1980 K20 TH400 problem

[Austin4wd]

So I got a K20 and its a TH400 truck. It runs like a top, just my trans doesn't seem to overly want to work. It drove forward about 50 feet and stopped. I put it in reverse and I got about 10 feet. Turned it off and let it sit a little bit and it does the exact same thing. Is my ATF to dirty and my filter is plugging up or is the trans gone?? I'm literally freaking out as I want to have it running before summer ends

[LTZ C20]

Have you checked the fluid level and condition?

[Austin4wd]

I don't know if it's gone or not, but it does hold ATF. Is it a clogged filter maybe?

[LTZ C20]

It's possible, you may have to drop the pan and examine and inspect the inside of filter and see if there are any obvious issue. Does fluid register anywhere on the dipstick?

[Austin4wd]

It registers on the cold operating range when I don't have it running and on the warm/hot side after its idles for an hour, so I'd say healthy operating range

[LTZ C20]

Transmission fluid should always be checked with the engine idling in park on a flat and level surface.

So you go out when the truck is cold, start it, let the engine RPM drop to idle and then check the fluid level, if it registers in the cold area you are good, if it's over cold, it's over filled, if it's under cold, your low.

So once you check it cold, let the engine continue to idle for an hour or so and recheck it, it should now be in the hot range. Remember that compared to the engine temp, it takes transmissions longer to come to their full operating temperature, but once there, they should be close to the operating temp of the engine. If I remember correctly, 10-20° variance in temp between engine and transmission is good.

If your readings match cold to hot, then the fluid level is most likely not an issue. Meaning, it registers cold when cold and hot when hot. If it registers low when cold and low when hot then it's low for sure. If it measures over when cold and over when hot then it's over filled for sure. If the readings don't match, like low when cold and over when hot or cold when cold but under when hot, then fluid level needs to be investigated further.

I suggest a recheck of the fluid level just to be sure as you said you checked it cold with the engine off. Over filling a transmission is just as damaging as under filling one.

Report back with the results of the recheck.

[Austin4wd]

I went and checked it did a cold reading and then waited for a while and did a hot reading, and the levels are where they're supposed to be. So my fluids are where they're supposed to be. I tried moving it and it only does the same thing. I'm reducing my problem to transmission is shot or the filter is clogged. Odd, a TH400 has never given me this amount of trouble

[LTZ C20]

Ok, sounds like you got good results. How was the color, consistency and smell of the fluid? Did this issue suddenly just happen or did you the truck this way?

[Austin4wd]

The colour was the usual red, it didn't smell like burning kitty litter, and the consistency was the same as fresh ATF. Yes, I bought the truck like this, and I'm not complaining for what I got for $500, with this being the only problem with it

[hatzie]

Sounds like the PO burned it up and changed the fluid in an attempt to bring it back to life.

[LTZ C20]

Sounds like the PO burned it up and changed the fluid in an attempt to bring it back to life.

I am believing that this probably the case. It would explain why it was sold so cheap if that's the only thing wrong with it. Luckily for you, it shouldn't be too terribly expensive to rebuilt depending on what parts have failed once it has been removed, disassembled and diagnosed.

[Austin4wd]

That's what I'm thinking, as my K10 also has a NP205 and that t-case has shredded 3-4 TH350s, so it makes sense it ate a TH400. I'll try replacing the filter, and if that doesn't work then I'll start hunting for a new trans

[LTZ C20]

I highly doubt that the transfer case caused the failure of the transmission. A TH400 is far stronger than a TH350, even in stock form. So I would investigate further into the cause of the transmission not working properly instead of condoning the transfer case as the reason for the failure. It is most likely NOT related to the transfer case, but far more likely internal to the transmission.

[Austin4wd]

I'm only placing my bet that the transfer case could have eaten the trans, but I'm not gonna quote on it. I'll do some more investigations once I get some other projects cleared up so I can drop the pan and check to see what's going on. I got a buddy who can help me rebuild it if I need too but it's still gonna run me up a couple hundred bucks. The guy I bought the truck from said it ran and drove fine then started acting up abou three years ago, and it was time to upgrade. On the plus side, I picked up a '90 Cavalier from the same guy cause he just wanted it out of his yard 😂😂😂

[LTZ C20]

Sounds like a plan. I really like the 400, it is an excellent transmission.