TH350 Dead

[1980K20]

I’ve been driving my 1980 k20 for nearly three years i think, haven’t posted in forever.  Fixing lots of little things in the way.

Today was 80’s day at the local car show and guess what, transmission died in the driveway.  Full rebuild in it, maybe 1400 miles.

Engine: 1990 sbc350, some power improvements but nothing crazy.  I liked the roller cam.
Transmission: th350, appears to be original to the truck. Full rebuild, heavy duty parts.  Torque converter probably 800rpm.  It was in bad shape when i got it.
Transfer case: np205, also rebuilt but no issues

There’s fluid in the transmission.  Linkage works.  When i shift there’s absolutely no sound other than the clicks of the linkage.

Thoughts?  Somebody told me to hit it with a rubber hammer, lol.

Maybe sheared the bolds off for the torque converter?  I didn’t hear anything.

Suggestion for debugging appreciated.  I’ll call transmission shop in Monday.

Thanks
  -Mark

[1980K20]

I went out to start it up, loosen a cooling line to see if it had pressure.  And it moved!  After running a bit it stopped moving.  I killed the engine, restarted, and i was able to move it again!

I checked fluid level, cold, half way inbetween add and full.

Update: 2400 miles on it since the rebuild

[VileZambonie]

Chock the wheels, drop the pan, inspect, check the manual valve, make sure the filter is properly installed. If you don't see anything obvious do a pressure test.

[1980K20]

Thanks VileZambonie,

I waited to talk to transmission guy, knowledgeable and i trust him.  He did not rebuild the transmission, but did rebuild the transfer case.  He’s since fixed some other cars for me with success.

He said it sounds like the filter is plugged.  If there’s a bunch of stuff in there, when you turn it off, you can get enough back pressure through the filter to get fluid through again.  But run it s bit and it plugs up again.  He said if i replace the filter, that will get me going, but with all of 2400 miles on it, whatever’s going wrong will likely continue and end up plugging the filter again.

I’ll take it in in two weeks.

Sigh.

[Shifty]

If there is that much crud in the pan, you have to wonder where it came from.  After changing the pan filter, I'd throw an inline filter that's far easier to change until the problem goes away-which it won't without replacing the clutches...

[1980K20]

If i just drained the pan, what do you think the chances if the gunk, be it metal or seals, would come out?  I’m super interested in what ails this tranny, but don’t like tranny fluids and wasting money.

[Shifty]

If the debris is too big to drain out of the drain hole (whether aftermarket pan, of the home-made version) then your trans is done anyway. 

[JohnnyPopper]

I agree with Shifty, but would pull the pan just to see.

You WILL see metallic flakes, hopefully not too bad.

I have the same trans in my K10, I'm keeping it alive by keeping it clean, in other words, I'm on borrowed time 

How many of you use rare earth magnets to catch the flakes? The best location is on the top of the drain plug, if you have one.

[1980K20]

Alright, my anxiety level is through the roof now.  Bought a filter and gasket and another gallon of red stuff that smells to me like vomit.  Trying to hold off to Sunday, big project on my international on Saturday to prepare for.

[VileZambonie]

Drain the fluid into a container so you can analyze it. Drop the pan, it's a piece of cake especially if you have a drain plug.

[ehjorten]

Another piece of advice...if you have metal in the pan, you will have metal in the converter. You can never drain that out. When a transmission grenades, you pretty much want to just replace the converter. Another thing you can do is install an inline magnetic filter on the cooler line. Replace that magnetic filter periodically. It is good insurance to keep metal particles from circulating through the system.
https://www.oregonperformancetransmission.com/product/NTP-22776F.html?srsltid=AfmBOorsjEJlJB8zAxaCRJrNT6WmyQ6QTyJrkXhZ6-REcjdGJtsWWowLa70

[1980K20]

Thanks for all the advice.  First the good news
1. The fluid didn’t smell near as bad as i remembered.
2. Perhaps the reason my transfer case always leaks is the hose vent was hitting the top of the truck.  Perhaps it put enough pressure to seal it and not let it vent properly.  Maybe.

Now for the bad news.

I thought i struck gold with that pretty metallic looking fluid.  The filter was certainly clogged with mostly small shavings and some big shavings.  I have videos but for now a few pictures.  We’ll sort, i don’t know how to make the pictures small enough from this fancy iphone. 

Started thinking about a new tremec.  Stick shift!  Just more cost and hassle.

[1980K20]

https://youtube.com/shorts/drVFOgP7OLk?si=Oh-yQinaDKk9BjVJ
https://youtube.com/shorts/EeVeYaXNbOQ?si=vMzfcBiIWcD9Tw2o
https://youtube.com/shorts/5ra9A9epQek?si=iknvZCSIZFMN2OwZ
https://youtu.be/9RAl2-R9i10?si=a4VAczwETKgh72UR

[JohnnyPopper]

With that much 'gold' I would think you're on borrowed time as well.

If your priority is to get her back on the road, you can gamble that you can clean it up enough with new filter and oil to get there. You won't know till you try, ergo the 'gamble'.

Or go for the replacement.

If option 1 is your path, I would strongly recommend some rare earth magnets to catch the 'dust' that will still be in circulation.

[Shifty]

Fortunately, a TH350 is the cheapest transmission out there, and hard to screw up.  In the 80's, I built a bunch of them for less than $100 with a shift kit (but minus the converter)