GMC transmission questions,(UPDATED)

Ok Folks,I'm kinda new in this aera so bear with me,

I just pruchased a XM 211 that I now have running,BUT It only has foward and reverse in LOW range,an it does not want to shift up in LOW either just stays in First. When it is put in HIGH range it seems to go in gear( the engine stalls down somewhat) but it doesn't move and the driveshaft from the trans to the T-case will not turn by hand at that time either. I have some expeierence in offroad heavy duty auto transmissions and Allison as well,but kinda wondering if maybe this is a simple problem.Any Input would be welcome.

RE: GMC transmission questions

It has been a while since I studied up on these, but it could be something simple like adjustment problems in the linkage. This transmission shift points are controlled by engine rpm if I remember correctly. Could have the wrong fluid in the transmission. They came out with motor oil, but could be changed to transmission fluid. I don't remember what had to be done to change from oil to transmission fluid.

RE: GMC transmission questions

Oil or Atf? I used 40 weight and have had better luck with that then Atf.
fill the oil with the truck running and in gear.
You could try adjusting the linkage from the carburetor to the transmission either forward or back just take note of where it was.
Hope this helps.
Best of luck.

RE: GMC transmission questions

the transmission may have had to be rebuilt to use ATF. Then you never know what has been done with these older trucks. One reason I moved on to stuff straight out of service.

tranny troubles

you bought a truck and didnt get a manual ? TM 9-8024,operators manual, TM 9-8025 transmision manual are both in the technical library for you to download,in TM 9-8024 pages163-170 ,trobleshooting,read this first,then read pages 355-363,transmision,manual and throttle linkage adjustment,READ EVERYTHING BEFORE YOU DO ANYTHING,THEN ADJUST THE TRANNSMISION AND ALL THE LINKAGES, these trannys are rather sensitive to the way the are adjusted,follow the adjustment procedure exactly,and maybe all will be well,good luck on your project,and on the subject of oil or atf,i use only 30wt NON DETERGENT OIL,the LO ,lube order calls for it,i buy mine at THE TRACTORE STORE,WHERE IT IS AVAILABLE IN 5 GAL CANS,i tried atf and switched back to 30wt,because it made tranny shift a little smoother,just my experience,and i have put 33k on my truck since i switched

This guy has TM 9-8024 in book form.....lots of copies...... E-bay # 350040319291........ good luck ......... Tim

When I first owned an M135, I talked with a reputable transmission shop owner that had worked on them in the military and also built the civilian version for racing. He adamantly stated to get the motor oil out of it and put in ATF because the motor oil has the wrong detergent and shear qualities. The military did it to reduce inventory of different oils. I did as he suggested, and used the truck in heavy farm service for 20 years with no transmission problems. I used it to haul grain, pull a 28 foot implement trailer, and had a 10 foot angling snow plow on it. I never had any seal problems. I think that is proof enough to debunk the tireless calling this a weak transmission by some.
As with automobile transmissions, the harshness or smoothness of the shift is an indicator of the transmission set-up. Smooth transmission shifts that keep mom's Sunday hat from quivering are accomplished by momentarily engaging two bands/clutches at once to smooth out the shift. One can at times notice the effect by hearing the motor and car seeming to momentarily drag as the shift occurs. Racing shift kits time the shift to make the split second disengage/engage upshift happen with no "overlap" of engagement. Yes, a harsher shift happens, but there is not the obvious wear of the stock way of having two gears momentarily engaged at the same time. You can understand how this causes wear! Your softer shifts with the engine oil are the result of the oil not getting out from between the frictions and the drum or clutch fast enough, allowing slipping for a moment, causing wear and heat build-up. Harsher shifts from the ATF mean the bands/clutches are engaging instantly with no slipping and less wear at each shift.
Regards Marti

UPDATE!!

Well,I finally got around to working on this GMC trans some more,according to the manual the linkages are adjusted correctly,And I pulled the trans pan off(heavy sucker!) to find the pickup screen gummed up and about 1/2 inch of sludge in the bottom!!

Needless to say I drained the converter and rear box on the trans(cleaned the pickup screen there too) and cleaned the trans oil pan+screen.Installed everything and refilled with the correct weight oil.Fired it up and it seens to go in gear much quicker and firmer,but it still just moves in LOW range only,When shifted into HIGH,the trans seems to go in gear,but it will not turn the output shaft and the truck will not move at all,seems like something is locking down in high range.Any Ideas?


AMGeneral

Sounds like the linkage between high and low range isn't allowing it to go into high range. My understanding is that the high/low range is external to the trans. Might try crawling underneath and having someone inside shift between high/low and see if you can see the linkage that controls it.

