1. Check the Obvious (and Safe):
* Transmission Fluid: Low or dirty transmission fluid is a very common cause of shifting problems. Check the fluid level (with the engine running and warmed up) and its condition. It should be reddish-pink and clear, not brown or burnt-smelling. If it's low or dirty, a fluid change is crucial. This is the most likely cause of your shifting issues.
* Battery and Electrical Connections: A weak battery or corroded battery terminals can affect many electrical systems, including the transmission controls and cruise control. Check the battery voltage and clean the terminals.
* Fuses and Relays: Consult your owner's manual to locate the fuses and relays related to the transmission and cruise control. Check them for signs of blowing or damage.
2. Investigate the Transmission Issues More Deeply:
* Shifting Pattern: Describe the difficulty. Does it only happen in certain gears? Is it sluggish, jerky, or does it refuse to shift at all? This helps pinpoint the problem.
* Check Engine Light (CEL): Is the CEL on? If so, get the trouble codes read using an OBD-II scanner (available at auto parts stores). The codes will give you valuable clues about the problem.
* Transmission Solenoids: Transmission solenoids control gear shifting. A faulty solenoid can cause shifting problems. This usually requires professional diagnosis and repair.
3. Address the Cruise Control:
* Cruise Control Fuse/Relay: Check the fuses and relays specifically for the cruise control system (your owner's manual will show you their locations).
* Cruise Control Switch: The switch on the steering wheel might be faulty. This often requires professional replacement.
4. Potential Underlying Problems (More Serious):
* Transmission Control Module (TCM): A malfunctioning TCM can cause a range of shifting issues. This requires professional diagnosis and may need replacement.
* Wiring Harness: Damaged wiring, particularly around the transmission or under the hood, can disrupt electrical signals to the transmission and cruise control. This is difficult to diagnose without professional help.
* Vehicle Speed Sensor (VSS): This sensor is important for both the cruise control and transmission. A faulty VSS can cause both systems to malfunction.
What to Do Now:
1. Check the transmission fluid level and condition immediately. This is the easiest and cheapest thing to do first.
2. Check the fuses and relays.
3. Get the trouble codes read if the CEL is on. This is essential for accurate diagnosis.
When to Seek Professional Help:
If checking the fluid, fuses, and relays doesn't solve the problem, or if you have a CEL, take your GMC Sierra to a qualified mechanic or transmission specialist. The problem could be something complex that requires specialized tools and diagnostic equipment. Attempting advanced repairs yourself without the proper knowledge can worsen the situation.
