New GMC Owner with shifting troubles

Hello squarebody community!

I'm new to the world of 80s tech/ vehicles, but I'm eager to learn the workings!
I've been working on my 1981 GMC G3500 5.7L V8 for about 8 or 9 months. Over the winter I had it stationary and parked so it hasn't been engaged with or running for a while.

The past couple days I finally decided to get it over with and did a full service (new radiator hose, trans filter +gasket, new battery, oil change, etc..). The dash needed to be put together (adding an updated stereo system), as it was left unassembled for quite a few months. It took some time trying to understand where everything needed to be plugged in, but I got most* of the important stuff sorted.. With that being said I gave her the first start since the beginning of winter - it took some cranking but she started up!

Now before the winter she was driving and shifting just fine, a little clunky when going into gear, but while driving my shifts felt fine! However, when I started her up and tried to shift in gear I noticed that I didn't need to press the brake peddle in order to shift into gear; and when in gear no amount of revving would get the wheels rolling.. She seems to be stuck in park. After some research I have a feeling that it might be my shifting cable, but I'm not sure what to look for or how to aid this ailment.
I do also want to mention that I filled the transmission with about 1.5L of the correct trans oil, but nothing is showing on my dipstick - (yes I am checking while the engine is running). Is it still just too low on oil, and could this be the reason for not going into gear?

Thank you for your help, and hopefully we can get her moving again - I'm very eager to give her a nice long drive!

WhosCacti said:

I do also want to mention that I filled the transmission with about 1.5L of the correct trans oil, but nothing is showing on my dipstick - (yes I am checking while the engine is running). Is it still just too low on oil, and could this be the reason for not going into gear?

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Yup. Get it filled to the proper amount running and warm, and check again.

1.5L ain't gonna cut it. Like @Bloodhound1981 said, put the proper amount in it. Then drive a short distance, park on level ground and check it again. Don't over-fill it and don't check it unless it's hot.

Welcome from Missouri.

Welcome from Austria. We want pics.

Welcome from the Texas Panhandle.

Thanks everyone for the welcome messages! Going out to the store to grab some more fluid and give that a go. Will update shortly!

Also added a pic of the exterior. Going to do some touch up painting/ rust repair later in the summer.

Snoots said:

1.5L ain't gonna cut it. Like @Bloodhound1981 said, put the proper amount in it. Then drive a short distance, park on level ground and check it again. Don't over-fill it and don't check it unless it's hot.

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Thanks for the tip!

Just added another 2L (all the store had) and not much is showing up on the dipstick (idling hot in park).

Now my head has been turned in another direction. After shutting her off and trying to turn the engine over, nothing. No cranking or anything. I’m wondering if my alternator is causing the root of the problem. I cant see any other reasons as to why it wouldn’t crank after about 15 mins of idling.

(For context I keep my batteries negative thermal disconnected because I’m still unsure if I have a parasitic draw (separate issue))

ali_c20 said:

Welcome from Austria. We want pics.

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Pic in thread! Hello from British Columbia Canada!

Welcome! Nice rig

Can you solder?

Snoots said:

Can you solder?

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If I need to know how to solder, I can solder haha