* Neutral Safety Switch (NSS): This is the most common culprit. The NSS prevents the engine from starting unless the transmission is in Park or Neutral. However, a failing NSS can also prevent the engine from running *while* in drive or reverse. A faulty switch might not be making proper contact, causing the engine to lose power when the transmission is engaged.
* Transmission Problems: Issues within the transmission itself could be causing the problem. This could include:
* Low Transmission Fluid: Check your transmission fluid level and condition. Low fluid can cause various problems, including stalling.
* Worn Transmission Components: Internal wear and tear within the transmission (e.g., worn clutches, bands, or other internal parts) can lead to power loss under load. This is more likely if you're experiencing other transmission issues like slipping or hard shifting.
* Torque Converter Issues: A faulty torque converter can cause the engine to stall or lose power.
* Ignition System Issues (less likely): While you can start the truck, a weak ignition coil, distributor cap, rotor, or wiring could fail under the increased load of driving. This is less likely if the problem only occurs in drive and reverse, but it's still possible.
* Wiring Issues: Damaged or corroded wiring related to the transmission, NSS, or ignition system could be interrupting the electrical signal needed to keep the engine running under load. This is often exacerbated by vibrations and the movement of the vehicle.
* Computer/Sensor Problems: While less probable, a problem with the engine control module (ECM) or related sensors could be causing the issue. This would require more advanced diagnostics.
Troubleshooting Steps:
1. Check Transmission Fluid: This is the easiest first step. Check the fluid level and condition. Low, dirty, or burnt fluid indicates a problem.
2. Inspect the Neutral Safety Switch: Locate the NSS (usually on the transmission itself). You might be able to visually inspect it for damage or corrosion. You could also try gently wiggling the shifter while the engine is running to see if it affects the engine's operation. This is risky, do not continue if your engine stalls.
3. Test the Ignition System (if other steps fail): Have a mechanic or experienced auto enthusiast test the ignition components for proper function.
4. Professional Diagnosis: If you can't pinpoint the problem yourself, take it to a qualified mechanic. They have diagnostic tools that can check for various sensor faults, electrical problems, and transmission issues.
The Neutral Safety Switch is the most likely culprit given the symptoms, but a thorough inspection is essential to rule out other possibilities. Don't drive the vehicle extensively while this problem persists as you could cause further damage.
