TH350, shifting weird, leaking and low.

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naminator

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Jul 21, 2020
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I have a 1978 GP, with an 81-85 307 Oldsmobile (Vin Y or Vin 9 with 5a heads), hooked into a TH350 which is hooked into a 7.5" rear end. Since the beginning of August, I have taken the car from not running and rotting away, to running, stopping, driving and so close to what I would consider done, prior to me selling the thing to the next owner.

Anyways, when I first looked at the car, I noticed it was leaking ATF pretty aggressively. Because I refused to toss cash at it without making sure it ran, I ended up tossing 2 different drip trays under it and forgetting about it for a bit. I continued to work on things on the car and the drip tray's I used both had a significant amount of fluid in them, however, some of that was water from a few rain storms, leaves from various trees, some oil, some brake fluid, some gasoline, some brake cleaner etc. Basically they where under a leaking car while I worked on it. The trans fluid was right in the hash marks, but started much higher when I got it (I figured it was just over filled).

I sprayed the pan with some brake clean and the leak actually ended up stopping after that. Fluid level still in the hash marks. I diagnosed some issues and took the car for an initial test run. I noticed that shifting from park to reverse was fine enough, but if you moved the shifter around (console shifter not column of course) it would drop into neutral. After that, you could shift to drive and it would sometimes engage, sometimes it would sit in neutral requiring another pullback to get it to engage. I drove it a little distance, slowly, getting up around maybe 25-30 mph, and it wouldn't shift up. So I limped home. The trans fluid is now at the bottom of the hash marks, but still in the correct level, getting close to "add 1qt" or whatever it was. Last night, I drove it again. This time getting it up to a blistering 35mph, which caused it to upshift 1 time during this process, but everything seemed fine. I drove it home, check the fluid and now I can't even see a level, appearing to be completely empty (no burnt smell no nothing and the fluid was nice and red before).

I know the pan is leaking. I am going to be replacing the gasket and cleaning the pan out, along with the filter/magnet (or replacing) however I have been told that cleaning out a neglected TH trans and putting in new fluid can cause more problems with sludge/debris being push around. As for the shifting issue, I am unsure if this is a vacuum modulation issue, the trans being warn/low, the Kick down cable being trashed (which wouldn't explain to me the weird engagement) or maybe something else I am missing. I am trying my very best to wrap up this project to sell, since I no longer really have space for the car, at least right now, and the snow is about to fly.
 
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g0thiac

G-Body Guru
Sep 6, 2020
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I know the pan is leaking. I am going to be replacing the gasket and cleaning the pan out, along with the filter/magnet (or replacing) however I have been told that cleaning out a neglected TH trans and putting in new fluid can cause more problems with sludge/debris being push around.
I'm pretty sure that is correct, as sometimes the old fluid has a viscosity that pretty much the transmission becomes conditioned to after a certain time of being used for awhile.

But I would still do it, because then at least you know right away if it needs a rebuild or replacement. Keeping the old fluid in only buys you a bit more time, it doesn't actually fix problems that already are an issue.
 
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naminator

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Jul 21, 2020
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I'm pretty sure that is correct, as sometimes the old fluid has a viscosity that pretty much the transmission becomes conditioned to after a certain time of being used for awhile.

But I would still do it, because then at least you know right away if it needs a rebuild or replacement. Keeping the old fluid in only buys you a bit more time, it doesn't actually fix problems that already are an issue.
That is a big concern. But since it appears that the trans fluid is low or non existent now, along with what was a fast leak of ATF and the busted kick down, I am almost being forced into doing the flush/clean. Hopefully, because the fluid looked decent when I saw fluid, it has had a leak for a long time and basically self flushed over the years.
 
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ELCAM

G-Body Guru
Jun 19, 2021
714
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Sounds like most of the problem is from low fluid level.

The pan certainly can be part of the problem but I would also check the sector shaft (shift shaft) seal and the speedometer gear adaptor. If it has a 350c to match the later engine the plastic speedometer gear adaptor does not have a seal in it and will leak as clearances increase. with wear.

The TH350 speedometer adaptor is steel and about 1" in diameter.

The TH350c adaptor is over 2" in diameter and will be plastic (most common) or sometimes is aluminum. The aluminum one has a lip seal that can be replaced the plastic one the whole adaptor has to be replaced.

When you get the fluid level high enough the pump is not cavitating then worry about other possible issues like the modulator or kickdown cable.

If the fluid is real clean I wouldn't worry about changing it, when it is completely trashed it is often the crud in the oil providing the friction material in the clutch packs. Putting new oil in it cleans off all the crud and no more friction material.
 

naminator

Apprentice
Thread starter
Jul 21, 2020
53
18
Sounds like most of the problem is from low fluid level.