There is a large clutch release spring in the rear of the transmission that is probably broken. You need to remove the reduction unit to get at it. Call Tom at Alfa - Heaven he will steer you in right direction. He has helped me a great deal by his knowlege. PM me for pictures, I have manuals. Bob C.

OK,I drained and flushed the trans system again and put Dextron III in it this time. Took it out for a spin and after a short drive I tried HIGH range again,IT WORKED!!!,I guess it is still kinda sludgy inside,so I guess I need to flush out the trans several more times to get it working properly.


I have the proper manuals coming.

When I had my M-135 tranny rebuilt about 15 years ago, the experienced mechanic had me run Dextron III and it has been fine ever since. Tom @ Snake River

tranny shifts

hey yall,didnt use the correct word in my previous post about tranny shifts,instead of "smooth ",i should have used the words " more consistent",since the tranny shifts are controlled by 2 things,engine rpm,which controls the governor vaccum,and the position of the shift lever,(HILLY),(LEVEL),in hilly the tranny shifts at max governed engine speed,and only 1-2 gear,level is different,it shifts at smaller amount of vaccum at the governor and uses all the gears ,1-4,after many tranny adjustments ,trying to get a more consistent shift,i finally realised that,i had a vaccum leak,the diaphgram on the carb governor was shot,and that was the reason i couldnt get tranny to shift ,simple to fix bought a carb rebuild kit and fixed the leaky diaphgram,and immiediatly the erratic shift problem was cured,this maybe of some help after you get up and running,another place to look for problems is at the 2 rubber lines at the distributor,if they are leaking ,well i dont have to tell you what that means,hope this helps,good luck to all on your projects

Update....

Been working on the trans in the M211 off and on lately,finally got to spend an entire day on it with the manual.
So far here is where i'm at...

All linkages checked and adjusted properly
Governer is working and allowing Max engine speed
Checked and adjusted the front band
Did my pressure check as per the manual, 120 psi in all FOWARD ranges(F-1,F-2 HIGH range,HIGH range REV press 210 psi)

IT still will not shift up from 1st(?) to a higher gear,in low or high range,according to the manual,all that is left is the rear oil pump,control valve,or the manual detent control valve lever.

I plan on calling the gentelman at Memphis on this Monday morning,but just asking all the GMC gurus here,have I missed anything?

tranny problem

hey.you had previously said tranny was full of sludge,maybe some of the valve/springs that are in the transmisiion control valve are stuck or sludged up so that they cant move,in TM9-8025-2 tranny manual you can remove and clean this without pulling the tranny,this manual is for the 303M tranny which is the later model of these trannys installed in the GMC trucks,there is another manual for these trannys it is TM 9-1819AB which covers the early model of these trannys,you should check the data plate on the tranny to see which model that you have,just from looking the valve assy/disassy procedures are the same for both tranny models,i had already asked raymond at memphis equip about this as i had a slow 3-4 shift and he told me to change the oil coupl of times and if that didnt fix it dissassemble the unit ,buy thev gasket kit from them and clean the unit ,look for a broken or weak spring or sludge/varnish buildup in thevalve body,or a sticking valve,i have both these manuals ,dont know if they both are in the SS library,TM 9-8024 says you are on the right track about what to look at,good luck

dumba## ,TM 9-1819AB is for the 302M model tranny,hope this helps,TM 9-8025-2 also covers the 301MG model tranny which doesnot have the hi lo 2 speed reduction unit

Ok,I have let this thread go too long with NO UPDATE!!!

Here goes, Well,after several cleaning sessions and more than 4 fluid changes and reading the 8024 and 8025 several times,I now have the trans shifting 1 to 2 consistently in both LOW and HIGH ranges. But after some testing It appears the servo piston seals are leaking pressure not allowing the trans to shift properly past 2nd gear.So next step is pulling both servos(front and rear) and re-sealing them. I'm getting good expierence with these transmissions!!

Hydramatic Servo

i have aNOS,NIB ,rear servo assy,part# 7411443,,NOS,NIB,front pump assy,part#7410903,governor assy,NOS,NIB,part# 7410974,#8333714,Piston front clutch,#7411523,Piston Servo,#7084053,Piston Trans,#8333720,Piston Reverse Clutch,all of this is NOS,NIB,plus 3 pgs of other NOS,NIB, Hydramatic tranny parts,PM if interested ,if you are going that far in the tranny,you should think about replacing assys instead of trying to fix them,IMHO,good luck on your project

PM sent!!

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