Seems as if all gears stuck in neutral

Hey 4runner owners. I'm new to the group (I've read the forums for over a year now though!),
I have a 2000 4runner and a 2001 4runner. I have different issues going on with each one.
My 01 is the one that I'm more stumped on. About 2 weeks after purchasing used from a seller on fb marketplace, I was headed to buy an impreza. I was going around a corner and up a small knoll, when all of a sudden my runner just started reving and wouldn't move. I pulled it to the side of the road and put it in park, turned the engine off. I started her back up and she started and shifted into gear as usual. So I obviously thought it was strange but continued on my way. Once starting up over a mountain road, she stopped dead center and wouldn't go, just revving as if in neutral. I shifted through the gears (the gears sounded like they were grinding? Never heard such a thing), and she caught and kept driving as normal. Also, she was leaking transmission fluid when we were stuck on the incline. I took my kids out of the truck because i just didnt know if she would end up loosing gears and rolling back down the steep mountain. My bf was annoyed at me but i of course didnt care as i hadn't ever experienced this issue so i didnt know what to expect! I was able to get up over the mountain to coast down the other side and park at a gas station. I got the subaru owner to meet me there. I had to leave my runner and get her a tow home.
Once home I did some extensive research and tried to examine a few things. It seems like she's stuck in neutral. Every gear seems as if its in neutral. I have been able to get her to drive around my yard and driveway a few times, just to end up doing the same thing in the end. I have owned many runners, this isn't a bad transmission, this is something simple but hard to diagnose.
Here's what I've come up with; shift solenoid? New radiator? I've already flushed the transmission fluid. (it was Hella dirty! Must have never been done before!) My bf has never owned nor experienced anything to do with 4runners. He keeps calling it junk and doesn't want to put any money into it because he thinks it's a junk transmission. I however know that's NOT the case. It's a simple fix that I just haven't figured out just yet. I've never experienced this.
Please, any and all personal experiences and/or knowledge is much appreciated! Help me get her on the road again. I want to show him how easy this fix is and how strong these 4runners truly are. I'll ask my other question in a different thread. Thank you for your time. I appreciate all of you!

 

If it was observably leaking transmission fluid, I suspect there isn't enough fluid left for it to work properly. It could have other problems as a result. I hope for the best.

 

It sounds like you’ve done some diagnostics already, but I’m not sure why you don’t think it’s the transmission.

You mentioned the radiator. I’ve heard the 3rd gens can suffer from pink milkshake syndrome, which is basically when the piping between the transmission cooler inside the radiator gets a leak and you end up with coolant and transmission oil mixed together. Do you see any evidence of that in the coolant, or in the transmission fluid you previously drained?

 

What he said. Its either low on fluid, bad transmission cooler/radiator, or transmission needs to be rebuilt. Or 2 out of 3.

 

I of course, drained the remaining fluid and replaced it with fresh fluid. Is that what you're saying? What was the cause or the fluid leak though?

 

I really don't think that it has anything to do with the transmission needing to be rebuilt. It starts right up and runs AMAZING. It sounds so strong! It idles so strong! It also goes into gear (D), since flushing the bad and replacing the new. It just won't go, ya know what I mean?

 

What they're saying is since it has a fluid leak that it could be low on fluid which will make the transmission slip and that could be the cause of your initial issue and the issue now if not refilled properly or the fluid has leaked enough out since refilling.

Did you check the fluid level properly when you refilled it?

There's no way for us to say why it's leaking without knowing where exactly it's leaking, it could be a front seal, a rear seal, pan gasket, drain plug or transmission lines leaking.

If you know for sure the fluid level is was correct and it was still slipping then I'm afraid the transmission is probably shot and news to be rebuilt/replaced

 

It does sound like it’s something related to the transmission. It’s literally not transmitting engine power to the wheels.

You said the old fluid was dirty. Do you recall seeing any metallic sparkles in the fluid? Did the drain plug magnet have anything chunky on it? The reason I ask is because you said it made gear grinding noises, which almost always is bad on an automatic transmission.

But maybe it has to do with the fluid level. How did you set the level? Some vehicles you need to check the level with the car running, but on others it has to be off. Not sure what the manual says on yours. I would double check the level before spending any money.

One more thing to check…check to make sure the transfer case didn’t accidentally get shifted into neutral (between 4hi and 4lo). Make sure it’s shifted into 2hi.

 

Thank you for reminding me, I knew about this at one point, but my memory is terrible and I did forget. I'm gonna go check her out first thing in the morning. I feel like I'm on to something again. Makes me super excited! I want Mt 4runner back on the road so bad I can almost taste it!