1984 CUCV transmission.

[Christianfishn]

Hello, first post here I've been hanging around for awhile but as I seriously face purchasing my first MV I thought some of you would have very helpful input.
I'm looking at a 1984 m1008. the truck is in decent shape with minor body rust and surface rust on the frame, Interior needs work but at my price-point all of this is expected. The truck starts great and goes true and I have good feelings about it. Only problem is the transmission (TH400) shifts good into drive, goes through first into second normal and then 3rd gear just doesn't happen at all. There is no grinding and it appear not to even try to shift up. Reverse works fine so I feel the internals are in working order. Am I wrong to think this? If I go through with purchasing it I would replace the governor and modulator to see if that would fix it which isn't a high investment at all. If that doesnt do the truck what should I check? Is there anything weird that I would be missing? If all else fails I will be looking at a rebuild or a swap. I can easily source a working TH400 from a gasser chevy 3500 4x4 at a fair price, would it be as simple as using the torque converter I have and installing that one in? or do I need to modify the internals. I have experience with manual trannys but very little automatic transmission experience. Any advice suggestions or thought on this would be great. Thanks for your time.

 

[NDT]

Welcome. My thoughts are your TH400 can be fixed without pulling it. If the modulator and related doesn't fix it, the control body sounds like it has a stuck spool.

 

[Christianfishn]

Is the control body the valve body? Just to make sure I know what you are talking about. Would an additive be able to free it up?

 

[NDT]

Yes valve body. I don't think an additive will fix it. Could also be the governor like you said, check that before the valve body.

 

[Christianfishn]

Ok awesome, thanks for the help.

 

[MarcusOReallyus]

You're all making this way too complicated. It might be as simple as some vacuum lines. Less than 5 bucks. That's the quick and easy fix for most shifting problems on these old trucks, and getting into modulators etc. before you take care of the vacuum lines is just throwing money way.

 

[Christianfishn]

I wasn't aware that the diesel 6.2L even ran a vacuum line. Ill definitely put that on the list to check out thanks.

 

[TexAndy]

Yep, down to the side of the transmission. Metal line connected by a little piece of rubber hose to the shift modulator or whatever that little cylinder is.

Mine was rotted completely in half. New piece of rubber was a little difficult to get on, but doable.

 

[MarcusOReallyus]

There's a vacuum generator (pump) where the distributor would be on a gasser. It's how the diesel makes vacuum to make the thing shift. That trans was designed for a gasser and shifts by vacuum, so when they put it behind a diesel they had to come up with a way to make the thing shift. So, vacuum pump. Therefore, vacuum lines. Old truck, old rubber, lots of leaks, lousy or no shifting, easy fix.

There ya go! It's the place to start. If that doesn't do it, THEN start looking at other things.

 

[Csm Davis]

Hello, first post here I've been hanging around for awhile but as I seriously face purchasing my first MV I thought some of you would have very helpful input.
I'm looking at a 1984 m1008. the truck is in decent shape with minor body rust and surface rust on the frame, Interior needs work but at my price-point all of this is expected. The truck starts great and goes true and I have good feelings about it. Only problem is the transmission (TH400) shifts good into drive, goes through first into second normal and then 3rd gear just doesn't happen at all. There is no grinding and it appear not to even try to shift up. Reverse works fine so I feel the internals are in working order. Am I wrong to think this? If I go through with purchasing it I would replace the governor and modulator to see if that would fix it which isn't a high investment at all. If that doesnt do the truck what should I check? Is there anything weird that I would be missing? If all else fails I will be looking at a rebuild or a swap. I can easily source a working TH400 from a gasser chevy 3500 4x4 at a fair price, would it be as simple as using the torque converter I have and installing that one in? or do I need to modify the internals. I have experience with manual trannys but very little automatic transmission experience. Any advice suggestions or thought on this would be great. Thanks for your time.

