1. Linkage and Cables:
* Broken or dislodged shift linkage: The mechanism connecting the gear selector to the transmission could be damaged, preventing proper engagement. Inspect the linkage visually for bends, breaks, or loose connections.
* Worn or broken shift cables: Similar to the linkage, the cables can fray, break, or become stretched, hindering the transmission's ability to shift.
2. Transmission Fluid:
* Low transmission fluid: Insufficient fluid deprives the transmission of lubrication and pressure necessary for shifting. Check the fluid level using the dipstick (ensure the engine is running and warmed up when checking). Low fluid suggests a leak which needs attention.
* Contaminated transmission fluid: Dirty or burnt fluid affects the transmission's performance. If the fluid is dark brown, smells burnt, or contains debris, a flush and filter change are needed.
3. Transmission Internal Problems:
* Low fluid pressure: A problem within the transmission itself, such as a worn pump, clogged filter, or failing valve body, can reduce fluid pressure, preventing the shift. This is a more serious problem.
* Internal damage: Worn clutches, bands, or other internal components can fail, making shifting impossible. This often results in other symptoms besides just failing to shift into Drive.
* Governor malfunction: The governor controls shifting based on vehicle speed. If faulty it could prevent shifting into drive.
4. Electrical Issues (Less Likely but Possible):
* Transmission control module (TCM) problems: Older vehicles had simpler electrical systems, but a malfunctioning TCM could still interfere with shifting. This is less common for a simple inability to shift into drive, but it's possible.
* Shift interlock solenoid: This solenoid prevents shifting out of park unless the brake pedal is pressed. A faulty solenoid or related wiring could prevent shifting into drive.
Troubleshooting Steps:
1. Check the transmission fluid: This is the simplest and most important first step.
2. Inspect the shift linkage and cables: Look for any visible damage or looseness.
3. Try manually shifting: Some Aerostars may have a manual valve body (check your specific model). If so, see if you can manually force the shift. *Caution: This should only be attempted if you are familiar with manual valve bodies, otherwise you risk further damage.*
4. Listen for unusual noises: Any unusual sounds coming from the transmission (whining, grinding, etc.) indicate internal problems.
Important Note: Without a proper diagnosis, attempting repairs on an automatic transmission can be risky and potentially lead to more extensive damage. If you are not experienced in auto transmission repair, it's best to consult a mechanic specializing in older vehicles. The 1986 Aerostar is an older vehicle with a likely less sophisticated transmission system than modern ones, making professional diagnosis fairly straightforward.