The pan certainly can be part of the problem but I would also check the sector shaft (shift shaft) seal and the speedometer gear adaptor. If it has a 350c to match the later engine the plastic speedometer gear adaptor does not have a seal in it and will leak as clearances increase. with wear.

The TH350 speedometer adaptor is steel and about 1" in diameter.

The TH350c adaptor is over 2" in diameter and will be plastic (most common) or sometimes is aluminum. The aluminum one has a lip seal that can be replaced the plastic one the whole adaptor has to be replaced.

When you get the fluid level high enough the pump is not cavitating then worry about other possible issues like the modulator or kickdown cable.

If the fluid is real clean I wouldn't worry about changing it, when it is completely trashed it is often the crud in the oil providing the friction material in the clutch packs. Putting new oil in it cleans off all the crud and no more friction material.

So I am thinking I will drop the pan, clean the pan both inside and out, put in a new gasket (with RTV if needed), bolt it back up and then fill with fresh fluid and probably some of that Lucas trans slip stuff, then test. Assuming it shifts better, the cleaning will reveal any leaks, if present, since it won't be covered in crud. Then I can deal with the other issues.
 
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ELCAM

G-Body Guru
Jun 19, 2021
714
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When I got my 79 Camino it was like this. The pan was covered in oil but it was not the pan leaking. The shift shaft is right above the pan and looks like the pan is leaking. The speedometer gear adaptor leaking also runs forward and down onto the pan. I have never pulled the pan off of it and it runs fine now that it is sealed up.

I would clean those areas and see if they are leaking. At 5 bux a quart i would rather nor waste perfectly good transmission fluid.

One of the things about a real leaky transmission like this is it has likely had a lot of new fluid put in it so the fluid may be basically brand new.
 
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naminator

Apprentice
Thread starter
Jul 21, 2020
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When I got my 79 Camino it was like this. The pan was covered in oil but it was not the pan leaking. The shift shaft is right above the pan and looks like the pan is leaking. The speedometer gear adaptor leaking also runs forward and down onto the pan. I have never pulled the pan off of it and it runs fine now that it is sealed up.

I would clean those areas and see if they are leaking. At 5 bux a quart i would rather nor waste perfectly good transmission fluid.

One of the things about a real leaky transmission like this is it has likely had a lot of new fluid put in it so the fluid may be basically brand new.

I was working on a separate, cooling based based issue last night. while running the car for that, I checked the trans dip stick. It had the barest hint of red on the very tip. So yeah, it drained out at some point, some how. Don't know if it was sitting and started draining backwards and over flowing out, or what, but it is clearly going to require a fill up and that probably means I will be swapping gaskets anyways. With how clean the fluid was/is, I suspect it has had a constant leak that made constant top ups necessary.
 

83MonteCarloSS

Greasemonkey
Oct 5, 2015
130
43
I don't flush the transmission for the reason stated before about old fluid. If you just drain, change the filter, and refill you are only doing a partial change. I haven't had any issues doing it this way on an overdue fluid change.

I wouldn't use the Lucas. I've tried it and never noticed a difference. If the clutches are gone they are gone. In my opinion you would be better off doing a shift kit to boost pressure and lessen the shift time rather than waisting money on a trans fix in a bottle. Shift kits are pretty cheap and work.

The last shift kit I put in a TH350 was a B&M one that I got on eBay for cheap. I was happy with it. It firmed up the shifts without being neck snapping like the previous transmission that had been done by a shop. I've had good luck also with Trans-go shift kits but only used them in a 200-4r and a front wheel drive vehicle. Good luck.
 

naminator

Apprentice
Thread starter
Jul 21, 2020
53
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Well, for whatever reason, I went and checked the engine/trans cold on Sunday, to try and work on a coolant flow issue. I checked the trans fluid and it was again, low. I added 1 bottle of lucas stop slip (I had it, it was on sale for $9 and clearly it fixes everything trans related) mixed with a bit of cheap atf. I idled the car for a bit after adding this, put it in gear and it seemed to engage the gears, decently well. I didn't want to really push it, but reverse, neutral, drive 2 and 1 all where there. I drove around the block, parked it, gave it a few revs, rowed the gears a few more times etc and shut it down. Popped the hood and checked the fluid and now it is over filled. I am guessing a bit of the trans fix is in the tube, so I am letting it sit and percolate down.

Maybe it just needs to be driven some more to really circulate the fluid. I desperately need to fix the temp gauge and maybe install a tach, so I know what everything is doing. You don't know how much you miss those things until you don't have them.
 

83MonteCarloSS

Greasemonkey
Oct 5, 2015
130
43
Ok, stupid question but are you checking the fluid level with the engine running?

You should be checking it with the fluid warm not hot, engine idling, in park, on a level surface.
 
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