Replacing gear shift cable

Here are some potential causes and solutions for your issue:

1. Shift cable: The shift cable might be stretched, damaged, or improperly adjusted. Inspect the cable for any visible damage and ensure that it moves smoothly. The cable may need to be replaced or adjusted to engage both forward and reverse gears reliably.

2. Control unit: The control unit, or shift lever, could be worn or have a damaged mechanism. Inspect the control unit for wear and tear, and ensure it is functioning smoothly. If it's damaged, it may need to be repaired or replaced.

3. Gear selector mechanism: The gear selector mechanism inside the outdrive may be worn or damaged, preventing reliable gear engagement. This issue would require a more in-depth inspection and potentially disassembly of the outdrive to diagnose and fix.

4. Clutch dog or gear wear: The clutch dog or gears inside the outdrive may be excessively worn, causing unreliable gear engagement. This issue would also require disassembly of the outdrive to inspect and potentially replace the worn components.

As with any mechanical issue, it's essential to ensure that you follow all safety precautions and consult a professional technician for accurate diagnosis and repair. Working on a boat's drive system can be dangerous and complicated if you're not experienced, so it's best to get professional help if you're unsure about any aspect of the repair.

Question edited for clarity.
Question moved to model category.

Video here

https://www.google.com/search?q=1999+Chevy+Silverado.+How+to+adjust+the+shift+cable

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check for broken selector cable or missing link bars

You first need to unhook the shift cable from the top of the gear box , then carefully remove it from the gear liver linkage and follow the reverse step to fix the new one. I am sharing a video of GM hope it will give some refrence .

replace transmission more likely cable need replacing old shift cable stretched more likely.

You might try to change the trany filter and oil.

i dont think the two things are related check your speedo cable inner cable could be frayed or kink on outer cable if not disconect cable from speedo end and lube it

The neddle is attached to the column via a thin metal wire and spring which then clips to the edge. Remove the trim above the column and look for the clip attached to the wire and a warn spot on the column and reattach it. :)

Only if its a 3 spd tranny. If O/D then check tv cable on carb. If good you can check gov also. Whats fluid look/ smell like?

Tha vacuum modulator will not keep the truck from shifting,it only helps with sift quallity.it sound like you have some internal problems and will need to take it to a transmission shop