Okay the one thing you didn't say was that you manually shifted from 1st to 2nd to drive to see each gear, a 1008 will shift out of first so fast many never even feel it. Top speed is about 55.

 

[nyoffroad]

The first place to start is with the dipstick not the vacum lines. Is the fluid at the correct level , is it pink or dark? Does the fluid smell burned? Is the dipstick even there and appear to be the correct one? (don't laugh it happens)
If all appears to be good and after buying it run threw the gears manually one at a time. Not being used to these trucks you may not even realize it's in 3rd gear, if it is then by all means check the vacuum system.
If you do need to rebuild or replace DO NOT REUSE THIS TORQUE CONVERTER! It would most likely have debris in it that will cause problems in a new tranny and void any warranty.

 

[timarr]

Like everyone said that shift wont happen because of a few cheap parts, modulator 13.00,vacuum tube 5.00,governor gear 15.00, or miss adjusted vacuum valve.

 

[Christianfishn]

Alrighty, picked up the truck today. All fluid levels are good and the cap is tight on the dipstick. Ill start going through starting with the vacuum assembly tomorrow. Does anyone know around what speeds the transmission should be shifting with stock gearing? thanks.

 

[Christianfishn]

Ok... replaced vacuum lines and made sure vacuum pum is making vacuum. I pulled the modulator and found transmission fluid in it that then dripped out not sure if it is supposed to do this... also in other posts people mention a retaining pin that should be loose in the modulator. I found no such pin... maybe I cant recognize it? I took a picture of the modulator and then inside the modulator housing in the tranny, this is what I saw...

 

[NDT]

I think I see the pin still in the transmission. If the pin were gone the transmission would have never shifted out of first before. You say it goes into second. Please double check which gears it really is going into by manually shifting the lever into 1, 2 and D. My experience has been that if it shifts out of first at all, the modulator and related are working properly.

 

[Christianfishn]

NDT you're right the pin is in there i was able to pull it out with soft nose needle pliers. When it came out it seemed pretty firmly placed, putting it back in was the same. It wasn't stuck per say but if i picked the whole truck and shook it on its side it wouldn't have fallen out haha Is this normal? Other than that the vacuum system seems good.

I can manually shift into 1st and 2nd without any issue and know for a fact it wont go into third. At 40mph the engine is revving very high.

I noticed a drip of atf coming out of what looks like the governor cover gasket and a small accumulation of atf on a few bolts on the cover... I am planning on ordering a governor and gasket, Should I drop the pan and replace the gasket or be weary of doing this? Thanks for the help.

 

[NDT]

I would clean up the modulator bore such that the pin moves super easy. Next I would pull the governor and see what the gear looks like. If "reving very high" is about 2400 rpm at 40 mph you ARE in 3rd gear. 55 mph is somewhere around 3000 rpm which is screaming for a diesel, but they can take it.

 

[Christianfishn]

Ok i will get that smoothed out and clean. Is there a chance the vacuum regulator is bad? Mine only has one bolt mounting it to the ip.

 

[cucvrus]

Yes that is possible. At this point in time anything is possible. That transmission could be 30 years old and never seen daylight. So at this point all you can do is service it with new fluid and a filter and check all the connections and adjust the vacuum modulator and make sure the vacuum lines have no holes in them and there is vacuum present. I just got my transmission back from the shop today. It shifted fine and everything was well except in cold weather it would take 30- 45 seconds to go into gear. I had it gone over. It had a bad torque converter and pump seals were dry. I guess that is what he told me. But don't be surprised if it is something minor or something major at this point. Everything is aged on these trucks. Good Luck and have a nice day.

 

[Christianfishn]

Ok, govenor is fine clean and moves freely and end gear is in good shape... I have vacuum to the modulator o-ring is good. Cleaned the pin and it easily moves as well. Can I bypass the vacuum regulator to see if it will shift into third? or is there no hope in trying that? Now I am at a standstill other than doing fluid and filter...