M 1008 Trannny Issue Thankful for any IDEAS

[mijon]

Hello Green Community,

I have 86 M 1008 that's been giving me some tranny Issues. Any Ideas/ Help would be greatly appreciated!!

Explanation of Problem:
-When I first start driving the truck it shifts into all gears fine and just as it usually should. However, when I get about 20 miles on her, it will not shift into 3rd gear. Or it will slip into 3rd gear after driving in 2nd for about a mile or so. I followed the TM procedure ( see below) and everything checks out. The vacuum houses are tight and clean and I just changed the Transmission Module. Problem still persists.

Before I get into rebuilding the tranny . I was wondering if you guys could think of any other trouble shouting ideas?? The truck has only 57,000 miles on it. Ive used it as a plow truck / work truck. But it seems like I shouldn't be having these Transmission issues with that low of miles??

Please let me know if you need any other information and thanks for your time !!

60. FIRST AND SECOND SPEEDS
ONLY; NO 2-3 SHIFT
Step
1. Check tailpipe, muffler, exhaust manifolds, and exhaust pipe for
restrictions. (Clear any restrictions)
Step
2. Check for damaged or loose vacuum hoses. Remove, clean, and inspect vacuum
modulator assembly and modulator valve. (See paragraph 5-3)
Step
3. Remove, disassemble, clean, and inspect governor assembly. 5-5)
Step
4. Remove, disassemble, clean, and inspect transmission control (See paragraph
5-7)
Step
5. Remove, disassemble, clean, and inspect direct clutch assembly,
Step
6. Remove, disassemble, clean, and inspect front servo assembly (see paragraph
5-1 O) and rear servo assembly (see paragraph 5-9).

 

[porkysplace]

What color is the fluid ?
Does it smell burnt.
Have you changed the fluid and filter recently ( or at all ) ?

It is a 30 year old truck , seals dry out and harden inside the transmission , mileage really doesn't matter.

 

[mijon]

Porky,

Thanks for the reply! I just changed the fluid and filter about a month ago. That was the first thing I changed when the issue came up. Before that though, the fluid and filter where original to the truck.

 

[GM72K10]

This helps too, especially if the truck was sitting for a while.
Check for damaged or loose vacuum hoses. Remove, clean, and inspect vacuum
modulator assembly and modulator valve

 

[porkysplace]

Was there much sludge in the pan ?

 

[Tinstar]

I would drop the pan and see if any pieces are in the bottom.
Filter flow issue?
What brand filter did you use?
Fram filters have been known to come apart internally and suck the filter debris inside the oil/fuel passages.
Fuel filters especially.
Thats why I only use Wix Filters.

Agree mileage doesn't matter on these.
Is your vacuum pump working?
Did you replace the rubber up there?

Plowing is very hard on these (according to old threads. I have no personal knowledge).

Sounds like a rebuild build is coming soon.
If all suggestions check out and problem still persist then time to spend some $$$.

 

[Christianfishn]

I had a similar issue when I first purchased my m1028, truck would not shift into third at all though. However I dropped the valve body and inspected all the parts. the direct clutch spring which holds pressure on the direct clutch piston was broken and the piston was cracked. If your piston which is plastic originally, is cracked or broken the th400 will no be able to or have trouble shifting into 3rd. i have more pictures somewhere around here if you need them. Its not hard to drop the VB with the tranny still in the truck. I would check this before I pulled the whole tranny, it will also emable you to check for vavle actuation and any clogging.

heres a link to the valve body diagram: https://goo.gl/images/Rj9gFy

 

[cucvrus]

If my opinion matters. it is transmission time. I have been there on several trucks. And i plow a lot of snow. A lot of snow. The transmission is over 30 years old. I have put new ones in out of the can that lasted less then 6 months. Plowing snow is about as hard as you can treat any vehicle. It tears things up. Save the head aches. Pull the transmission and have it rebuilt. i just paid $1800. to have a transmission and transfer case completely over hauled. It is like a new truck again. I don't even put a used transmission in anymore before I have it at the shop and checked out. Saves me a lot of time and labor. Sometimes the bill is $200. sometimes it is $1000. But in the end I know what I have and how dependable the truck is going to be. A broke down plow truck costs thousands of dollars a day to down time and lost commerce. OK good Luck. Save the hassle and get it done right the first time. Happy New Year.

 

[Chevybrit]

I've just had the same problem .. Vac pressure was good to modulator, no major debris in the pan. I decided to rebuild myself and it turned out to be the front accumulator piston $9.
Along with the 2/3 valve this controls the 3rd gear shift.
I did not want to shell out the kind of money for someone else to rebuild mine so I went ahead and did it. I pulled the tranny out on the 23rd and i was running around in the truck today at the supermarket.
You may find this helpful starting P5
http://www.steelsoldiers.com/showthread.php?27303-To-Rebuild-or-not-to-Rebuild-TH400/page5

 

[Chevybrit]

Christianfishn mentioned the same thing above .. here was mine (pink) it had a hair crack in it. It's possible to replace it insitu (with an aluminium one) paying close attention that you loose none of the check valves when you spilt the valve body.. Easy on the bench and you can check the plates and bands whilst your in there..

 

[mijon]

In Conclusion!!

Turns out the 3rd clutch was toast and a couple teeth where gone on the fly wheel. Thanks everyone for your thoughts and ideas!!

I hope this post can be of some help to someone else someday!!

HAPPY NEW YEAR!!