GMC Automatic Transmission Problems 

Vehicle:   GMC
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GMC trucks, vans, and SUVs manufactured by General Motors (GM) include models such as Yukon, Terrain, Acadia, Canyon, Sierra, Savana. 

In this article, we focus on some of the most common problems and issues we notice with GMC vehicles. Here we document a number of GMC transmission issues, common causes, and tips on how to troubleshoot GMC transmission problems yourself.

Symptoms

GMC transmission problems
  • GMC won't move when placed in drive 
  • Shifter won't shift out of Park
  • Can not move shifter back to Park
  • Check engine light stays on
  • Vehicle stuck in fail-safe mode. Also known as the limp mode. The vehicle will not change gears. 
  • Vehicle won't move in any gear
  • Transmission slips 
  • Harsh shifting

Common Causes

GMC Valve Body
  • Low transmission fluid level - One of the most common issues that prevent a GMC vehicle from shifting when placed in the Drive (D) or Reverse (R) is low transmission fluid level. 
  • Shifter link - The shifter link connects the shifter to the transmission valve body. A misaligned shifter link or rod can prevent the transmission from moving when placed in Drive. That's because even though you have selected Drive or Reverse, the transmission is not placed on the same gear. 
  • Brake light switch - If you can not move the shifter out of the park or back into park position the problem can be due to a faulty brake light switch. Other possible cues include bad ignition interlock or  faulty shifter. 
  • Valve Body - A faulty valve body can cause a number of problems on a GMC vehicle. You may notice harsh shifting, delayed shifts, cars shifts only at high RPMs or no shifting at all.
  • Vehicle Speed Sensor - Transmission vehicle speed sensor does not only cause erratic shifting, it often leads to transmsion getting stuck in one gear (limp mode) and triggers the check engine light. 
  • Overheating - Overheating of transmission fluid is another common problem that can lead to harsh shifting issues and premature failure of a GMC transmsion. Overheating is a concern on GMC trucks that are used for towing or hauling heavy loads frequently. 

Common GMC Transmission Problems

GMC Transmission

Delayed or incorrect shifting

GMC vehicles that use a 4-speed automatic 4L60E or 4L65E transmission can develop a problem with gear shifts. This issue will manifest itself as improper gear changes that take too long to happen. In some cases, the transmission will not downshift during accelerations. These symptoms can be constant or intermittent and may trigger the check engine light along with a transmisison related code such as P0700. 

Possible causes and solutions: 

  • Faulty throttle position sensor, which sends incorrect data to the TCU. This is likely to trigger a check engine light. There will be a fault code stored in Engine Control Module (ECU) memory as well. The operation of the throttle position sensor can be monitored and verified using most diagnostic tools. 
  • Worn Torque Converter Clutch (TCC) regulator valve, will cause harsh shifting from 1st to 2nd gear. This will trigger a check engine light and may also trigger fault code P1870 code. The regulator valve is inside the valve body, and there are numerous rebuild kits with upgraded components. See GMC Transmission Rebult Kits
  • Clogged or faulty shift solenoids, resulting in electrical glitches or jammings. If the fault is electrical, it may trigger a check engine light. Mechanical issues and clogged solenoids will rarely result in a check engine light.  

Missing higher gears or transmission stuck in 1st gear

GMC vehicles that use a 4-speed automatic 4L60E or 4L65E transmission can have intermittent or constant problems when switching to higher gears. There are two usual scenarios. In one of them, the vehicle will refuse to shift from 2nd to 3rd gear.

In the other one, only the 1st gear will be available, and the vehicle will refuse to engage any 2nd-4th gears and reverse. 

Possible causes and solutions: 

  • Worn clutch pack for 3th and 4th gear, resulting in the inability to engage 3rd gear. The wear happens over time because of a leak within the piston that engages the clutch. The solution is to replace both the engagement piston and the clutch pack.
  • Broken drive shell collar or worn collar splines, limiting the transmission to 1st gear only. Other drive gears and reverse are no longer available. In most cases, this happens out of sudden and stays that way. Replacing the drive shell is the only solution. 

No reverse available 

GMC vehicles with a 4-speed automatic 4L60E or 4L65E transmission can have intermittent or constant problems with selecting reverse. This will manifest itself as an inability to engage reverse gear, while there are no issues with the engagement of other gears. There will be no other symptoms. 

Possible causes and solutions: 

  • The worn or faulty servo engages the reverse gear. When it starts leaking, it will not be able to create oil pressure needed for gear engagement. Replacing it is an easy job, but it requires the oil pan and valve body to be removed. 

Incorrect gear shifts, gear slipping or overheating 

Newer GMC trucks equipped with a 6-speed 6L80 transmission can suffer from various intermittent or constant transmission issues. Symptoms include harsh or delayed shifting, inability to engage certain gears and slipping when shifting gears. These problems can even lead to transmission overheating in some cases. 

