires a clean signal path to the PCM to relay input shaft speed. If its signal is degraded—due to corrosion or wiring issues—the PCM often enters limp mode, triggering codes like P0717 and P0751 gmforum.comgrandprixforums.com.
- The 1–2 Shift Solenoid (Sol A) is similarly routed through this harness and controls first-gear engagement. Its misbehavior can be symptomatic, but many times is secondary to signal loss from the ISS.
- Factory wiring diagrams for your car, including complete pinouts and color codes, are available through:
- Paid PDF wiring manuals (e.g., Service Manual or Electrical Diagram & Repair Manual) servicepartmanuals.cominstantcolorwiring.com;
- Automotive electricians' portals that list components like ISS high/low signal, solenoid control, fuses, PCM/Class 2 data lines Portal Diagnostov.
What You Can Do Next
A. Obtain Full Wiring Reference
Acquire a proper wiring diagram:
- Purchase the official 2006 Buick LaCrosse electrical wiring manual for full detail (pinouts, wire colors, harness layout) instantcolorwiring.comservicepartmanuals.com.
- Alternatively, access it via subscription to a platform like Portal-Diagnostov, which catalogs A/T wiring including ISS, solenoids, PCM connections Portal Diagnostov.
B. Locate ISS & Solenoid Wires
Once you have the manual:
- Identify which pin number(s) correspond to the ISS high and low signals.
- Trace out the 1–2 shift solenoid control pin and its reference ground or control path.
C. Testing & Diagnosis Workflow
- Backprobe the PCM connector:
- Check continuity and proper voltage/low voltage present.
- Use multimeter to ensure reference voltage and return path are in spec.
- Wiggle harness while capturing live data:
- Observe ISS RPM and solenoid duty.
- Wiggle around the connector or wiring; a glitch in live data—or complete signal loss—indicates wiring or pin issues.
- Inspect physical connector:
- Remove the connector on the trans case.
- Look for ATF leaks, corrosion, bent or pitted pins—common failure point due to fluid seepage and heat.
- Ground & Voltage Integrity Checks:
- Reclean and torque major ground points (G102, G105).
- Measure voltage under load; loose grounds or low voltage can cause CAN comm issues and cascade multiple codes (e.g., U1300, EBCM faults).
Why This Matters
- Your recurring limp mode (starting in 2nd, 1st/4th gear missing) matches what happens when the ISS signal drops out or solenoid commands fail—despite replacing every component, poor wiring or ground likely underpins everything.
- Fixing wiring integrity ensures stable communication between transmission and PCM, preventing limp behavior and associated error codes.
Let me know once you have access to the pin-to-feature mapping—or if you'd like help interpreting a specific page or tracing a wire color—happy to guide you through it step-by-step!
Can only imagine the frustration you must have........
Amen we are getting wizard level advice for free God bless this guy.
That's not true, The industry is full of incompetent people, If a transmission shop can't fix a 4T65E after firing the parts cannon.....They need to shut the doors & go get a job in finance.
Where's the accountability & integrity? Misdiagnosing something happens to every mechanic no matter how good they are, It's how you handle it that matters.
Mr06Lacrosse.....
You need to scope the 1-2 Shift Solenoid control circuit & the 2 ISS Sensor circuits.
*Current & Voltage on the 1-2 SS
*AC sine wave on the ISS
This article touches on shift solenoid electrical testing, Pintle Hump & Inductive kick aspects of the waveforms. HERE
P0717 will freeze shift adapts.
P0751 diagnostic routine WILL NOT run with a P0717 set!!
The above facts don't align with the AI generated procedures you were given.....Auto repair/diag requires good vehicle specific repair information at times!!