M-1009 transmission woe's

[underdog]

All stock.
This truck runs and drives good.
But has a strange problem i just cannot get my head around.
Ok so here is my discription of truck.
Motor and trans are clean no or negledgable leaks.
Changed transmission filter and fluid.
Checked vacume hose to modulator no trans fluid there.
Vacume hoses about rotted off it but still there.
This is it"s problem.
It stinks of burning fluid like trans type when driving.
Seems the transfer case is leaking towards the trans.
May be slinging fluid onto exhaust.
Anyway it will just lose drive and reverse. Just driving along and like it went into nuteral.
You can pull it into 4 low and continue on for a short distance then shift back to two wheel drive and all is well till it quits again. Or you can shut it down and wait a few minutes and it will usually be fine.
So what does 4 Low have to do with it ???

 

[dilvoy]

The t case shift fork may have the plastic tips broken off and it won't properly position the slider gear and you are slipping out of gear. Might be something else though.

 

[ARMYMAN30YearsPlus]

any leak of vacume will cause bad problems on shifting but the smell of burning trans fluid is not normally related to a leaking rubber vacume hose. The smell could be from driving it without shifting a clutch inside is burning up

 

[underdog]

Yes i know i need to change the vacume lines. But that should cause different problems.
But the truck drives and shiftes fine while it is working.
I remember sometimes if a modulator valve goes bad it can send trans fluid to the motor via the vacume line.

Interesting about the transfercase may be dropping into neutural.
I think i remember trying 4 high and no go also.
I think the problem is with the transfercase also.
If it was the trans once it quit it would be all over till you repiared or replaced something.

 

[doghead]

Underdog, you have an M1009 not an M109.

 

[underdog]

Yes M-1009.
I are spastic ..................
Thank you for understanding.
Hey i got it in the right section? I think??

 

[85-m1028]

did you account for the fluid in converter when changing it out? check the trans fluid level on flat ground, engine running, and shifted in neutral..

check the vac pull at the vac pump should be 20" replace those vac hoses and the canister on the side of the trans mission

I did have a problem with the t-case and trans swaping fluid but only out on the trail sitting at steep angles for long periods of time.

 

[maybefixit]

One note - the vacuum modulator on the transmission often has an effect on the entire transmission's internal pressures. Having them too low / high could lead to seal problems, and can also cause rapid clutch pack wear. Definitely replace rotted vacuum hoses - they do make a big difference, including those in other places on the vehicle. There's not a lot of vacuum source with a diesel, so the effects of leaks will be multiplied.

The modulator has a diaphragm which, if it cracks or leaks, will allow trans fluid to be sucked out of the trans. Fortunately the modulator is usually kinda easy to replace. Doesn't sound like yours is bad, tho.....

Have you looked up at the radiator area? There are rubber lines connecting the metal lines from the frame to the radiator's internal transmission cooler. If your vacuum lines are rotten then those probably are also. Might be 'misting' trans fluid into the fan area while driving, but not leaking enough to notice otherwise.
I think those would be 3/8" or 5/16" lines, probably only a couple feet to replace. If you've been trying to figure out a 'power steering leak' but that level was mysteriously good, then this could be part of the issue. I think you could replace the lines with reinforced rubber fuel line hose, which should have an imbedded string woven in it for strength. Don't use a low pressure (under 10 psi rating) hose that lacks the reinforcement - transmissions use high pressure and a burst hose could dump your trans fluid while driving.
There is also another set of rubber lines connecting the frame's metal lines to the transmission, might check those too if the ones at the radiator are shot.

It's a long shot, but maybe if the leak at the cooler is allowing an air bubble to go through the trans, that stopping the truck and shifting to neutral (or park) is clearing the bubble, and the 'shift to low range' is not really involved. However, it could be that there's a transfer case issue on top of the fluid leak.

If you do find badly rotted hoses, maybe crawl under and see how the fuel hoses look. Don't want those springing a leak!

 

[CCATLETT1984]

the trans cooler on the cucv has all metal lines, no rubber.

 

[superburban]

Here are some of the possibilities, as I understand your description:

1.) Transfer case slipping into neutral.
I dont think this is it, from the description, because if this were the case, shifting into 4-Hi would solve the problem.

2.) Clutches in transmission are worn and slipping.
This would be consistent with your description. What I think could be happening is that as you are driving, the offending clutch heats up from a little slippage, causing a lot of slippage. When you shift into 4-Lo, it does not require as much torque to move the vehicle, so it gets moving again. I think your transmission needs R&R-ed.

BTW, I happen to have a good running TH400 sitting in my garage gathering dust!

 

[taylordc15]

That is what mine was doing, it would run fine for about 30min and then would quit shifting, if you let it cool down it would run fine again for a while until it heated back up. It was worn clutches as described above.

 

[Mike_Pop]

BTW, I happen to have a good running TH400 sitting in my garage gathering dust

And I thought I was the only one who kept a spare TH400 tranny in my garage!!! Mine is completely rebuilt though.

 

[appnut1]

I can tell you that if you shift the transfer case from 2 hi to 4 lo and back again and it drives away the problem is a worn out gear in the transfer case. Had an '86 K10 do this to me. As far as the oil swap there is a seal in the transfer case that will let the oil out of the case and into the transmission, there is also an adapter between the two that likes to come a little loose and leak as well.