Possible causes and solutions: 

  • Worn pressure regulator system, which ensures correct hydraulic pressure needed for proper operation of the transmission. When parts inside this system wear out, the pressure drops and causes various shifting issues. The solution is to rebuild or replace the pressure regulator system. 

Shaking and shuddering during gear changes

GMC vehicles with 8-speed 8L90 and 8L45 transmission have a widespread problem known as ‘Chevy shudder’. The symptoms that may happen during gear shifts and typically include shaking, juddering and harsh shifting. In severe conditions, it may be followed by a noticeable clunk. Issues are usually present during any gear change, although more noticeable in 1st and 2nd gear. 

Possible causes and solutions: 

  •  Incorrect transmission fluid type, which is a well known and documented problem with all GM vehicles that use this 8-speed transmission. There is a Technical Service Bulletin (TSB 18-NA-355), which describes that replacing factory-installed transmission fluid with Mobil 1 Synthetic LV ATF HP fluid solves the issue.  
  • Glazed torque converter as a result of prolonged usage with shudder problem present. This will cause similar symptoms even after the transmission fluid has been flushed, and replacing the torque converter is the only solution. 

Improbable transmission temperature 

GMC vehicles with 8-speed 8L90 and 8L45 transmission can develop intermittent problems regarding the transmission temperature. Symptoms that drivers can experience include improbable temperature readings to overheating, slipping, and harsh shifting. These issues will trigger a check engine light, so there will be a fault code stored in DTC memory. 

Possible causes and solutions: 

  • Faulty transmission fluid temperature sensors, which send incorrect and implausible data. The TCU will recognize this and trigger a P0711 code. Besides the sensor, both wiring or connector might be broken. 
  • Low transmission fluid level or dirty filter, which can cause overheating. High fluid temperatures will cause other issues with transmission operation. 

Troubleshooting GMC Transmission Problems

Most GMC transmission problems such as harsh shifting delayed shifting, no gear, stuck in gear, no reverse or drive are quite often caused by low transmission fluid level. 

If your GMC vehicle with automatic transmission is not shifting properly, check the transmission fluid first. See instructions below on how to check the transmission fluid level on a GMC. 

If transmission fluid level is correct, use a transmission scanner to read fault codes via the OBD-II port. See the instructions below. 

How to Check GMC Transmission Fluid Level

 

  1. Park vehicle on level ground. Park GMC
  2. Set the parking brakes and shifter in Park.  Pull the hood release and open the hood.  Open GMC Hood
  3. Secure the hood open. GMC hood open

     

  4. Locate the transmission dipstick. GMC Transmission Dipstick

     

  5. Remove the dipstick and clean it with a clean cloth. 
  6. Reinsert the dipstick in the transmission. Ensure the transmission is fully inserted then remove it. 
  7. Look carefully at the dipstick to determine the current transmission fluid level. The level should be between MIN and MAX marks for the COLD (lower) markings. 
  8. If the level is low, add transmission fluid level. 
  9. Next, reinsert the dipstick, close the hood and drive vehicle for 15 minutes to warm up the transmssion fluid.
  10. Park on a level ground and repeat the procedure above to recheck the transmission fluid once the transmission has warmed up.

Not all GMC vehicles have a transmission dipstick. If the dipstick is not present, the vehicle will need to be raised on a lift, and the level can be checked via the fill hole. 

Read GMC Transmission Fault Codes

Diagnose GMC transmission problem

The next step is to read fault codes from the transmission control module or what is known as the TCU. To read these codes, you will need a GMC OBD-II Transmission Scanner. Basic code readers are not recommended because they will show a generic code or may not show a fault code at all.

  1. Park vehicle and turn off the ignition. Set parking brakes. 
  2. Locate diagnostic port under the dashboard, driver's side. 
  3. Plugin your OBD-II scanner then turn on ignition without starting the engine. 
  4. The scanner will turn on. Allow it to communicate with the vehicle. Select GM then your particular model. 
  5. Select Control Units then Transmission. 
  6. Select Read Fault Codes from the menu.  

These instructions work on all 2000 and newer GMC trucks, including Yukon, Terrain, Acadia, Canyon, Sierra, Savana, etc. 

The scanner will display a fault code, for example: 

  •  P0700 - This is a generic code meaning there is a malfunction with the transmission. The check engine light will be on, and transmission may be in failsafe mode. 
  • P0729 - is defined as the Incorrect  Gear Ratio. 
  • P0741  - There is a signification difference between the torque converter and the transmission input shaft.

These general codes. If you are using a transmission scanner that can read GMC specific codes, you will get more detailed descriptions. Make sure to research each code and keep in mind the same code could be caused by many possible problems. 

By YOUCANIC Automotive Experts
Aug 2020